[LLVMdev] Duplicate loading of double constants

[Nick Lewycky]

On 18 August 2013 22:38, Eugene Toder <eltoder at gmail.com> wrote:

> Hi,
>
> I found that in some cases llvm generates duplicate loads of double
> constants,
> e.g.
>
> $ cat t.c
> double f(double* p, int n)
> {
>     double s = 0;
>     if (n)
>         s += *p;
>     return s;
> }
> $ clang -S -O3 t.c -o -
> ...
> f:                                      # @f
>         .cfi_startproc
> # BB#0:
>         xorps   %xmm0, %xmm0
>         testl   %esi, %esi
>         je      .LBB0_2
> # BB#1:
>         xorps   %xmm0, %xmm0
>         addsd   (%rdi), %xmm0
> .LBB0_2:
>         ret
> ...
>

Thanks. Please file a bug for this on llvm.org/bugs .

The crux of the problem is that machine CSE runs before register allocation
and is consequently extremely conservative when doing CSE to avoid
potentially increasing register pressure. Of course, with such a small
testcase, register pressure isn't a problem. MachineCSE might be able to do
a better job here.

Nick

Note that there are 2 xorps instructions, the one in BB#1 being clearly
> redundant
> as it's dominated by the first one. Two xorps come from 2 FsFLD0SD
> generated by
> instruction selection and never eliminated by machine passes. My guess
> would be
> machine CSE should take care of it.
>
> A variation of this case without indirection shows the same problem, as
> well as
> not commuting addps, resulting in an extra movps:
>
> $ cat t.c
> double f(double p, int n)
> {
>     double s = 0;
>     if (n)
>         s += p;
>     return s;
> }
> $ clang -S -O3 t.c -o -
> ...
> f:                                      # @f
>         .cfi_startproc
> # BB#0:
>         xorps   %xmm1, %xmm1
>         testl   %edi, %edi
>         je      .LBB0_2
> # BB#1:
>         xorps   %xmm1, %xmm1
>         addsd   %xmm1, %xmm0
>         movaps  %xmm0, %xmm1
> .LBB0_2:
>         movaps  %xmm1, %xmm0
>         ret
> ...
>
> Thanks,
> Eugene
>
>
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