Re: IRA undoing scheduling decisions

[Richard Guenther <richard dot guenther at gmail dot com>]

On Wed, Aug 26, 2009 at 10:47 PM, Peter Bergner<bergner@vnet.ibm.com> wrote:
> On Mon, 2009-08-24 at 23:56 +0000, Charles J. Tabony wrote:
>> I am seeing a performance regression on the port I maintain, and I would appreciate some pointers.
>>
>> When I compile the following code
>>
>> void f(int *x, int *y){
>> ? *x = 7;
>> ? *y = 4;
>> }
>>
>> with GCC 4.3.2, I get the desired sequence of instructions. ?I'll call it sequence A:
>>
>> r0 = 7
>> r1 = 4
>> [x] = r0
>> [y] = r1
>>
>> When I compile the same code with GCC 4.4.0, I get a sequence that is lower performance for my target machine. ?I'll call it sequence B:
>>
>> r0 = 7
>> [x] = r0
>> r0 = 4
>> [y] = r0
>
> This is caused by update_equiv_regs() which IRA inherited from local-alloc.c.
> Although with gcc 4.3 and earlier, you don't see the problem, it is still there,
> because if you look at the 4.3 dumps, you will see that update_equiv_regs()
> unordered them for us. ?What is saving us is that sched2 reschedules them
> again for us in the order we want. ?With 4.4, IRA happens to reuse the same
> register for both pseudos, so sched2 is hand tied and cannot schedule them
> back again for us.
>
> Looking at update_equiv_regs(), if I disable the replacement for regs
> that are local to one basic block (patch below) like it existed before
> John Wehle's patch way back in Oct 2000:
>
> ?http://gcc.gnu.org/ml/gcc-patches/2000-09/msg00782.html
>
> then we get the ordering we want. ?Does anyone know why John removed
> that part of the test in his patch? ?Thoughts anyone?

Hmm.  I suppose if you conditionalize it on flag_schedule_insns it might be
an overall win.  Care to SPEC test that change?

Thanks,
Richard.

>
> Peter
>
>
> Index: ira.c
> ===================================================================
> --- ira.c ? ? ? (revision 151111)
> +++ ira.c ? ? ? (working copy)
> @@ -2510,6 +2510,7 @@ update_equiv_regs (void)
> ? ? ? ? ? ? ? ? ? ? calls. ?*/
>
> ? ? ? ? ? ? ? ? ?if (REG_N_REFS (regno) == 2
> + ? ? ? ? ? ? ? ? ? ? && REG_BASIC_BLOCK (regno) < NUM_FIXED_BLOCKS
> ? ? ? ? ? ? ? ? ? ? ?&& (rtx_equal_p (x, src)
> ? ? ? ? ? ? ? ? ? ? ? ? ?|| ! equiv_init_varies_p (src))
> ? ? ? ? ? ? ? ? ? ? ?&& NONJUMP_INSN_P (insn)
>
>
>
>