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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86, FMA] use FMA instructions for negation instead of loading a constant and xor'ing?"
href="https://llvm.org/bugs/show_bug.cgi?id=24366">24366</a>
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<th>Summary</th>
<td>[x86, FMA] use FMA instructions for negation instead of loading a constant and xor'ing?
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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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
</td>
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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>
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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>With the addition of FMA instructions, x86 can negate FP values without loading
a constant:
$ cat nmul.ll
define float @nmul(float %a, float %b) {
%mul = fmul float %a, %b
%neg = fsub float -0.0, %mul
ret float %neg
}
$ ./llc -o - nmul.ll -mattr=fma
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 14, 4
.section __TEXT,__literal16,16byte_literals
.align 4
LCPI0_0:
.long 2147483648 ## 0x80000000
.long 2147483648 ## 0x80000000
.long 2147483648 ## 0x80000000
.long 2147483648 ## 0x80000000
.section __TEXT,__text,regular,pure_instructions
.globl _nmul
.align 4, 0x90
_nmul: ## @nmul
.cfi_startproc
## BB#0:
vmulss %xmm1, %xmm0, %xmm0
vxorps LCPI0_0(%rip), %xmm0, %xmm0
retq
-------------------------------------------------------------------------------
Better speed/size, but does require a temp register:
vxorps %xmm2, %xmm2, %xmm2 <--- make zero cheaply for add operand
vfnmadd213ss %xmm2, %xmm1, %xmm0
I don't think this causes any IEEE-754 problems, but if it does, it should
still be possible to do this with -ffast-math?</pre>
</div>
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