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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