wiki:Commentary/Compiler/Backends/NCG

Version 1 (modified by p_tanski, 7 years ago) (diff)

--

Native Code Generator

Overview: Files, Phases

After GHC has produced Cmm (use -ddump-cmm or -ddump-opt-cmm to view), the Native Code Generator (NCG) transforms Cmm into architecture-specific assembly code. The NCG is located in compiler/nativeGen and is separated into eight modules:

and one header file:

The NCG runs through two main phases: a machine-independent phase and a machine-dependent phase.

The machine-independent phase begins with Cmm blocks. A Cmm block is roughly parallel to a Cmm function or procedure in the same way as a compiler may generate a C function into a block of assembler instructions. Cmm blocks are held as lists of Cmm statements ([CmmStmt], defined in compiler/cmm/Cmm.hs, or type CmmStmts, defined in compiler/cmm/CmmUtils.hs). A machine-specific (assembler) instruction is represented as a Instr, defined in compiler/nativeGen/MachInstrs.hs. During this phase:

  1. each Cmm block is lazily converted to machine-specific instructions (Instr) operating on an infinite number of registers;

  2. for each basic block (a, contiguous block of instructions with no branches (jumps) in each Cmm block), real registers are lazily allocated based on the number of available registers on the target machine (say, 32 integer and 32 floating-point registers on the PowerPC architecture),

    if a basic block simultaneously requires more registers than are available on the target machine and the temporary variable needs to be used (would sill be live) after the current instruction, it will be moved (spilled) into memory; and,

  3. each Cmm block is optimised by reordering its basic blocks from the original order (the Instr order from the Cmm) to minimise the number of branches between basic blocks, in other words, by maximising fallthrough of execution from one basic block to the next.