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<base href="https://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 --- - incorrect comparison result between INT_MAX and INT_MIN with optimizations" href="https://urldefense.proofpoint.com/v2/url?u=https-3A__llvm.org_bugs_show-5Fbug.cgi-3Fid-3D23757&d=AwMBaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=pF93YEPyB-J_PERP4DUZOJDzFVX5ZQ57vQk33wu0vio&m=cGe0r1aYLC31EEOwanT2mjzG0wcbkE54Y4piDsBQd4c&s=-87fU5bBE66PK-eC8uhxw8KhtJUmt7H0ZmsuQZp7U7o&e=">23757</a>
</td>
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<tr>
<th>Summary</th>
<td>incorrect comparison result between INT_MAX and INT_MIN with optimizations
</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>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>todd@cloudera.com
</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>The following program gives different results with -O1 vs -O0:
#define __STDC_LIMIT_MACROS
#include <stdint.h>
#include <iostream>
#include <string.h>
using namespace std;
bool Increment32(int32_t* cell_ptr) {
int32_t orig = *cell_ptr;
int32_t inc;
// Signed overflow is undefined in C. So, we'll use a branch here
// instead of counting on undefined behavior.
if (orig == INT32_MAX) {
inc = INT32_MIN;
} else {
inc = orig + 1;
}
*cell_ptr = inc;
return inc > orig;
}
int main(int argc, char** argv) {
int32_t input = INT32_MAX;
cout << "input: " << input << endl;
bool ret = Increment32(&input);
cout << "output: " << input << endl;
if (ret) {
cout << "BAD ANSWER" << endl;
}
return 0;
}
I narrowed it down to the following set of passes on llvm 3.4:
todd@todd-ThinkPad-T540p:/tmp$ $I/clang++ -O0 -c -emit-llvm test.cc
todd@todd-ThinkPad-T540p:/tmp$ $I/opt -sroa -simplifycfg -instcombine -o
test-opt.bc test.bc ; $I/clang++ -o test test-opt.bc ; ./test
input: 2147483647
output: -2147483648
WRONG ANSWER
I also verified that a 3.7 build (from the chromium toolchain) has the same
bug.
Compiling with -fwrapv or -fsanitize=undefined makes the error go away. gcc
doesn't have the same issue.</pre>
</div>
</p>
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