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<base href="http://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 --- - FP constants not combined (fast-math)"
href="http://llvm.org/bugs/show_bug.cgi?id=21962">21962</a>
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</tr>
<tr>
<th>Summary</th>
<td>FP constants not combined (fast-math)
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<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>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>$ cat fadd.c
double foo(double x, double y) {
return -( x + 1.0 + y + 2.0 );
}
Or as IR:
define double @foo(double %x, double %y) {
%add = fadd fast double %x, 1.000000e+00
%add1 = fadd fast double %add, %y
%add2 = fadd fast double %add1, 2.000000e+00
%sub = fsub fast double -0.000000e+00, %add2
ret double %sub
}
$ ./clang -march=haswell -O1 -ffast-math -fomit-frame-pointer fadd.c -S -o -
LCPI0_0:
.quad -4616189618054758400 ## double -1
LCPI0_1:
.quad -4611686018427387904 ## double -2
...
vmovsd LCPI0_0(%rip), %xmm2
vsubsd %xmm0, %xmm2, %xmm0
vsubsd %xmm1, %xmm0, %xmm0
vaddsd LCPI0_1(%rip), %xmm0, %xmm0
retq
-----------------------------------------------------------------------
This should be reassociated/combined to "-3.0 - x - y" (one constant, two
subtracts):
vmovsd LCPI0_0(%rip), %xmm2
vsubsd %xmm0, %xmm2, %xmm3
vsubsd %xmm1, %xmm3, %xmm0
retq</pre>
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