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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [X86][SSE] Slow sqrt libcall branch due to dependency on sqrtsd result"
href="https://llvm.org/bugs/show_bug.cgi?id=31455">31455</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[X86][SSE] Slow sqrt libcall branch due to dependency on sqrtsd result
</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>Windows NT
</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>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>llvm-dev@redking.me.uk
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, mkuper@google.com, spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>#include <math.h>
double sqrt_add(double a, double b) {
return sqrt(a) + b;
}
define double @sqrt_add(double, double) {
%3 = tail call double @sqrt(double %0)
%4 = fadd double %3, %1
ret double %4
}
declare double @sqrt(double)
Found while looking at c-ray performance (PR22657), llvm tests for an
non-finite result of the (slow) sqrtsd instruction to decide whether to
fallback on the sqrt libcall, while gcc tests the input for a negative or zero
input, giving the branch logic more time to work.
llvm (3.9) output:
sqrt_add(double, double): # @sqrt_add(double, double)
vmovapd %xmm0, %xmm2
vxorps %xmm0, %xmm0, %xmm0
vsqrtsd %xmm2, %xmm0, %xmm0
vucomisd %xmm0, %xmm0
jnp .LBB0_2
pushq %rax
vmovapd %xmm2, %xmm0
vmovsd %xmm1, (%rsp) # 8-byte Spill
callq sqrt
vmovsd (%rsp), %xmm1 # 8-byte Reload
addq $8, %rsp
.LBB0_2:
vaddsd %xmm1, %xmm0, %xmm0
retq
gcc (7 snapshot) output:
sqrt_add(double, double):
vsqrtsd %xmm0, %xmm3, %xmm3
subq $24, %rsp
vxorpd %xmm2, %xmm2, %xmm2
vucomisd %xmm0, %xmm2
vmovsd %xmm3, (%rsp)
jbe .L2
vmovsd %xmm1, 8(%rsp)
call sqrt
vmovsd 8(%rsp), %xmm1
.L2:
vaddsd (%rsp), %xmm1, %xmm0
addq $24, %rsp
ret</pre>
</div>
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