[fpc-devel] Merging identical procedure proposals

[J. Gareth Moreton]

Hi everyone,

So one optimisation that has cropped up a couple of times is finding 
ways to merge subroutines that, while containing different source code, 
compile into the exact same assembly language.  For example, 
TStream.WriteData has implementations for numerous input types, and the 
compilation of "function TStream.WriteData(const Buffer: WideChar): 
NativeInt;", "function TStream.WriteData(const Buffer: Int16): 
NativeInt;", "function TStream.WriteData(const Buffer: UInt16): 
NativeInt;" all produce the following:

     leaq    -40(%rsp),%rsp
.seh_stackalloc 40
.seh_endprologue
     movq    %rcx,%rax
     movw    %dx,32(%rsp)
     leaq    32(%rsp),%rdx
     movl    $2,%r8d
     movq    (%rax),%rax
     call    *264(%rax)
     movslq    %eax,%rax
     nop
     leaq    40(%rsp),%rsp
     ret
.seh_endproc

As you can imagine, having the compiler only keep one copy of the code 
and redirect all relevant calls to it will reduce code size 
significantly for no speed penalty.

One thought I had was to keep a "code hash" for each subroutine that's 
computed via digesting the assembly language, although I'm not quite 
sure how it should handle labels.  If two hashes are identical, the 
procedures are checked more closely to determine if they are actually 
identical and it wasn't just a collision.

I figure this would be a whole-program optimization though due to 
inter-unit calls and comparisons and the like.

Gareth aka. Kit


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