Version 17 (modified by simonpj, 7 years ago) (diff)


Note To Reader

This page was written with more detail than usual since you may need to know how to work with Cmm as a programming language. Cmm is the basis for the future of GHC, Native Code Generation, and if you are interested in hacking Cmm at least this page might help reduce your learning curve. As a finer detail, if you read the Compiler pipeline wiki page or glanced at the diagram there you may have noticed that whether you are working backward from an intermediate C (Haskell-C "HC", .hc) file or an Assembler file you get to Cmm before you get to the STG language, the Simplifier or anything else. In other words, for really low-level debugging you may have an easier time if you know what Cmm is about. Cmm also has opportunities for implementing small and easy hacks, such as little optimisations and implementing new Cmm Primitive Operations.

A portion of the RTS is written in Cmm: rts/Apply.cmm, rts/Exception.cmm, rts/HeapStackCheck.cmm, rts/PrimOps.cmm, rts/StgMiscClosures.cmm, rts/StgStartup.cmm and StgStdThunks.cmm. (For notes related to PrimOps.cmm see the PrimOps page; for much of the rest, see the HaskellExecution page.) Cmm is optimised before GHC outputs either HC or Assembler. The C compiler (from HC, pretty printed by compiler/cmm/PprC.hs) and the Native Code Generator (NCG) Backends are closely tied to data representations and transformations performed in Cmm. In GHC, Cmm roughly performs a function similar to the intermediate Register Transfer Language (RTL) in GCC.

Table of Contents

  1. Additions in Cmm
  2. Compiling Cmm with GHC
  3. Basic Cmm
    1. Code Blocks in Cmm
    2. Variables, Registers and Types
      1. Local Registers
      2. Global Registers and Hints
      3. Declaration and Initialisation
      4. Memory Access
    3. Literals and Labels
    4. Sections and Directives
    5. Expressions
    6. Statements and Calls
    7. Operators and Primitive Operations
      1. Operators
      2. Primitive Operations
  4. Cmm Design: Observations and Areas for Potential Improvement

The Cmm language

Cmm is the GHC implementation of the C-- language; it is also the extension of Cmm source code files: .cmm (see What the hell is a .cmm file?). The GHC Code Generator (CodeGen) compiles the STG program into C-- code, represented by the Cmm data type. This data type follows the definition of `C--` pretty closely but there are some remarkable differences. For a discussion of the Cmm implementation noting most of those differences, see the Basic Cmm section, below.

Additions in Cmm

Although both Cmm and C-- allow foreign calls, the .cmm syntax includes the

foreign "C" cfunctionname(R1) [R2];

The [R2] part is the (set of) register(s) that you need to save over the call.

Other additions to C-- are noted throughout the Basic Cmm section, below.

Compiling Cmm with GHC

GHC is able to compile .cmm files with a minimum of user-effort. To compile .cmm files, simply invoke the main GHC driver but remember to:

  • add the option -dcmm-lint if you have handwritten Cmm code;
  • add appropriate includes, especially includes/Cmm.h if you are using Cmm macros or GHC defines for certain types, such as W_ for bits32 or bits64 (depending on the machine word size)--Cmm.h is in the /includes directory of every GHC distribution, i.e., usr/local/lib/ghc-6.6/includes; and,
  • if you do include GHC header files, remember to pass the code through the C preprocessor by adding the -cpp option.

For additional fun, you may pass GHC the -keep-s-file option to keep the temporary assembler file in your compile directory. For example:

ghc -cpp -dcmm-lint -keep-s-file -c Foo.cmm -o Foo.o

This will only work with very basic Cmm files. If you noticed that GHC currently provides no -keep-cmm-file option and -keep-tmp-files does not save a .cmm file and you are thinking about redirecting output from -ddump-cmm, beware. The output from -ddump-cmm contains equal-lines and dash-lines separating Cmm Blocks and Basic Blocks; these are unparseable. The parser also cannot handle const sections. For example, the parser will fail on the first 0 or alphabetic token after const:

section "data" {
        const rOG_info;	// parse error `rOG_info'
        const 0;	// parse error `0'
        const 0;
        const 0;

Although GHC's Cmm pretty printer outputs C-- standard parenthetical list of arguments after procedure names, i.e., (), the Cmm parser will fail at the ( token. For example:

__stginit_Main_() {	// parse error `('
        Sp = Sp + 4;
        jump (I32[Sp + (-4)]);

The Cmm procedure names in rts/PrimOps.cmm are not followed by a (possibly empty) parenthetical list of arguments; all their arguments are Global (STG) Registers, anyway, see Variables, Registers and Types, below. Don't be confused by the procedure definitions in other handwritten .cmm files in the RTS, such as rts/Apply.cmm: all-uppercase procedure invocations are special reserved tokens in compiler/cmm/CmmLex.x and compiler/cmm/CmmParse.y. For example, INFO_TABLE is parsed as one of the tokens in the Alex info predicate:

info	:: { ExtFCode (CLabel, [CmmLit],[CmmLit]) }
	: 'INFO_TABLE' '(' NAME ',' INT ',' INT ',' INT ',' STRING ',' STRING ')'
		-- ptrs, nptrs, closure type, description, type
		{ stdInfo $3 $5 $7 0 $9 $11 $13 }

GHC's Cmm parser also cannot parse nested code blocks. For example:

s22Q_ret() {
	s22Q_info {  	// parse error `{'
		const Main_main_srt-s22Q_info+24;
		const 0;
		const 2228227;
	R2 = base_GHCziHandle_stdout_closure;
	R3 = 10;
	Sp = Sp + 4;    /* Stack pointer */
	jump base_GHCziIO_zdwhPutChar_info;

The C-- specification example in section 4.6.2, "Procedures as section contents" also will not parse in Cmm:

section "data" { 
	const PROC = 3; 	// parse error `PROC' (no parse after 'const')
	bits32[] {p_end, PROC}; // parse error `[' (only bits8[] is allowed)
				// parse error `{' (no {...} variable initialisation)

	p (bits32 i) {	// parse error `{' (Cmm thinks "p (bits32 i)" is a statement)
			i = i-1; 
		if (i >= 0) { goto loop ; }	// no parse error 
						// (if { ... } else { ... } *is* parseable)

Note that if p (bits32 i) { ... } were written as a Cmm-parseable procedure, as p { ... }, the parse error would occur at the closing curly bracket for the section "data" { ... p { ... } }<- here.

Basic Cmm

Cmm is a high level assembler with a syntax style similar to C. This section describes Cmm by working up from assembler--the C-- papers and specification work down from C. At the least, you should know what a "high level" assembler is, see What is a High Level Assembler?. Cmm is different than other high level assembler languages in that it was designed to be a semi-portable intermediate language for compilers; most other high level assemblers are designed to make the tedium of assembly language more convenient and intelligible to humans. If you are completely new to C--, I highly recommend these papers listed on the C-- Papers page:

Cmm is not a stand alone C-- compiler; it is an implementation of C-- embedded in the GHC compiler. One difference between Cmm and a C-- compiler like Quick C-- is this: Cmm uses the C preprocessor (cpp). Cpp lets Cmm integrate with C code, especially the C header defines in includes, and among many other consequences it makes the C-- import and export statements irrelevant; in fact, according to compiler/cmm/CmmParse.y they are ignored. The most significant action taken by the Cmm modules in the Compiler is to optimise Cmm, through compiler/cmm/CmmOpt.hs. The Cmm Optimiser generally runs a few simplification passes over primitive Cmm operations, inlines simple Cmm expressions that do not contain global registers (these would be left to one of the Backends, which currently cannot handle inlines with global registers) and performs a simple loop optimisation.

Code Blocks in Cmm

The Haskell representation of Cmm separates contiguous code into:

  • modules (compilation units; a .cmm file); and
  • basic blocks

Cmm modules contain static data elements (see Literals and Labels) and Basic Blocks, collected together in Cmm, a type synonym for GenCmm, defined in compiler/cmm/Cmm.hs:

newtype GenCmm d i = Cmm [GenCmmTop d i]

type Cmm = GenCmm CmmStatic CmmStmt

data GenCmmTop d i
  = CmmProc
     [d]	       -- Info table, may be empty
     CLabel            -- Used to generate both info & entry labels
     [LocalReg]        -- Argument locals live on entry (C-- procedure params)
     [GenBasicBlock i] -- Code, may be empty.  The first block is
                       -- the entry point.  The order is otherwise initially 
                       -- unimportant, but at some point the code gen will
                       -- fix the order.

		       -- the BlockId of the first block does not give rise
		       -- to a label.  To jump to the first block in a Proc,
		       -- use the appropriate CLabel.

  -- some static data.
  | CmmData Section [d]	-- constant values only

type CmmTop = GenCmmTop CmmStatic CmmStmt

CmmStmt is described in Statements and Calls;
Section is described in Sections and Directives;
the static data in [d] is [CmmStatic] from the type synonym Cmm;
CmmStatic is described in Literals and Labels.

Basic Blocks and Procedures

Cmm procedures are represented by the first constructor in GenCmmTop d i:

    CmmProc [d] CLabel [LocalReg] [GenBasicBlock i]

For a description of Cmm labels and the CLabel data type, see the subsection Literals and Labels, below.

Cmm Basic Blocks are labeled blocks of Cmm code ending in an explicit jump. Sections (see Sections and Directives) have no jumps--in Cmm, Sections cannot contain nested Procedures (see, e.g., Compiling Cmm with GHC). Basic Blocks encapsulate parts of Procedures. The data type GenBasicBlock and the type synonym CmmBasicBlock encapsulate Basic Blocks; they are defined in compiler/cmm/Cmm.hs:

data GenBasicBlock i = BasicBlock BlockId [i]

type CmmBasicBlock = GenBasicBlock CmmStmt

newtype BlockId = BlockId Unique
  deriving (Eq,Ord)

instance Uniquable BlockId where
  getUnique (BlockId u) = u

The BlockId data type simply carries a Unique with each Basic Block. For descriptions of Unique, see

Variables, Registers and Types

Like other high level assembly languages, all variables in C-- are machine registers, separated into different types according to bit length (8, 16, 32, 64, 80, 128) and register type (integral or floating point). The C-- standard specifies little more type information about a register than its bit length: there are no distinguishing types for signed or unsigned integrals, or for "pointers" (registers holding a memory address). A C-- standard compiler supports additional information on the type of a register value through compiler hints. In a foreign call, a "signed" bits8 would be sign-extended and may be passed as a 32-bit value. Cmm diverges from the C-- specification on this point somewhat (see below). C-- and Cmm do not represent special registers, such as a Condition Register (CR) or floating point unit (FPU) status and control register (FPSCR on the PowerPC, MXCSR on Intel x86 processors), as these are a matter for the Backends.

C-- and Cmm hide the actual number of registers available on a particular machine by assuming an "infinite" supply of registers. A backend, such as the NCG or C compiler on GHC, will later optimise the number of registers used and assign the Cmm variables to actual machine registers; the NCG temporarily stores any overflow in a small memory stack called the spill stack, while the C compiler relies on C's own runtime system. Haskell handles Cmm registers with three data types: LocalReg, GlobalReg and CmmReg. LocalRegs and GlobalRegs are collected together in a single Cmm data type:

data CmmReg 
  = CmmLocal  LocalReg
  | CmmGlobal GlobalReg
  deriving( Eq )

Local Registers

Local Registers exist within the scope of a Procedure:

data LocalReg
  = LocalReg !Unique MachRep

For a list of references with information on Unique, see the Basic Blocks and Procedures section, above.

A MachRep, the type of a machine register, is defined in compiler/cmm/MachOp.hs:

data MachRep
  = I8		-- integral type, 8 bits wide (a byte)
  | I16		-- integral type, 16 bits wide
  | I32		-- integral type, 32 bits wide
  | I64		-- integral type, 64 bits wide
  | I128	-- integral type, 128 bits wide (an integral vector register)
  | F32		-- floating point type, 32 bits wide (float)
  | F64		-- floating point type, 64 bits wide (double)
  | F80		-- extended double-precision, used in x86 native codegen only.
  deriving (Eq, Ord, Show)

There is currently no register for floating point vectors, such as F128. The types of Cmm variables are defined in the Happy parser file compiler/cmm/CmmParse.y and the Alex lexer file compiler/cmm/CmmLex.x. (Happy and Alex will compile these into CmmParse.hs and CmmLex.hs, respectively.) Cmm recognises the following C-- types as parseable tokens, listed next to their corresponding defines in includes/Cmm.h and their STG types:

Cmm Token Cmm.h #define STG type
bits8 I8 StgChar or StgWord8
bits16 I16 StgWord16
bits32 I32, CInt, CLong StgWord32; StgWord (depending on architecture)
bits64 I64, CInt, CLong, L_ StgWord64; StgWord (depending on architecture)
float32 F_ StgFloat
float64 D_ StgDouble

includes/Cmm.h also defines L_ for bits64, so F_, D_ and L_ correspond to the GlobalReg data type constructors FloatReg, DoubleReg and LongReg. Note that although GHC may generate other register types supported by the MachRep data type, such as I128, they are not parseable tokens. That is, they are internal to GHC. The special defines CInt and CLong are used for compatibility with C on the target architecture, typically for making foreign "C" calls.

Note: Even Cmm types that are not explicit variables (Cmm literals and results of Cmm expressions) have implicit MachReps, in the same way as you would use temporary registers to hold labelled constants or intermediate values in assembler functions. See:

Global Registers and Hints

These are universal both to a Cmm module and to the whole compiled program. Variables are global if they are declared at the top-level of a compilation unit (outside any procedure). Global Variables are marked as external symbols with the .globl assembler directive. In Cmm, global registers are used for special STG registers and specific registers for passing arguments and returning values. The Haskell representation of Global Variables (Registers) is the GlobalReg data type, defined in compiler/cmm/Cmm.hs:

data GlobalReg
  -- Argument and return registers
  = VanillaReg			-- pointers, unboxed ints and chars
	{-# UNPACK #-} !Int	-- register number, such as R3, R11

  | FloatReg		        -- single-precision floating-point registers
	{-# UNPACK #-} !Int	-- register number

  | DoubleReg		        -- double-precision floating-point registers
	{-# UNPACK #-} !Int	-- register number

  | LongReg	                -- long int registers (64-bit, really)
	{-# UNPACK #-} !Int	-- register number

  -- STG registers
  | Sp			-- Stack ptr; points to last occupied stack location.
  | SpLim		-- Stack limit
  | Hp			-- Heap ptr; points to last occupied heap location.
  | HpLim		-- Heap limit register
  | CurrentTSO		-- pointer to current thread's TSO
  | CurrentNursery	-- pointer to allocation area
  | HpAlloc		-- allocation count for heap check failure

  -- We keep the address of some commonly-called 
  -- functions in the register table, to keep code
  -- size down:
  | GCEnter1		-- stg_gc_enter_1
  | GCFun		-- stg_gc_fun

  -- Base offset for the register table, used for accessing registers
  -- which do not have real registers assigned to them.  This register
  -- will only appear after we have expanded GlobalReg into memory accesses
  -- (where necessary) in the native code generator.
  | BaseReg

  -- Base Register for PIC (position-independent code) calculations
  -- Only used inside the native code generator. It's exact meaning differs
  -- from platform to platform  (see compiler/nativeGen/PositionIndependentCode.hs).
  | PicBaseReg

For a description of the Hp and Sp virtual registers, see The Haskell Execution Model page. General GlobalRegs are clearly visible in Cmm code according to the following syntax defined in compiler/cmm/CmmLex.x:

GlobalReg Constructor Syntax Examples
VanillaReg Int R ++ Int R1, R10
FloatReg Int F ++ Int F1, F10
DoubleReg Int D ++ Int D1, D10
LongReg Int L ++ Int L1, L10

General GlobalRegs numbers are decimal integers, see the parseInteger function in compiler/utils/StringBuffer.lhs. The remainder of the GlobalReg constructors, from Sp to BaseReg are lexical tokens exactly like their name in the data type; PicBaseReg does not have a lexical token since it is used only inside the NCG. See Position Independent Code and Dynamic Linking for an in-depth description of PIC implementations in the NCG.

GlobalRegs are a very special case in Cmm, partly because they must conform to the STG register convention and the target C calling convention. That the Cmm parser recognises R1 and F3 as GlobalRegs is only the first step. The main files to look at for more information on this delicate topic are:

All arguments to out-of-line PrimOps in rts/PrimOps.cmm are STG registers.

Cmm recognises all C-- syntax with regard to hints. For example:

"signed" bits32 x;  // signed or unsigned int with hint "signed"

foreign "C" labelThread(R1 "ptr", R2 "ptr") [];

"ptr" info = foreign "C" lockClosure(mvar "ptr") [];

Hints are represented in Haskell as MachHints, defined near MachRep in compiler/cmm/MachOp.hs:

data MachHint
  = NoHint	-- string: "NoHint"	Cmm syntax: [empty]
  | PtrHint	-- string: "PtrHint"    Cmm syntax: "ptr"    (C-- uses "address")
  | SignedHint	-- string: "SignedHint"	Cmm syntax: "signed"
  | FloatHint	-- string: "FloatHint"	Cmm syntax: "float" 

Although the C-- specification does not allow the C-- type system to statically distinguish between floats, signed ints, unsigned ints or pointers, Cmm does. Cmm MachReps carry the float or int kind of a variable, either within a local block or in a global register. GlobalReg includes separate constructors for Vanilla, Float, Double and Long. Cmm still does not distinguish between signed ints, unsigned ints and pointers (addresses) at the register level, as these are given hint pseudo-types or their real type is determined as they run through primitive operations. MachHints still follow the C-- specification and carry kind information as an aide to the backend optimisers.

Global Registers in Cmm currently have a problem with inlining: because neither compiler/cmm/PprC.hs nor the NCG are able to keep Global Registers from clashing with C argument passing registers, Cmm expressions that contain Global Registers cannot be inlined into an argument position of a foreign call. For more thorough notes on inlining, see the comments in compiler/cmm/CmmOpt.hs.

Declaration and Initialisation

Cmm variables hold the same values registers do in assembly languages but may be declared in a similar way to variables in C. As in C--, they may actually be declared anywhere in the scope for which they are visible (a block or file)--for Cmm, this is done by the loopDecls function in compiler/cmm/CmmParse.y. In compiler/rts/PrimOps.cmm, you will see Cmm variable declarations like this one:

W_ w, code, val;  // W_ is a cpp #define for StgWord, 
		  // a machine word (32 or 64-bit--general register size--unsigned int)

Remember that Cmm code is run through the C preprocessor. W_ will be transformed into bits32, bits64 or whatever is the bitssize of the machine word, as defined in includes/Cmm.h. In Haskell code, you may use the compiler/cmm/MachOp.hs functions wordRep and halfWordRep to dynamically determine the machine word size. For a description of word sizes in GHC, see the Word page.

The variables w, code and val should be real registers. With the above declaration the variables are uninitialised. Initialisation requires an assignment statement. Cmm does not recognise C-- "{ literal, ... }" initialisation syntax, such as bits32{10} or bits32[3] {1, 2, 3}. Cmm does recognise initialisation with a literal:

string_name:	bits8[] "twenty character string\n\0";

variable_num:	bits32 10::bits32;

The typical method seems to be to declare variables and then initialise them just before their first use. (Remember that you may declare a variable anywhere in a procedure and use it in an expression before it is initialised but you must initialise it before using it anywhere else--statements, for example.)

Memory Access

If the value in w were the address of a memory location, you would obtain the value at that location similar to Intel assembler syntax. In Cmm, you would write:

code = W_[w];  // code is assigned the W_ value at memory address w

compare the above statement to indirect addressing in Intel assembler:

mov	al, [eax]  ; move value in memory at indirect address in register eax, 
		   ; into register al

The code between the brackets (w in [w], above) is an expression. See the Expressions section. For now, consider the similarity between the Cmm-version of indexed memory addressing syntax, here:

R1 = bits32[R2 + R3];	// R2 (memory address), R3 (index, offset), result: type bits32

// note: in Cmm 'R2' and 'R3' would be parsed as global registers
// this is generally bad form; instead, 
// declare a local variable and initialise it with a global, such as:
bits32 adr, ofs, res;
adr = R2;
ofs = R3;
res = bits32[adr + ofs];
R1 = res;

// using local variables will give the NCG some leeway to avoid clobbering the globals
// should you call another procedure somewhere in the same scope

and the corresponding Intel assembler indexed memory addressing syntax, here:

mov	al, ebx[eax]	; ebx (base), eax (index)
; or
mov	al, [ebx + eax]

You will generally not see this type of syntax in either handwritten or GHC-produced Cmm code, although it is allowed; it simply shows up in macros. C-- also allows the * (multiplication) operator in addressing expressions, for an approximation of scaled addressing ([base * (2^n)]); for example, n (the "scale") must be 0, 1, 2 or 4. C-- itself would not enforce alignment or limits on the scale. Cmm, however, could not process it: since the NCG currently outputs GNU Assembler syntax, the Cmm or NCG optimisers would have to reduce n in (* n) to an absolute address or relative offset, or to an expression using only + or -. This is not currently the case and would be difficult to implement where one of the operands to the * is a relative address not visible in the code block. includes/Cmm.h defines macros to perform the calculation with a constant. For example:

/* Converting quantities of words to bytes */
#define WDS(n) ((n)*SIZEOF_W)  // SIZEOF_W is a constant

is used in:

#define Sp(n)  W_[Sp + WDS(n)]

The function cmmMachOpFold in compiler/cmm/CmmOpt.hs will reduce the resulting expression Sp + (n * SIZEOF_W) to Sp + N, where N is a constant. A very large number of macros for accessing STG struct fields and the like are produced by includes/mkDerivedConstants.c and output into the file includes/DerivedConstants.h when GHC is compiled.

Of course, all this also holds true for the reverse (when an assignment is made to a memory address):

section "data" {
	num_arr: bits32[10];

proc1 {
	// ...
	bits32[num_arr + (2*3)] = 5::bits32;  // in C: num_arr[(2*3)] = 5;
	// ...

or, for an example of a macro from DerivedConstants.h:

StgAtomicallyFrame_code(frame) = R1;

this will be transformed to:

REP_StgAtomicallyFrame_code[frame + SIZEOF_StgHeader + OFFSET_StgAtomicallyFrame_code] = R1;
// further reduces to (on Darwin PPC arch):
I32[frame + SIZEOF_StgHeader + 0] = R1;

Literals and Labels

Cmm literals are exactly like C-- literals, including the Haskell-style type syntax, for example: 0x00000001::bits32. Cmm literals may be used for initialisation by assignment or in expressions. The CmmLit and CmmStatic data types, defined in compiler/cmm/Cmm.hs together represent Cmm literals, static information and Cmm labels:

data CmmLit
  = CmmInt Integer  MachRep
	-- Interpretation: the 2's complement representation of the value
	-- is truncated to the specified size.  This is easier than trying
	-- to keep the value within range, because we don't know whether
	-- it will be used as a signed or unsigned value (the MachRep doesn't
	-- distinguish between signed & unsigned).
  | CmmFloat  Rational MachRep
  | CmmLabel    CLabel			-- Address of label
  | CmmLabelOff CLabel Int		-- Address of label + byte offset
        -- Due to limitations in the C backend, the following
        -- MUST ONLY be used inside the info table indicated by label2
        -- (label2 must be the info label), and label1 must be an
        -- SRT, a slow entrypoint or a large bitmap (see the Mangler)
        -- Don't use it at all unless tablesNextToCode.
        -- It is also used inside the NCG during when generating
        -- position-independent code. 
  | CmmLabelDiffOff CLabel CLabel Int   -- label1 - label2 + offset

Note how the CmmLit constructor CmmInt Integer MachRep contains sign information in the Integer, the representation of the literal itself: this conforms to the C-- specification, where integral literals contain sign information. For an example of a function using CmmInt sign information, see cmmMachOpFold in compiler/cmm/CmmOpt.hs, where sign-operations are performed on the Integer.

The MachRep of a literal, such as CmmInt Integer MachRep or CmmFloat Rational MachRep may not always require the size defined by MachRep. The NCG optimiser, compiler/nativeGen/MachCodeGen.hs, will test a literal such as 1::bits32 (in Haskell, CmmInt (1::Integer) I32) for whether it would fit into the bit-size of Assembler instruction literals on that particular architecture with a function defined in compiler/nativeGen/MachRegs.lhs, such as fits16Bits on the PPC. If the Integer literal fits, the function makeImmediate will truncate it to the specified size if possible and store it in a NCG data type, Imm, specifically Maybe Imm. (These are also defined in compiler/nativeGen/MachRegs.lhs.)

The Haskell representation of Cmm separates unchangeable Cmm values into a separate data type, CmmStatic, defined in compiler/cmm/Cmm.hs:

data CmmStatic
  = CmmStaticLit CmmLit	
	-- a literal value, size given by cmmLitRep of the literal.
  | CmmUninitialised Int
	-- uninitialised data, N bytes long
  | CmmAlign Int
	-- align to next N-byte boundary (N must be a power of 2).
  | CmmDataLabel CLabel
	-- label the current position in this section.
  | CmmString [Word8]
	-- string of 8-bit values only, not zero terminated.

Note the CmmAlign constructor: this maps to the assembler directive .align N to set alignment for a data item (hopefully one you remembered to label). This is the same as the align directive noted in Section 4.5 of the C-- specification (PDF). In the current implementation of Cmm the align directive seems superfluous because compiler/nativeGen/PprMach.hs translates Sections to assembler with alignment directives corresponding to the target architecture (see Sections and Directives, below).


Remember that C--/Cmm names consist of a string where the first character is:

  • ASCII alphabetic (uppercase or lowercase);
  • an underscore: _ ;
  • a period: . ;
  • a dollar sign: $ ; or,
  • a commercial at: @ .

Cmm labels conform to the C-- specification. C--/Cmm uses labels to refer to memory locations in code--if you use a data directive but do not give it a label, you will have no means of referring to the memory! For GlobalRegs (transformed to assembler .globl), labels serve as both symbols and labels (in the assembler meaning of the terms). The Haskell representation of Cmm Labels is contained in the CmmLit data type, see Literals section, above. Note how Cmm Labels are CLabels with address information. The Clabel data type, defined in compiler/cmm/CLabel.hs, is used throughout the Compiler for symbol information in binary files. Here it is:

data CLabel
  = IdLabel	    		-- A family of labels related to the
	Name			-- definition of a particular Id or Con

  | DynIdLabel			-- like IdLabel, but in a separate package,
	Name			-- and might therefore need a dynamic
	IdLabelInfo		-- reference.

  | CaseLabel			-- A family of labels related to a particular
				-- case expression.
	{-# UNPACK #-} !Unique	-- Unique says which case expression

  | AsmTempLabel 
	{-# UNPACK #-} !Unique

  | StringLitLabel
	{-# UNPACK #-} !Unique

  | ModuleInitLabel 
	Module			-- the module name
	String			-- its "way"
	Bool			-- True <=> is in a different package
	-- at some point we might want some kind of version number in
	-- the module init label, to guard against compiling modules in
	-- the wrong order.  We can't use the interface file version however,
	-- because we don't always recompile modules which depend on a module
	-- whose version has changed.

  | PlainModuleInitLabel	-- without the vesrion & way info
	Bool			-- True <=> is in a different package

  | ModuleRegdLabel

  | RtsLabel RtsLabelInfo

  | ForeignLabel FastString	-- a 'C' (or otherwise foreign) label
	(Maybe Int) 		-- possible '@n' suffix for stdcall functions
		-- When generating C, the '@n' suffix is omitted, but when
		-- generating assembler we must add it to the label.
	Bool			-- True <=> is dynamic

  | CC_Label  CostCentre
  | CCS_Label CostCentreStack

      -- Dynamic Linking in the NCG:
      -- generated and used inside the NCG only,
      -- see module PositionIndependentCode for details.
  | DynamicLinkerLabel DynamicLinkerLabelInfo CLabel
        -- special variants of a label used for dynamic linking

  | PicBaseLabel                -- a label used as a base for PIC calculations
                                -- on some platforms.
                                -- It takes the form of a local numeric
                                -- assembler label '1'; it is pretty-printed
                                -- as 1b, referring to the previous definition
                                -- of 1: in the assembler source file.

  | DeadStripPreventer CLabel
    -- label before an info table to prevent excessive dead-stripping on darwin

  | HpcTicksLabel Module       -- Per-module table of tick locations
  | HpcModuleNameLabel         -- Per-module name of the module for Hpc
  | HpcModuleOffsetLabel Module-- Per-module offset of the module for Hpc (dynamically generated)

  deriving (Eq, Ord)

Sections and Directives

The Haskell representation of Cmm Section directives, in compiler/cmm/Cmm.hs as the first part of the "Static Data" section, is:

data Section
  = Text
  | Data
  | ReadOnlyData
  | RelocatableReadOnlyData
  | UninitialisedData
  | ReadOnlyData16	-- .rodata.cst16 on x86_64, 16-byte aligned
  | OtherSection String

Cmm supports the following directives, corresponding to the assembler directives pretty-printed by the pprSectionHeader function in compiler/nativeGen/PprMach.hs:

Section Constructor Cmm section directive Assembler Directive
Text "text" .text
Data "data" .data
ReadOnlyData "rodata" .rodata
(generally; varies by arch,OS)
RelocatableReadOnlyData no parse (GHC internal), output: "relreadonly" .const_data
.section .rodata
(generally; varies by arch,OS)
UninitialisedData "bss", output: "uninitialised" .bss
ReadOnlyData16 no parse (GHC internal), output: none .const
.section .rodata
(generally; on x86_64:
.section .rodata.cst16)

You probably already noticed I omitted the alignment directives (for clarity). For example, pprSectionHeader would pretty-print ReadOnlyData as

.align 2

on an i386 with the Darwin OS. If you are really on the ball you might have noticed that the PprMach.hs output of ".section .data" and the like is really playing it safe since on most OS's, using GNU Assembler, the .data directive is equivalent to .section __DATA .data, or simply .section .data. Note that OtherSection String is not a catch-all for the Cmm parser. If you wrote:

section ".const\n.align 2\n\t.section .rodata" { ... }

The Cmm parser (through GHC) would panic, complaining, "PprMach.pprSectionHeader: unknown section."

While the C-- specification allows a bare data keyword directive, Cmm does not:

// this is valid C--, not Cmm!
data { }

// all Cmm directives use this syntax:
section [Cmm section directive] { }

Cmm does not recognise the C-- "stack" declaration for allocating memory on the system stack.

GHC-produced Cmm code is replete with data sections, each of which is stored in .data section of the binary code. This contributes significantly to the large binary size for GHC-compiled code.

Target Directive

The C-- specification defines a special target directive, in section 4.7. The target directive is essentially a code block defining the properties of the target architecture:

	memsize	N	// bit-size of the smallest addressable unit of memory
	byteorder	[big,little]	// endianness
	pointersize	N	// bit-size of the native pointer type
	wordsize	N	// bit-size of the native word type

This is essentially a custom-coded version of the GNU Assembler (as) .machine directive, which is essentially the same as passing the -arch [cpu_type] option to as.

Cmm does not support the target directive. This is partly due GHC generally lacking cross-compiler capabilities. Should GHC move toward adding cross-compilation capabilities, the target might not be a bad thing to add. Target architecture parameters are currently handled through the Build System?, which partly sets such architectural parameters through includes/mkDerivedConstants.c and includes/ghcconfig.h.


Expressions in Cmm follow the C-- specification. They have:

Cmm expressions may include

  • a literal or a name (CmmLit contains both, see Literals and Labels, above);
  • a memory reference (CmmLoad and CmmReg, see Memory Access, above);
  • an operator (a MachOp, in CmmMachOp, below); or,
  • another expression (a [CmmExpr], in CmmMachOp, below).

These are all included as constructors in the CmmExpr data type, defined in compiler/cmm/Cmm.hs:

data CmmExpr
  = CmmLit CmmLit               -- Literal or Label (name)
  | CmmLoad CmmExpr MachRep     -- Read memory location (memory reference)
  | CmmReg CmmReg		-- Contents of register
  | CmmMachOp MachOp [CmmExpr]  -- operation (+, -, *, etc.)
  | CmmRegOff CmmReg Int

Note that CmmRegOff reg i is only shorthand for a specific CmmMachOp application:

CmmMachOp (MO_Add rep) [(CmmReg reg),(CmmLit (CmmInt i rep))]
	where rep = cmmRegRep reg

The function cmmRegRep is described below. Note: the original comment following CmmExpr in compiler/cmm/Cmm.hs is erroneous (cf., mangleIndexTree in compiler/nativeGen/MachCodeGen.hs) but makes the same point described here. The offset, (CmmLit (CmmInt i rep)), is a literal (CmmLit), not a name (CLabel). A CmmExpr for an offset must be reducible to a CmmInt in Haskell; in other words, offsets in Cmm expressions may not be external symbols whose addresses are not resolvable in the current context.

Boolean comparisons are not boolean conditions. Boolean comparisons involve relational operators, such as >, < and ==, and map to MachOps that are converted to comparison followed by branch instructions. For example, < would map to MO_S_Lt for signed operands, compiler/nativeGen/MachCodeGen.hs would transform MO_S_Lt into the LTT constructor of the Cond union data type defined in compiler/nativeGen/MachInstrs.hs and compiler/nativeGen/PprMach.hs would transform LTT to the distinguishing comparison type for an assembler comparison instruction. You already know that the result of a comparison instruction is actually a change in the state of the Condition Register (CR), so Cmm boolean expressions do have a kind of side-effect but that is to be expected. In fact, it is necessary since at the least a conditional expression becomes two assembler instructions, in PPC Assembler:

cmplwi   r3, 0  ; condition test
blt      Lch    ; branch instruction

This condition mapping does have an unfortunate consequence: conditional expressions do not fold into single instructions. In Cmm, as in C--, expressions with relational operators may evaluate to an integral (0, nonzero) instead of evaluating to a boolean type. For certain cases, such as an arithmetic operation immediately followed by a comparison, extended mnemonics such as addi. might eliminate the comparison instruction. See Cmm Design: Observations and Areas for Potential Improvement for more discussion and potential solutions to this situation.

Boolean conditions include: &&, ||, ! and parenthetical combinations of boolean conditions. The if expr { } and if expr { } else { } statements contain boolean conditions. The C-- type produced by conditional expressions is bool, in Cmm, type BoolExpr in compiler/cmm/CmmParse.y:

data BoolExpr
  = BoolExpr `BoolAnd` BoolExpr
  | BoolExpr `BoolOr`  BoolExpr
  | BoolNot BoolExpr
  | BoolTest CmmExpr

The type BoolExpr maps to the CmmCondBranch or CmmBranch constructors of type CmmStmt, defined in compiler/cmm/Cmm.hs, see Statements and Calls.

The CmmExpr constructor CmmMachOp MachOp [CmmExpr] is the core of every operator-based expression; the key here is MachOp, which in turn depends on the type of MachRep for each operand. See Fundamental and PrimitiveOperators. In order to process CmmExprs, the data type comes with a deconstructor function to obtain the relevant MachReps, defined in compiler/cmm/Cmm.hs:

cmmExprRep :: CmmExpr -> MachRep
cmmExprRep (CmmLit lit)      = cmmLitRep lit
cmmExprRep (CmmLoad _ rep)   = rep
cmmExprRep (CmmReg reg)      = cmmRegRep reg
cmmExprRep (CmmMachOp op _)  = resultRepOfMachOp op
cmmExprRep (CmmRegOff reg _) = cmmRegRep reg

The deconstructors cmmLitRep and cmmRegRep (with its supporting deconstructor localRegRep) are also defined in compiler/cmm/Cmm.hs.

In PPC Assembler you might add two 32-bit integrals by:

add	r3, r1, r2	; r3 = r1 + r2

while in Cmm you might write:

res = first + second;

Remember that the assignment operator, =, is a statement since it has the "side effect" of modifying the value in res. The + expression in the above statement, for a 32-bit architecture, would be represented in Haskell as:

CmmMachOp (MO_Add I32) [CmmReg (CmmLocal uniq I32), CmmReg (CmmLocal uniq I32)]

The expr production rule in the Cmm Parser compiler/cmm/CmmParse.y maps tokens to "values", such as + to an addition operation, MO_Add. The mkMachOp function in the Parser determines the MachOp type in CmmMachOp MachOp [CmmExpr] from the token value and the MachRep type of the head variable. Notice that the simple + operator did not contain sign information, only the MachRep. For expr, signed and other MachOps, see the machOps function in compiler/cmm/CmmParse.y. Here is a table of operators and the corresponding MachOps recognised by Cmm (listed in order of precedence):

Operator MachOp
/ MO_U_Quot
* MO_Mul
% MO_U_Rem
- MO_Sub
+ MO_Add
>> MO_U_Shr
<< MO_Shl
& MO_And
^ MO_Xor
| MO_Or
>= MO_U_Ge
> MO_U_Gt
<= MO_U_Le
< MO_U_Lt
!= MO_Ne
== MO_Eq
~ MO_Not
- MO_S_Neg

Quasi-operator Syntax

If you read to the end of expr in compiler/cmm/CmmParse.y, in the next production rule, expr0, you will notice that Cmm expressions also recognise a set of name (not symbol) based operators that would probably be better understood as quasi-operators. The syntax for these quasi-operators is in some cases similar to syntax for Cmm statements and generally conform to the C-- specification, sections 3.3.2 (expr) and 7.4.1 (syntax of primitive operators), except that 3. and, by the equivalence of the two, 1. may return multiple arguments. In Cmm, quasi-operators may have side effects. The syntax for quasi-operators may be:

  1. expr0 `name` expr0
    (just like infix-functions in Haskell);
  2. type[ expression ]
    (the memory access quasi-expression described in Memory Access; the Haskell representation of this syntax is CmmLoad CmmExpr MachRep);
  3. %name( exprs0 )
    (standard prefix form, similar to C-- statement syntax for procedures but with the distinguishing prefix %; in Cmm this is also used as statement syntax for calls, which are really built-in procedures, see Cmm Calls)

A expr0 may be a literal (CmmLit) integral, floating point, string or a CmmReg (the production rule reg: a name for a local register (LocalReg) or a GlobalReg).

Note that the name in expr0 syntax types 1. and 3. must be a known primitive (primitive operation), see Operators and Primitive Operations. The first and third syntax types are interchangeable:

bits32 one, two, res;
one = 1::bits32;
two = 2::bits32;

res = one `lt` two;

// is equivalent to:

res = %lt(one, two);

The primitive operations allowed by Cmm are listed in the machOps production rule, in compiler/cmm/CmmParse.y, and largely correspond to MachOp data type constructors, in compiler/cmm/MachOp.hs, with a few additions. The primitive operations distinguish between signed, unsigned and floating point types.

Cmm adds some expression macros that map to Haskell Cmm functions. They are listed under exprMacros in compiler/cmm/CmmParse.y and include:


Statements and Calls

Cmm Statements generally conform to the C-- specification, with a few exceptions noted below. Cmm Statements implement:

  • no-op; the empty statement: ;
  • C-- (C99/C++ style) comments: // ... \n and /* ... */
  • the assignment operator: =
  • store operation (assignment to a memory location): type[expr] =
  • control flow within procedures (goto) and between procedures (jump, returns) (note: returns are only Cmm macros)
  • foreign calls (foreign "C" ...) and calls to Cmm Primitive Operations (%)
  • procedure calls and tail calls
  • conditional statement (if ... { ... } else { ... })
  • tabled conditional (switch)

Cmm does not implement the C-- specification for Spans (sec. 6.1) or Continuations (sec. 6.7).
Although Cmm supports primitive operations that may have side effects (see Primitive Operations, below), it does not parse the syntax %% form mentioned in section 6.3 of the C-- specification. Use the %name(arg1,arg2) expression-syntax instead.
Cmm does not implement the return statement (C-- spec, sec. 6.8.2) but provides a set of macros that return a list of tuples of a CgRep and a CmmExpr: [(CgRep,CmmExpr)]. For a description of CgRep, see comments in compiler/codeGen/SMRep.lhs. The return macros are defined at the end of the production rule stmtMacros in compiler/cmm/CmmParse.y:

  • RET_P
  • RET_N
  • RET_PP
  • RET_NN
  • RET_NP

In the above macros, P stands for PtrArg and N stands for NonPtrArg; both are CgRep constructors. These return macros provide greater control for the CodeGen and integrate with the RTS but limit the number and type of return arguments in Cmm: you may only return according to these macros! The returns are processed by the emitRetUT function in compiler/cmm/CmmParse.y, which in turn calls several functions from compiler/codeGen/CgMonad.lhs, notably emitStmts, which is the core Code Generator function for emitting CmmStmt data.

The Haskell representation of Cmm Statements is the data type CmmStmt, defined in compiler/cmm/Cmm.hs:

data CmmStmt
  = CmmNop
  | CmmComment FastString

  | CmmAssign CmmReg CmmExpr	 -- Assign to register

  | CmmStore CmmExpr CmmExpr     -- Assign to memory location.  Size is
                                 -- given by cmmExprRep of the rhs.

  | CmmCall	 		 -- A foreign call, with 
     [(CmmReg,MachHint)]	 -- zero or more results
     [(CmmExpr,MachHint)]	 -- zero or more arguments
     (Maybe [GlobalReg])	 -- Global regs that may need to be saved
				 -- if they will be clobbered by the call.
				 -- Nothing <=> save *all* globals that
				 -- might be clobbered.

  | CmmBranch BlockId             -- branch to another BB in this fn

  | CmmCondBranch CmmExpr BlockId -- conditional branch

  | CmmSwitch CmmExpr [Maybe BlockId]   -- Table branch
	-- The scrutinee is zero-based; 
	--	zero -> first block
	--	one  -> second block etc
	-- Undefined outside range, and when there's a Nothing

  | CmmJump CmmExpr [LocalReg]    -- Jump to another function, with these 
				  -- parameters.

Note how the constructor CmmJump contains [LocalReg]: this is the Cmm implementation of the C-- jump statement for calling another procedure where the parameters are the arguments passed to the other procedure. None of the parameters contain the address--in assembler, a label--of the caller, to return control to the caller. The CmmCall constructor also lacks a parameter to store the caller's address. Cmm implements C-- jump nesting and matching returns by tail calls, as described in section 6.8 of the C-- specification. Tail calls are managed through the CodeGen, see compiler/codeGen/CgTailCall.lhs. You may have already noticed that the call target of the CmmJump is a CmmExpr: this is the Cmm implementation of computed procedure addresses, for example:

proc1 {

 jump (bits32[x+4])( ... );


The computed procedure address, in this case (bits32[x+4]), should always be the first instruction of a Cmm procedure. You cannot obtain the address of a code block within a procedure and jump to it, as an alternative way of computing a continuation.

CmmBranch BlockId represents an unconditional branch to another Basic Block in the same procedure. There are two unconditional branches in Cmm/C--:

  1. goto statement; and
  2. a branch from the else portion of an if-then-else statement.

CmmCondBranch CmmExpr BlockId represents a conditional branch to another Basic Block in the same procedure. This is the if expr statement where expr is a CmmExpr, used in both the unary if and if-then-else statements. CmmCondBranch maps to more complex Assembler instruction sets or HC code (compiler/cmm/PprC.hs). For assembler, labels are created for each new Basic Block. During parsing, conditional statements map to the BoolExpr data type which guides the encoding of assembler instruction sets.

CmmSwitch represents the switch statement. It is parsed and created as with the doSwitch function in compiler/cmm/CmmParse.y or created from case expressions with the emitSwitch and mk_switch functions in compiler/codeGen/CgUtils.hs. In the NCG, a CmmSwitch is generated as a jump table using the genSwitch function in compiler/nativeGen/MachCodeGen.hs. There is currently no implementation of any optimisations, such as a cascade of comparisons for switches with a wide deviation in values or binary search for very wide value ranges--for output to HC, earlier versions of GCC could not handle large if-trees, anyway.

Cmm Calls

Cmm calls include both calls to foreign functions and calls to Cmm quasi-operators using expression syntax (see Quasi-operator Syntax). Although Cmm does not implement any of the control flow statements of C-- specification (section 6.8.1), foreign calls from Cmm are one of the most complex components of the system due to various differences between the Cmm and C calling conventions.

The data type, CmmCallTarget is defined in compiler/cmm/Cmm.hs as:

data CmmCallTarget
  = CmmForeignCall		-- Call to a foreign function
	CmmExpr 		-- literal label <=> static call
				-- other expression <=> dynamic call
	CCallConv		-- The calling convention

  | CmmPrim			-- Call to a "primitive" (eg. sin, cos)
	CallishMachOp		-- These might be implemented as inline
				-- code by the backend.

CCallConv is defined in compiler/prelude/ForeignCall.lhs; for information on register assignments, see comments in compiler/codeGen/CgCallConv.hs.

CallishMachOp is defined in compiler/cmm/MachOp.hs; see, also, below Primitive Operations. CallishMachOps are generally used for floating point computations (without implementing any floating point exceptions). Here is an example of using a CallishMachOp (not yet implemented):

  add, carry = %addWithCarry(x, y);

Operators and Primitive Operations

Cmm generally conforms to the C-- specification for operators and "primitive operations". The C-- specification, in section 7.4, refers to both of these as "primitive operations" but there are really two different types:

  • operators, as I refer to them, are:
    • parseable tokens, such as +,-,* or /;
    • generally map to a single machine instruction or part of a machine instruction;
    • have no side effects; and,
    • are represented in Haskell using the MachOp data type;
  • primitive operations (Cmm quasi-operators) are special, usually inlined, procedures, represented in Haskell using the CallishMachOp data type; primitive operations may have side effects.

The MachOp and CallishMachOp data types are defined in compiler/cmm/MachOp.hs.

Both Cmm Operators and Primitive Operations are handled in Haskell as Inline PrimOps, though what I am calling Cmm primitive operations may be implemented as out-of-line foreign calls.


data MachOp

  -- Integer operations
  = MO_Add    MachRep
  | MO_Sub    MachRep
  | MO_Eq     MachRep
  | MO_Ne     MachRep
  | MO_Mul    MachRep		-- low word of multiply
  | MO_S_MulMayOflo MachRep 	-- nonzero if signed multiply overflows
  | MO_S_Quot MachRep		-- signed / (same semantics as IntQuotOp)
  | MO_S_Rem  MachRep		-- signed % (same semantics as IntRemOp)
  | MO_S_Neg  MachRep		-- unary -
  | MO_U_MulMayOflo MachRep	-- nonzero if unsigned multiply overflows
  | MO_U_Quot MachRep		-- unsigned / (same semantics as WordQuotOp)
  | MO_U_Rem  MachRep		-- unsigned % (same semantics as WordRemOp)

  -- Signed comparisons (floating-point comparisons also use these)
  | MO_S_Ge MachRep
  | MO_S_Le MachRep
  | MO_S_Gt MachRep
  | MO_S_Lt MachRep

  -- Unsigned comparisons
  | MO_U_Ge MachRep
  | MO_U_Le MachRep
  | MO_U_Gt MachRep
  | MO_U_Lt MachRep

  -- Bitwise operations.  Not all of these may be supported at all sizes,
  -- and only integral MachReps are valid.
  | MO_And   MachRep
  | MO_Or    MachRep
  | MO_Xor   MachRep
  | MO_Not   MachRep
  | MO_Shl   MachRep
  | MO_U_Shr MachRep	-- unsigned shift right
  | MO_S_Shr MachRep	-- signed shift right

  -- Conversions.  Some of these will be NOPs.
  -- Floating-point conversions use the signed variant.
  | MO_S_Conv MachRep{-from-} MachRep{-to-}	-- signed conversion
  | MO_U_Conv MachRep{-from-} MachRep{-to-}	-- unsigned conversion

Each MachOp generally corresponds to a machine instruction but may have its value precomputed in the Cmm, NCG or HC optimisers.

Primitive Operations

Primitive Operations generally involve more than one machine instruction and may not always be inlined.

-- These MachOps tend to be implemented by foreign calls in some backends,
-- so we separate them out.  In Cmm, these can only occur in a
-- statement position, in contrast to an ordinary MachOp which can occur
-- anywhere in an expression.
data CallishMachOp
  = MO_F64_Pwr
  | MO_F64_Sin
  | MO_F64_Cos
  | MO_F64_Tan
  | MO_F64_Sinh
  | MO_F64_Cosh
  | MO_F64_Tanh
  | MO_F64_Asin
  | MO_F64_Acos
  | MO_F64_Atan
  | MO_F64_Log
  | MO_F64_Exp
  | MO_F64_Sqrt
  | MO_F32_Pwr
  | MO_F32_Sin
  | MO_F32_Cos
  | MO_F32_Tan
  | MO_F32_Sinh
  | MO_F32_Cosh
  | MO_F32_Tanh
  | MO_F32_Asin
  | MO_F32_Acos
  | MO_F32_Atan
  | MO_F32_Log
  | MO_F32_Exp
  | MO_F32_Sqrt
  | MO_WriteBarrier

For an example, the floating point sine function, sinFloat# in compiler/prelude/primops.txt.pp is piped through the callishOp function in compiler/codegen/CgPrimOp.hs to become Just MO_F32_Sin. The CallishMachOp constructor MO_F32_Sin is piped through compiler/nativeGen/MachCodeGen.hs, where the function genCCall will (for most architectures) call outOfLineFloatOp to issue a call to a C function such as sin.

Cmm Design: Observations and Areas for Potential Improvement

"If the application of a primitive operator causes a system exception, such as division by zero, this is an unchecked run-time error. (A future version of this specification may provide a way for a program to recover from such an exception.)" C-- spec, Section 7.4. Cmm may be able to implement a partial solution to this problem, following the paper: A Single Intermediate Language That Supports Multiple Implementations of Exceptions (2000). (TODO write notes to wiki and test fix.)

The IEEE 754 specification for floating point numbers defines exceptions for certain floating point operations, including:

  • range violation (overflow, underflow);
  • rounding errors (inexact);
  • invalid operation (invalid operand, such as comparison with a NaN value, the square root of a negative number or division of zero by zero); and,
  • zero divide (a special case of an invalid operation).

Many architectures support floating point exceptions by including a special register as an addition to other exception handling registers. The IBM PPC includes the FPSCR ("Floating Point Status Control Register"); the Intel x86 processors use the MXCSR register. When the PPC performs a floating point operation it checks for possible errors and sets the FPSCR. Some processors allow a flag in the Foating-Point Unit (FPU) status and control register to be set that will disable some exceptions or the entire FPU exception handling facility. Some processors disable the FPU after an exception has occurred while others, notably Intel's x86 and x87 processors, continue to perform FPU operations. Depending on whether quiet NaNs (QNaNs) or signaling NaNs (SNaNs) are used by the software, an FPU exception may signal an interrupt for the software to pass to its own exception handler.

Some higher level languages provide facilities to handle these exceptions, including Ada, Fortran (F90 and later), C++ and C (C99, fenv.h, float.h on certain compilers); others may handle such exceptions without exposing a low-level interface. There are three reasons to handle FPU exceptions, and these reasons apply similarly to other exceptions:

  • the facilities provide greater control;
  • the facilities are efficient--more efficient than a higher-level software solution; and,
  • FPU exceptions may be unavoidable, especially if several FPU operations are serially performed at the machine level so the higher level software has no opportunity to check the results in between operations.

A potential solution to the problem of implementing Cmm exceptions, especially for floating point operations, is at Cmm: Implementing Exception Handling.

The C-- Language Specification mentions over 75 primitive operators. The Specification lists separate operators for integral and floating point (signed) arithmetic (including carry, borrow and overflow checking), logical comparisons and conversions (from one size float to another, from float to integral and vice versa, etc.). C-- also includes special operators for floating point number values, such as NaN, mzerok and pzerok, and rounding modes; integral kinds also include bitwise operators, unsigned variants, and bit extraction for width changing and sign or zero-extension. A C-- implementation may conveniently map each of these operators to a machine instruction, or to a simulated operation on architectures that do not support a single instruction. There seem to be two main problems with the current GHC-implementation of Cmm:

  1. not enough operators
  2. no implementation of vector (SIMD) registers (though there is a I128 MachRep)

If a particular architecture supports it, assembler includes instructions such as mnemonics with the . ("dot") suffix (add., fsub.), which set the Condition Register (CR) thereby saving you at least one instruction. (Extended mnemonics can save you even more.) Extended mnemonics with side effects may be implemented as new CallishMachOps, see Primitive Operations and Cmm Calls. Assembler also supports machine exceptions, especially exceptions for floating-point operations, invalid storage access or misalignment (effective address alignment). The current implementation of Cmm cannot model such exceptions through flow control because no flow control is implemented, see Cmm Calls.

Hiding the kinds of registers on a machine eliminates the ability to handle floating point exceptions at the Cmm level and to explicitly vectorize (use SIMD extensions). The argument for exposing vector types may be a special case since such low-level operations are exposed at the C-level, as new types of variables or "intrinsics," that are C-language extensions provided by special header files and compiler support (vector unsigned int or __m128i, vector float or __m128) and operations (vec_add(), + (with at least one vector operand), _mm_add_epi32()).