[llvm-dev] Unnecessary spill/fill issue

[Jason via llvm-dev]

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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