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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 --- - don't unroll a loop that has no hope for improvement"
href="https://llvm.org/bugs/show_bug.cgi?id=26532">26532</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>don't unroll a loop that has no hope for improvement
</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>All
</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>Loop Optimizer
</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>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>I'm not sure where this should be solved, but the "unroll-runtime" pass should
be reined in / less aggressive.
I was looking at the example in <a class="bz_bug_link
bz_status_NEW "
title="NEW --- - 30% performance deficiency vs GCC because of excessive vectorization on -march=slm"
href="show_bug.cgi?id=25108">bug 25108</a> and wondered what happened with
simpler cases:
int foo(int x, int n) {
for (int i=0; i<n; i++)
x = x * x;
return x;
}
With:
$ ./clang -v
clang version 3.9.0 (trunk 260122)
Target: x86_64-apple-darwin15.3.0
$ ./clang -O2 25108.c -S -emit-llvm -o -
This gets unrolled 8x + prologue code. The net effect is that code which should
be simple like this (-Os or gcc 5.3 with -O2 is similar):
00 testl %esi, %esi
02 jle 0xb
04 imull %edi, %edi
07 decl %esi
09 jne 0x4
0b movl %edi, %eax
0d retq
...is bloated out to 64 bytes with almost no chance that this could be better
for performance on any recent x86:
00 testl %esi, %esi
02 jle 0x3d
04 leal -0x1(%rsi), %ecx
07 xorl %eax, %eax
09 movl %esi, %edx
0b andl $0x7, %edx
0e je 0x19
10 imull %edi, %edi
13 incl %eax
15 cmpl %eax, %edx
17 jne 0x10
19 cmpl $0x7, %ecx
1c jb 0x3d
1e subl %eax, %esi
20 imull %edi, %edi
23 imull %edi, %edi
26 imull %edi, %edi
29 imull %edi, %edi
2c imull %edi, %edi
2f imull %edi, %edi
32 imull %edi, %edi
35 imull %edi, %edi
38 addl $-0x8, %esi
3b jne 0x20
3d movl %edi, %eax
3f retq</pre>
</div>
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