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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - wrong vectorization in the presence of C++11 atomics"
href="http://llvm.org/bugs/show_bug.cgi?id=22306">22306</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>wrong vectorization in the presence of C++11 atomics
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</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>new bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>sohachak@mpi-sws.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Created <span class=""><a href="attachment.cgi?id=13726" name="attach_13726" title="contains testcase .cpp, .ll, .opt.bc files">attachment 13726</a> <a href="attachment.cgi?id=13726&action=edit" title="contains testcase .cpp, .ll, .opt.bc files">[details]</a></span>
contains testcase .cpp, .ll, .opt.bc files
Hi,
LLVM introduces data race in the following compilation in SLP vectorization
phase.
Source
----------
atomic<int> x[4];
int a[4];
void writeA() {
for(int i=0;i<4;i++) {
a[i] = 0;
x[i].store(i,memory_order_release);
}
}
Compilation command
---------------------
clang++ -std=c++11 -emit-llvm -pthread <filename>.cpp -S;opt -O3 <filename>.ll
-o <filename>.opt.bc -S
<filename>.opt.bc - optimized code
--------
define void @_Z6writeAv() #3 {
entry:
store atomic i32 0, i32* getelementptr inbounds ([4 x %"struct.std::atomic"]*
@x, i64 0, i64 0, i32 0, i32 0) release, align 16
store atomic i32 1, i32* getelementptr inbounds ([4 x %"struct.std::atomic"]*
@x, i64 0, i64 1, i32 0, i32 0) release, align 4
store atomic i32 2, i32* getelementptr inbounds ([4 x %"struct.std::atomic"]*
@x, i64 0, i64 2, i32 0, i32 0) release, align 8
store <4 x i32> zeroinitializer, <4 x i32>* bitcast ([4 x i32]* @a to <4 x
i32>*), align 16
store atomic i32 3, i32* getelementptr inbounds ([4 x %"struct.std::atomic"]*
@x, i64 0, i64 3, i32 0, i32 0) release, align 4
ret void
}
The transformation steps are as follows
for(int i=0;i<4;i++) {a[i] = 0; x[i] = i;}
1. loop unrolling => a[0] = 0; x[0].store(0,release); a[1] = 0;
x[1].store(1,release); a[2] = 0; x[2].store(2,release); a[3] = 0;
x[3].store(3,release);
2. statement reordering => x[0].store(0,release); x[1].store(1,release);
x[2].store(2,release); a[0] = 0; a[1] = 0; a[2] = 0; a[3] = 0;
x[3].store(3,release); // wrong reordering
3. SLP vectorization => x[0].store(0,release); x[1].store(1,release);
x[2].store(2,release); a[0:3] = 0; x[3].store(3,release);
Movement of non-atomic shared variable a[] after the release write of atomic
variable x is unsafe in step 2.
Consider the following thread is running in parallel
int readA() {
int r=0;
if(x[2].load(memory_order_acquire) == 2){
r = a[2];
}
return r;
}
The source program does not have any data race as the write(a[2])
happens-before read(a[2]) due to the synchronization between
(x[2].store(2,release) , x[2].load(acquire)).
But the target program is racy since the write(a[2]); x[2].store(2,release) is
reordered.
The reordering in the SLP vectorization is introducing the error.
Attached are the testcase cpp, and LLVM IR files.
Regards,
soham</pre>
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