From: user5857@newsgrouper.org.invalid
EricP posted:
> Anton Ertl wrote:
> > EricP writes:
> >> That shows about 12% instructions are conditional branch and 9% CMP.
> >> That says to me that almost all Bcc are paired with a CMP,
> >> and very few use the flags set as a side effect of ALU ops.
> >>
> >> I would expect those two numbers to be closer as even today compilers
don't
> >> know about those side effect flags and will always emit a CMP or TST
first.
> >
> > Compilers certainly have problems with single flag registers, as they
> > run contrary to the base assumption of register allocation. But you
> > don't need full-blown tracking of flags in order to make use of flags
> > side effects in compilers. Plain peephole optimization can be good
> > enough. E.g., if you have
> >
> > if (a+b<0) ...
> >
> > the compiler may naively translate this to
> >
> > add tmp = a, b
> > tst tmp
> > bge cont
> >
> > The peephole optimizer can have a rule that says that this is
> > equivalent to
> >
> > add tmp = a, b
> > bge cont
> >
> > When I compile
> >
> > long foo(long a, long b)
> > {
> > if (a+b<0)
> > return a-b;
> > else
> > return a*b;
> > }
> >
> > with gcc-12.2.0 -O -c on AMD64, I get
> >
> > 0000000000000000 :
> > 0: 48 89 f8 mov %rdi,%rax
> > 3: 48 89 fa mov %rdi,%rdx
> > 6: 48 01 f2 add %rsi,%rdx
> > 9: 78 05 js 10
> > b: 48 0f af c6 imul %rsi,%rax
> > f: c3 ret
> > 10: 48 29 f0 sub %rsi,%rax
> > 13: c3 ret
> >
> > Look, Ma, no tst.
> >
> > - anton
>
> This could be 1 MOV shorter.
> It didn't need to MOV %rdi, %rdx as it already copied rdi to rax.
> Just ADD %rsi,%rdi and after that use the %rax copy.
foo:
ADD R3,R1,R2
PLT0 R3,TF
ADD R1,R1,-R2
MUL R1,R1,R2
RET
5 inst versus 8.
> For that optimization { ADD CMP Bcc } => { ADD Bcc }
> to work those three instructions must be adjacent.
> In this case it wouldn't make a difference but in general
> I think they would want the freedom to move code about and not have
> the ADD bound to the Bcc too early so this would have to be about
> the very last optimization so it didn't interfere with code motion.
>
> The Microsoft compiler uses LEA to do the add which doesn't change flags
> so even if it has a flags optimization it would not detect it:
>
> long foo(long,long) PROC ; foo, COMDAT
> lea eax, DWORD PTR [rcx+rdx]
> test eax, eax
> jns SHORT $LN2@foo
> sub ecx, edx
> mov eax, ecx
> ret 0
> $LN2@foo:
> imul ecx, edx
> mov eax, ecx
> ret 0
5 versus 9
> Also if MS had moved ecx to eax first as GCC does then it could have
> the function result land in eax and eliminate the final two MOV eax,ecx.
still 5 versus 7
--- SoupGate-Win32 v1.05
* Origin: you cannot sedate... all the things you hate (1:229/2)
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