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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - An optimization error about loop unrolling"
href="https://bugs.llvm.org/show_bug.cgi?id=42382">42382</a>
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<th>Summary</th>
<td>An optimization error about loop unrolling
</td>
</tr>
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<th>Product</th>
<td>new-bugs
</td>
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<th>Version</th>
<td>7.0
</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>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>new bugs
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>witstorm@163.com
</td>
</tr>
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<th>CC</th>
<td>htmldeveloper@gmail.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>In the experiment, when clang uses O2 and O3 optimization options, the pass of
Loop unrolling will cause an result error.The procedure is as follows：
typedef int A __attribute__((vector _size(64),aligned(64)));
A Aa,Ab,Ai;
static __inline A __attribute__((__always_inline__))
SET_A (int __A,int __B,int __C,int __D,int __E,int __F,int __G,int __H,int
__I,int __J,int __K,int __L,int __M,int __N,int __O,int __P){
union{
int __a[16] __attribute__((aligned(64)));
A __v;
}__u;
__u.__a[0]=__A;
__u.__a[1]=__B;
__u.__a[2]=__C;
__u.__a[3]=__D;
__u.__a[4]=__E;
__u.__a[5]=__F;
__u.__a[6]=__G;
__u.__a[7]=__H;
__u.__a[8]=__I;
__u.__a[9]=__J;
__u.__a[10]=__K;
__u.__a[11]=__L;
__u.__a[12]=__M;
__u.__a[13]=__N;
__u.__a[14]=__O;
__u.__a[15]=__P;
return __u.__v;
}
int main(){
int
r[16]={2047,2047,2047,2047,2047,2047,2047,2047,2047,2047,2047,2047,2047,2047,2047,2047};
int i;
for(i=1024;i>=1;i>>=1){
Ai=SET_A(i,i,i,i,i,i,i,i,i,i,i,i,i,i,i,i);
Aa^=Aa>>i;
Ab+=Aa+Ai;
}
}
You can reproduce the error with the following command：
clang -arch=x86 -static -O2 -emit-llvm -S a.c
Loop unrolling optimization produces incorrect values in IR.</pre>
</div>
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