[llvm-dev] Unnecessary spill/fill issue

[Sanjay Patel via llvm-dev]

It sounds bad, but I can't tell from the posted info how to diagnose it.

Can you post (a possibly reduced) example to demonstrate what you're
seeing? A bug report would be even better, so we can track if there are
multiple problems:
https://llvm.org/bugs/

On Mon, May 9, 2016 at 3:41 PM, Jason via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Does anyone have any insight into this problem? Is there a way to minimize
> excessive spill/fill for this kind of scenario?
> Thanks,
> Jason
>
>
> On Fri, May 6, 2016 at 10:44 AM, Jason <thesurprises at gmail.com> wrote:
>
>> Hi, I am using mcjit in llvm 3.6 to jit kernels to x86 avx2. I've noticed
>> some inefficient use of the stack around constant vectors. In one example,
>> I have code that computes a series of constant vectors at compile time.
>> Each vector has a single use. In the final asm, I see a series of spills at
>> the top of the function of all the constant vectors immediately to stack,
>> then each use references the stack pointer directly:
>>
>> Lots of these at top of function:
>>
>> movabsq $.LCPI0_212, %rbx
>> vmovaps (%rbx), %ymm0
>> vmovaps %ymm0, 2816(%rsp)       # 32-byte Spill
>>
>> Later on, each use references the stack pointer:
>>
>> vpaddd 2816(%rsp), %ymm4, %ymm1 # 32-byte Folded Reload
>>
>> It seems the spill to stack is unnecessary. In one particularly bad
>> kernel, I have 128 8-wide constant vectors, and so there is 4KB of stack
>> use just for these constants. I think a better approach could be to load
>> the constant vector pointers as needed:
>>
>> movabsq $.LCPI0_212, %rbx
>> vpaddd (%rbx), %ymm4, %ymm1
>>
>>
>> Thanks,
>> Jason
>>
>
>
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