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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 --- - MemorySanitizer false positives in optimized code"
href="https://llvm.org/bugs/show_bug.cgi?id=28054">28054</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>MemorySanitizer false positives in optimized code
</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>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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>eugeni.stepanov@gmail.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>$ cat 1.cc
#include <stdio.h>
#include <sanitizer/msan_interface.h>
struct Map {
unsigned Small;
long Buckets;
long *Ptr;
unsigned NumBuckets;
Map() {
Small = true;
Buckets = 42;
__msan_allocated_memory(&NumBuckets, sizeof(NumBuckets));
}
long *getBuckets() {
return Small ? &Buckets : Ptr;
}
unsigned getNumBuckets() {
return Small ? 1 : NumBuckets;
}
};
Map M;
int main() {
for (int i = 0; i < 1; ++i) {
if (M.getNumBuckets() != 0) {
if (42 == *M.getBuckets())
return 1;
}
}
if (!M.Small)
printf("zz\n");
}
$ clang++ 1.cc -std=c++14 -g -O3 -fsanitize=memory && ./a.out
WARNING: MemorySanitizer: use-of-uninitialized-value
#0 0x4879c3 in main 1.cc:30:9
#1 0x7f12aeff9f44 in __libc_start_main
#2 0x419b11 in _start
Looking at the bitcode, the first thing the program does is compare NumBuckets
with 0 and branch on the result. That's what MSan is complaining about. The
source code reads NumBuckets only when Small == 0, which is impossible.
This happens only at -O3.
$ clang++ 1.cc -std=c++14 -g -O3 -S -emit-llvm -o -
...
%struct.Map = type <{ i32, [4 x i8], i64, i64*, i32, [4 x i8] }>
...
define i32 @main() #0 {
entry:
%0 = load i32, i32* getelementptr inbounds (%struct.Map, %struct.Map* @M, i64
0, i32 0), align 8
%tobool.i20 = icmp eq i32 %0, 0
%1 = load i32, i32* getelementptr inbounds (%struct.Map, %struct.Map* @M, i64
0, i32 4), align 8
%cmp1 = icmp eq i32 %1, 0
%2 = load i64*, i64** getelementptr inbounds (%struct.Map, %struct.Map* @M,
i64 0, i32 3), align 8
br i1 %cmp1, label %entry.split.us, label %entry.split</pre>
</div>
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