[PATCH, i386] Add prefixes avoidance tuning for silvermont target

[Uros Bizjak]

On Tue, Jul 15, 2014 at 1:01 PM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
> On 15 Jul 10:42, Uros Bizjak wrote:
>> On Tue, Jul 15, 2014 at 10:25 AM, Ilya Enkovich <enkovich.gnu@gmail.com> wrote:
>>
>> >>>>> Also fully restrict xmm8-15 does not seem right.  It is just costly
>> >>>>> but not fully disallowed.
>> >>>>
>> >>>> As said earlier, you can try "Ya*x" as a constraint.
>> >>>
>> >>> I tried it. It does not seem to affect allocation much. I do not see
>> >>> any gain on targeted tests.
>> >>
>> >> Strange, because the documentation claims:
>> >>
>> >> '*'
>> >>      Says that the following character should be ignored when choosing
>> >>      register preferences.  '*' has no effect on the meaning of the
>> >>      constraint as a constraint, and no effect on reloading.  For LRA
>> >>      '*' additionally disparages slightly the alternative if the
>> >>      following character matches the operand.
>> >>
>> >> Let me rethink this a bit. Prehaps we could reconsider Jakub's
>> >> proposal with "Ya,!x" (with two alternatives). IIRC this approach was
>> >> needed for some MMX alternatives, where we didn't want RA to allocate
>> >> a MMX register when the value could be passed in integer regs, but the
>> >> value was still allowed in MMX register.
>> >
>> > That's is what my patch already does, but with '?' instead of '!'.
>>
>> Yes, I know. The problem is, that Ya*x type conditional allocation
>> worked OK in the past for "not preferred, but still alowed regclass"
>> registers, There are several patterns in i386.md that live by this
>> premise, including movsf_internal and movdf_internal. If this approach
>> doesn't work anymore, then we have to either figure out what is the
>> reason, or invent a new strategy that will be applicable to all cases.
>>
>> Can you please post a small test that illustrates the case where Ya,!x
>> works, but Ya*x doesn't?
>
> It's hard to compose a small testcase which will have SSE4 instructions generated with required register usage.  I use tcpjumbo test from TCPmark for initial check of how my patch works.  This test has a lot of pmovzxwd instructions generated and many of them use xmm8-15.  I tried two versions of a simple patch which modifies only pmovzxwd instruction.
>
> Patch1:
>
> diff --git a/gcc/config/i386/sse.md b/gcc/config/i386/sse.md
> index d907353..6b03b72 100644
> --- a/gcc/config/i386/sse.md
> +++ b/gcc/config/i386/sse.md
> @@ -11852,10 +11852,10 @@
>     (set_attr "mode" "OI")])
>
>  (define_insn "sse4_1_<code>v4hiv4si2"
> -  [(set (match_operand:V4SI 0 "register_operand" "=x")
> +  [(set (match_operand:V4SI 0 "register_operand" "=Yr,!x")
>         (any_extend:V4SI
>           (vec_select:V4HI
> -           (match_operand:V8HI 1 "nonimmediate_operand" "xm")
> +           (match_operand:V8HI 1 "nonimmediate_operand" "Yr,!xm")
>             (parallel [(const_int 0) (const_int 1)
>                        (const_int 2) (const_int 3)]))))]
>    "TARGET_SSE4_1"
>
> Patch2:
>
> diff --git a/gcc/config/i386/sse.md b/gcc/config/i386/sse.md
> index d907353..b3721c4 100644
> --- a/gcc/config/i386/sse.md
> +++ b/gcc/config/i386/sse.md
> @@ -11852,10 +11852,10 @@
>     (set_attr "mode" "OI")])
>
>  (define_insn "sse4_1_<code>v4hiv4si2"
> -  [(set (match_operand:V4SI 0 "register_operand" "=x")
> +  [(set (match_operand:V4SI 0 "register_operand" "=Yr*x")
>         (any_extend:V4SI
>           (vec_select:V4HI
> -           (match_operand:V8HI 1 "nonimmediate_operand" "xm")
> +           (match_operand:V8HI 1 "nonimmediate_operand" "Yr*xm")
>             (parallel [(const_int 0) (const_int 1)
>                        (const_int 2) (const_int 3)]))))]
>    "TARGET_SSE4_1"
>
>
> Here are results of looking for pmovzxwd in resulting binaries:
> #objdump -d tcpjumbo-orig | grep pmovzxwd | grep "xmm8\|xmm9\|xmm10\|xmm11\|xmm12\|xmm13\|xmm14\|xmm15" | wc -l
> 76
> #objdump -d tcpjumbo-patch1 | grep pmovzxwd | grep "xmm8\|xmm9\|xmm10\|xmm11\|xmm12\|xmm13\|xmm14\|xmm15" | wc -l
> 0
> #objdump -d tcpjumbo-patch2 | grep pmovzxwd | grep "xmm8\|xmm9\|xmm10\|xmm11\|xmm12\|xmm13\|xmm14\|xmm15" | wc -l
> 76
>
> Therefore I make a conclusion that Yr*x does not really differ much from x.

Just FTR:

Using "Yr,*x" is also a viable option:

#objdump -d tcpjumbo-patch3 | grep pmovzxwd | grep
"xmm8\|xmm9\|xmm10\|xmm11\|xmm12\|xmm13\|xmm14\|xmm15" | wc -l
0

I believe that the above is the way to go with LRA. Vladimir, what do you think?

Uros.