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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86] suboptimal codegen for vector with string of set bits (-1)"
href="https://llvm.org/bugs/show_bug.cgi?id=26301">26301</a>
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<th>Summary</th>
<td>[x86] suboptimal codegen for vector with string of set bits (-1)
</td>
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<th>Product</th>
<td>libraries
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
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<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>I noticed this while looking at vmaskmov codegen because that (unnecessarily)
uses vector masks with elements of all ones or zeros:
define <4 x i32> @high_64_ones() {
ret <4 x i32><i32 0, i32 0, i32 -1, i32 -1>
}
define <2 x i64> @high_64_ones_alt() {
ret <2 x i64><i64 0, i64 -1>
}
$ ./llc high_ones.ll -o -
_high_64_ones: ## @high_64_ones
movaps LCPI0_0(%rip), %xmm0 ## xmm0 = [0,0,4294967295,4294967295]
retq
_high_64_ones_alt: ## @high_64_ones_alt
movq $-1, %rax
movd %rax, %xmm0
pslldq $8, %xmm0 ## xmm0 =
zero,zero,zero,zero,zero,zero,zero,zero,xmm0[0,1,2,3,4,5,6,7]
retq
-----------------------------------------------------------------------------
Some might argue that the 1st case is fine; don't burden the vector units
because loads (and memory space?) are free on big Intel core systems. But I
think the 2nd would be better as:
pcmpeqd %xmm0, %xmm0 // splat 1 bits all the way across
pslldq $8, %xmm0 // shift in the zeros
...to avoid the move from integer to SSE register. And that's the codegen we
should produce by default for both cases.</pre>
</div>
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