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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization in math expression: log of pow or exp of log"
href="https://bugs.llvm.org/show_bug.cgi?id=35742">35742</a>
</td>
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<th>Summary</th>
<td>Missed optimization in math expression: log of pow or exp of log
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</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>zheltonozhskiy@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#include <cmath>
double f(double a)
{
return exp(log(a)) ;
}
double f2(double a, double b)
{
return log(pow(a,b)) ;
}
clang(trunk) -g0 -O3 -march=bdver4 -ffast-math generates following assembly:
f(double): # @f(double)
pushq %rax
callq log
popq %rax
jmp exp # TAILCALL
f2(double, double): # @f2(double, double)
pushq %rax
callq pow
popq %rax
jmp log # TAILCALL
While gcc generates
f(double):
ret
f2(double, double):
subq $24, %rsp
vmovsd %xmm1, 8(%rsp)
call __log_finite
vmovsd 8(%rsp), %xmm1
addq $24, %rsp
vmulsd %xmm0, %xmm1, %xmm0
ret
replacing exponent of natural logarithm with the number itself and ln(a^b) with
b*ln(a) (same for log10/log2)</pre>
</div>
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