https://bugs.llvm.org/show_bug.cgi?id=51108
Bug ID: 51108
Summary: Poor unrolling prevents vectorization opportunities
Product: libraries
Version: trunk
Hardware: PC
OS: Windows NT
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: llvm-...@redking.me.uk
CC: andrea.dibia...@gmail.com, florian_h...@apple.com,
lebedev...@gmail.com, llvm-bugs@lists.llvm.org,
ryta1...@gmail.com, spatel+l...@rotateright.com
https://godbolt.org/z/jE1e9rT5j
(NOTE: disabled fma on gcc to prevent fmul+fadd->fma diff(
constexpr int SIZE = 128;
float A[SIZE][16];
float B[SIZE][16];
__attribute__((__noinline__))
float foo()
{
float sum = 0.0f;
for (int i = 1; i < 32; ++i)
for (int j = 0; j < 4; ++j)
sum += A[i][j] * B[i][j];
return sum;
}
clang -g0 -O3 -march=znver2
_Z3foov:
vxorps %xmm0, %xmm0, %xmm0
movq $-1984, %rax # imm = 0xF840
.LBB0_1:
vmovss A+2048(%rax), %xmm1 # xmm1 = mem[0],zero,zero,zero
vmovsd B+2052(%rax), %xmm2 # xmm2 = mem[0],zero
vmulss B+2048(%rax), %xmm1, %xmm1
vaddss %xmm1, %xmm0, %xmm0
vmovsd A+2052(%rax), %xmm1 # xmm1 = mem[0],zero
vmulps %xmm2, %xmm1, %xmm1
vaddss %xmm1, %xmm0, %xmm0
vmovshdup %xmm1, %xmm1 # xmm1 = xmm1[1,1,3,3]
vaddss %xmm1, %xmm0, %xmm0
vmovss A+2060(%rax), %xmm1 # xmm1 = mem[0],zero,zero,zero
vmulss B+2060(%rax), %xmm1, %xmm1
addq $64, %rax
vaddss %xmm1, %xmm0, %xmm0
jne .LBB0_1
retq
The clang code has several issues:
1 - if we'd used a better indvar we could have avoided some very large offsets
on the address math (put A and B in registers and use a better range/increment
for %rax).
2 - GCC recognises that the array is fully dereferencable allowing it to use
fewer (vector) loads and then extract/shuffle the elements that it requires
3 - we fail to ensure the per-loop reduction is in a form that we can use
HADDPS (on targets where its fast)
4 - the LoopMicroOpBufferSize in the znver3 model has a VERY unexpected effect
on unrolling - I'm not sure clang's interpretation of the buffer size is the
same as just copying AMD's hardware specs
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