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Re: [PATCH] x86: emit tzcnt unconditionally
On Mon, May 07, 2012 at 09:36:38AM +0200, Uros Bizjak wrote:
> On Mon, May 7, 2012 at 7:43 AM, Jakub Jelinek <jakub@redhat.com> wrote:
>
> >> Index: i386.md
> >> ===================================================================
> >> --- i386.md ? (revision 187217)
> >> +++ i386.md ? (working copy)
> >> @@ -12112,9 +12112,22 @@
> >> ? ? (set (match_operand:SWI48 0 "register_operand" "=r")
> >> ? ? ? (ctz:SWI48 (match_dup 1)))]
> >> ? ?""
> >> - ?"bsf{<imodesuffix>}\t{%1, %0|%0, %1}"
> >> +{
> >> + ?if (optimize_function_for_size_p (cfun))
> >> + ? ?return "bsf{<imodesuffix>}\t{%1, %0|%0, %1}";
> >> + ?else if (TARGET_BMI)
> >> + ? ?return "tzcnt{<imodesuffix>}\t{%1, %0|%0, %1}";
> >> + ?else
> >> + ? ?/* tzcnt expands to rep;bsf and we can use it even if !TARGET_BMI. ?*/
> >> + ? ?return "rep; bsf{<imodesuffix>}\t{%1, %0|%0, %1}";
> >> +}
> >
> > Shouldn't that be done only for generic tuning? ?If somebody uses
> > -mtune=native, then emitting rep; bsf is overkill, the code is intended
> > to be run on a CPU without (or with TARGET_BMI with) tzcnt insn support.
>
> Yes, this is a good idea.
>
> Something like attached patch?
Yeah, but probably with the TARGET_BMI and optimize_function_for_size_p
lines swapped.
Because i386.h defines
CTZ_DEFINED_VALUE_AT_ZERO to 1 for TARGET_BMI, therefore it isn't
undefined for TARGET_BMI, so even for -Os you can't just use bsf.
Jakub