Re: Move insn out of the way

[Richard Guenther <richard.guenther@gmail.com>]

On Wed, Aug 10, 2011 at 3:46 PM, Paulo J. Matos <paulo@matos-sorge.com> wrote:
> On 10/08/11 12:40, Richard Guenther wrote:
>>
>> On x86 we expand the code to ((xl&  al) ^ al) | ((xh&  ah) ^ ah) == 0
>> which is then if-converted.  Modified testcase:
>>
>> long long x;
>> _Bool __attribute__((regparm(2))) mask (long long a)
>> {
>>   return (x&  a) == a;
>> }
>>
>> on i?86 gets you
>>
>> mask:
>> .LFB0:
>>         .cfi_startproc
>>         pushl   %ebx
>>         .cfi_def_cfa_offset 8
>>         .cfi_offset 3, -8
>>         movl    %eax, %ebx
>>         andl    x, %ebx
>>         movl    %edx, %ecx
>>         andl    x+4, %ecx
>>         xorl    %ebx, %eax
>>         xorl    %ecx, %edx
>>         orl     %edx, %eax
>>         sete    %al
>>         popl    %ebx
>>         .cfi_restore 3
>>         .cfi_def_cfa_offset 4
>>         ret
>>
>> so I wonder if you should investigate why the xor variant doesn't trigger
>> for you?
>
> I can reproduce this result in GCC 4.6.1 for x86.
> I can't understand what you mean by this though. From inspecting the logs it
> seems that the if-conversion is done manually at expand time. The final pass
> before expand shows the original (x & a) == a, however, after expand the rtl
> already contains xor, ior, etc. So I guess I would need to do something
> similar in my backend. I can't however, find in the i386(.md|.c) where this
> is actually happening.
>
>> On i?86 if-conversion probably solves your specific issue,
>> but I guess the initial expansion is where you could improve placement
>> of the 1 (after all, the 0 is after the jumps).
>>
>
> This is happening on my own backend so I guess anything that is implemented
> to do if-conversion on i386 needs to be implemented also on my backend. Can
> you point me to the code on i386 so I can take a look at it?

I think it's all happening in generic code via do_store_flag.

Richard.

> Cheers,
>
> --
> PMatos
>
>