Changes between Version 3 and Version 4 of Commentary/Compiler/Backends/NCG

Nov 13, 2006 11:36:57 PM (11 years ago)



  • Commentary/Compiler/Backends/NCG

    v3 v4  
    3535The '''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 block''s are held as lists of {{{Cmm}}} statements ({{{[CmmStmt]}}}, defined in [[GhcFile(compiler/cmm/Cmm.hs)]], or {{{type CmmStmts}}}, defined in [[GhcFile(compiler/cmm/CmmUtils.hs)]]).  A machine-specific (assembler) instruction is represented as a {{{Instr}}}. During this phase:
    36  1. each Cmm block is ''lazily'' converted to machine-specific instructions ({{{Instr}}}) operating on an infinite number of registers;[[BR]][[BR]]
     36 1. each Cmm block is ''lazily'' converted to abstract machine instructions ({{{Instr}}}) operating on an infinite number of registers--since the NCG Haskell files only contain instructions for the host computer on which GHC was compiled, these {{{Instr}}} are machine-specific;[[BR]][[BR]]
    3737 1. 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).[[BR]]''Note'': 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,[[BR]][[BR]]
    3838 1. 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.