Hi everyone,
So I've been studying the disassembler dumps of the RTL for some new
possible optimisations, and I've found a potential one that's a little
trickier than normal:
...
# Peephole Optimization: MovxMov2Movx
movslq %eax,%rbx
jmp .Lj83
.p2align 4,,10
.p2align 3
.Lj82:
movq %rdi,%rdx
movq %rsi,%rcx
call SYSUTILS_$$_ANSICOMPARETEXT$ANSISTRING$ANSISTRING$$LONGINT
# Peephole Optimization: MovxMov2Movx
movslq %eax,%rbx
.Lj83:
movq %rbx,%rax
...
Here, %rbx is not used afterwards, and the premise is that for the two
"movslq %eax,%rbx" calls, you can change them to "movslq %eax,%rax" and
the "movq %rbx,%rax" can be removed completely. It also works if the
destination register in the final instruction is not %rax, but a third
register (thus the sign-extension operations become "movslq %eax,%reg3").
The hard part though is tracking it, since the optimiser will have to
potentially evaluate many different branches and if there's even just a
single branch where %rbx (in this case) is set through some other means
or the final destination register is in use while %rbx is being
manipulated, the optimisation must not be made.
This optimisation would require at least two passes and probably only
something to do under -O4. The difficult part is storing information in
between the passes to indicate which mov/s/z instructions can get
modified. The answer I've come up with so far is to add some extra
information into the tai_label object that gets updated whenever a
"mov/s/z; jmp" pair is found to that label, and if the relevant register
is immediately loaded elsewhere then deallocated.
Does anyone have any alternative approaches that would be better?
Otherwise, would it be okay that I add additional functionality to
tai_label for x86? I'm trying to avoid adding new 'hint' entries into
the AsmList, since this will slow down overall compilation and is asking
for trouble.
Gareth aka. Kit
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