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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 - Missed vectorization in SPEC benchmark - emulated gather capability?"
href="https://bugs.llvm.org/show_bug.cgi?id=52358">52358</a>
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<th>Summary</th>
<td>Missed vectorization in SPEC benchmark - emulated gather capability?
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
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<pre>Extracted from 450.soplex:
typedef double Real;
struct Element {
Real val;
int idx;
};
struct Vector {
int dimen;
Real* val;
};
Real foo(Element* e, int n, const Vector& w) {
Real x = 0;
while (n--) {
x += e->val * w.val[e->idx];
e++;
}
return x;
}
Flags: -Ofast -mavx2
Godbolt: <a href="https://godbolt.org/z/7f9nsf98f">https://godbolt.org/z/7f9nsf98f</a>
LLVM just unrolls this loop:
.LBB0_7: # =>This Inner Loop Header: Depth=1
movsxd rcx, dword ptr [rdi + 8]
vmovsd xmm1, qword ptr [rax + 8*rcx] # xmm1 = mem[0],zero
vmulsd xmm1, xmm1, qword ptr [rdi]
vaddsd xmm0, xmm1, xmm0
movsxd rcx, dword ptr [rdi + 24]
vmovsd xmm1, qword ptr [rax + 8*rcx] # xmm1 = mem[0],zero
vmulsd xmm1, xmm1, qword ptr [rdi + 16]
movsxd rcx, dword ptr [rdi + 40]
vmovsd xmm2, qword ptr [rax + 8*rcx] # xmm2 = mem[0],zero
vmulsd xmm2, xmm2, qword ptr [rdi + 32]
vaddsd xmm1, xmm1, xmm2
movsxd rcx, dword ptr [rdi + 56]
vmovsd xmm2, qword ptr [rax + 8*rcx] # xmm2 = mem[0],zero
vmulsd xmm2, xmm2, qword ptr [rdi + 48]
vaddsd xmm0, xmm0, xmm1
add rdi, 64
vaddsd xmm0, xmm2, xmm0
add esi, -4
jne .LBB0_7
New GCC can vectorize it since this commit:
Add emulated gather capability to the vectorizer
This adds a gather vectorization capability to the vectorizer
without target support by decomposing the offset vector, doing
sclar loads and then building a vector from the result. This
is aimed mainly at cases where vectorizing the rest of the loop
offsets the cost of vectorizing the gather.</pre>
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