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From: "marcin.krotkiewski at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug rtl-optimization/60086] suboptimal asm generated for a loop (store/load false aliasing)
Date: Fri, 07 Feb 2014 16:43:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60086

--- Comment #8 from Marcin Krotkiewski <marcin.krotkiewski at gmail dot com> ---
(In reply to Andrey Belevantsev from comment #5)
> At this point insn 461 is dead but we do not notice, and it doesn't look
> easy.  I think there was some suggestion in the original research for
> killing dead insn copies left after renaming but I don't remember offhand.

Following Alexanders suggestion, I compiled the test code with -mavx -O3
-fselective-scheduling2 -frename-registers. This seems to get rid of the dead
instructions and yields the desired scheduling:

.L5:
    vmovapd    (%rbx,%rdi), %ymm0
     addq    $1, %rsi
    vmovapd    (%r12,%rdi), %ymm3
     vaddpd    0(%r13,%rdi), %ymm0, %ymm2
    vaddpd    (%r14,%rdi), %ymm3, %ymm4
     vmovapd    %ymm2, (%rbx,%rdi)
     vmovapd    %ymm4, (%r12,%rdi)
     addq    $32, %rdi
    cmpq    %rsi, %rdx
    ja    .L5

Alexander, I should maybe clarify that the 'good' code was prepared by hand,
modifying the 'bad' asm I got from gcc 4.7. Asm generated by gcc 4.4 was the
same. If that is what you were refering to.

I am a bit confused now. It seems that all fine and the desired asm can be
generated, so there is no real bug. But why is the original code compiled with
-O3 -mavx bad then? Is -fschedule-insns not enabled at -O2?