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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - failure to convert FP addition loop into a constant"
href="https://llvm.org/bugs/show_bug.cgi?id=27899">27899</a>
</td>
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<tr>
<th>Summary</th>
<td>failure to convert FP addition loop into a constant
</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>All
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>If we continue on the path from the realistic (<a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Code explosion when LoopVectorize vectorizes a loop with a stride of 8 floats, part 2"
href="show_bug.cgi?id=27881">bug 27881</a>, <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - failure to convert FP addition loop into multiplication with fast-math"
href="show_bug.cgi?id=27894">bug 27894</a>) to the
absurd, I think we end here. :)
float OneMeelion() {
float sum = 0.0f;
for (int i = 0; i < 1000; i++)
sum += 1000.0f;
return sum;
}
We don't need fast-math to know this answer.
$ ./clang -O2 OneMeelion.c -S -o -
...
xorps %xmm0, %xmm0
movl $1000, %eax ## imm = 0x3E8
movss LCPI0_0(%rip), %xmm1 ## xmm1 = mem[0],zero,zero,zero
.p2align 4, 0x90
LBB0_1: ## %for.body
## =>This Inner Loop Header: Depth=1
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addss %xmm1, %xmm0
addl $-10, %eax
jne LBB0_1
Surprisingly, this didn't vectorize!
That's because "-Rpass-analysis=loop-vectorize":
remark: loop not vectorized: cannot prove it is safe to reorder floating-point
operations</pre>
</div>
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