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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Signed division with power of two can be faster"
href="https://bugs.llvm.org/show_bug.cgi?id=43197">43197</a>
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<th>Summary</th>
<td>Signed division with power of two can be faster
</td>
</tr>
<tr>
<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>
</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>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<p>
<div>
<pre>int foo(int i) {
return i/(1 << 4);
}
foo(int): # @foo(int)
mov eax, edi
sar eax, 31
shr eax, 28
add eax, edi
sar eax, 4
ret
Dispatch Width: 6
uOps Per Cycle: 3.81
IPC: 2.86
Block RThroughput: 1.5
GCC:
foo(int):
test edi, edi
lea eax, [rdi+15]
cmovns eax, edi
sar eax, 4
ret
Dispatch Width: 6
uOps Per Cycle: 4.38
IPC: 3.13
Block RThroughput: 1.2
GCC's perf is better, microbenchmark confirms it:
int main(void) {
int s = 0;
for (int i = -60000000; i < 1000000000; ++i)
s+= i/(1 << 4);
return s;
}
Compiled with -O1:
clang -O1 -march=native lop.c
time ./a.out
real 0m0,658s
user 0m0,655s
sys 0m0,004s
gcc-9 -O1 -march=native lop.c
time ./a.out
real 0m0,605s
user 0m0,605s
sys 0m0,000s</pre>
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