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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - The loop will not be unrolled by writing the -fno-unroll-loops option before or after -O2."
href="https://bugs.llvm.org/show_bug.cgi?id=47033">47033</a>
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<th>Summary</th>
<td>The loop will not be unrolled by writing the -fno-unroll-loops option before or after -O2.
</td>
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<th>Product</th>
<td>clang
</td>
</tr>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>-New Bugs
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>fj8765ah@aa.jp.fujitsu.com
</td>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<p>
<div>
<pre>a.c:
void foo(double * restrict a,
double * restrict b,
double * restrict c,
int n) {
for (int i=0;i<n;i++)
c[i] = a[i] + b[i];
return;
}
The loop will not be unrolled by writing the -fno-unroll-loops option before or
after -O2.
# For AArch64, this problem does not occur unless the -mcpu option is
specified, for example, thunderx2t99.
% clang -target x86_64-unkown-linux-gnu -O2 a.c -Rpass=unroll -S
a.c:6:3: remark: unrolled loop by a factor of 2 with run-time trip count
[-Rpass=loop-unroll]
for (int i=0;i<n;i++)
^
% clang -target x86_64-unkown-linux-gnu -fno-unroll-loops -O2 a.c -Rpass=unroll
-S
% clang -target x86_64-unkown-linux-gnu -O2 -fno-unroll-loops a.c -Rpass=unroll
-S
However, the loop is not vectorized only if the -fno-vectorize option is
written after -O2.
% clang -target x86_64-unkown-linux-gnu -O2 a.c -Rpass=vector -S
a.c:6:3: remark: vectorized loop (vectorization width: 2, interleaved count: 2)
[-Rpass=loop-vectorize]
for (int i=0;i<n;i++)
^
% clang -target x86_64-unkown-linux-gnu -fno-vectorize -O2 a.c -Rpass=vector -S
a.c:6:3: remark: vectorized loop (vectorization width: 2, interleaved count: 2)
[-Rpass=loop-vectorize]
for (int i=0;i<n;i++)
^
% clang -target x86_64-unkown-linux-gnu -O2 -fno-vectorize a.c -Rpass=vector -S
%
I think it is because I did not check the positional relation with -O2 in
clang/lib/Driver/ToolChains/Clang.cpp.
I think -fno-unroll-loops should behave like -fno-vectorize.
How do you like it?</pre>
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