On Wed, Apr 10, 2013 at 11:46 AM, Konstantin Vladimirov
<konstantin.vladimi...@gmail.com> wrote:
> Hi,
>
> I have this problem in private backend, but it is reproducible on
> x86-gcc also, so I suppose core GCC probems. Lets consider simple
> example:
>
> unsigned int buffer[10];
>
> __attribute__((noinline)) void
> myFunc(unsigned int a, unsigned int b, unsigned int c)
> {
> unsigned int tmp;
> if( a & 0x2 )
> {
> tmp = 0x3221;
> tmp |= (a & 0xF) << 24;
> tmp |= (a & 0x3) << 2;
> }
> else
> {
> tmp = 0x83621;
> tmp |= (a & 0xF) << 24;
> tmp |= (a & 0x3) << 2;
> }
> buffer[0] = tmp;
> }
>
> And compile it with -Os to assembler. It yields:
>
> movl %edi, %eax
> andl $15, %eax
> sall $24, %eax
> testb $2, %dil
> je .L2
> andl $3, %edi
> sall $2, %edi
> orl %edi, %eax
> orl $12833, %eax
> jmp .L3
> .L2:
> andl $3, %edi
> sall $2, %edi
> orl %edi, %eax
> orl $538145, %eax
> .L3:
> movl %eax, buffer(%rip)
> ret
>
> There is a big common code fragment:
>
> andl $3, %edi
> sall $2, %edi
> orl %edi, %eax
>
> That potentially can be moved upper and reduce code size. But it
> doesn't. Also on O2.
>
> Things are even worse in my backend, because my target have
> conditional or and all code possibly may be linearized in this case
> with good performance impact, and only thing why not is because GCC
> fails to move common part out of the if-else switch upper prior to
> trying conditional execution.
>
> Can someone advice me how to tune target machine options to signal,
> that moving common parts out of the conditional expressions is
> profitable? Or the only way is to write custom pass?
Apart from RTL cross-jumping which is quite limited there is no existing
pass that does this transformation. There is PR23286 for this which
also has some patches.
Richard.
> ---
> With best regards, Konstantin
