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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Poor unrolling prevents vectorization opportunities"
href="https://bugs.llvm.org/show_bug.cgi?id=51108">51108</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Poor unrolling prevents vectorization opportunities
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>andrea.dibiagio@gmail.com, florian_hahn@apple.com, lebedev.ri@gmail.com, llvm-bugs@lists.llvm.org, ryta1203@gmail.com, spatel+llvm@rotateright.com
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<div>
<pre><a href="https://godbolt.org/z/jE1e9rT5j">https://godbolt.org/z/jE1e9rT5j</a>
(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</pre>
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