[LLVMbugs] [Bug 12051] New: Miscompilation with -O1 and above: Wrong branch taken

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12051

             Bug #: 12051
           Summary: Miscompilation with -O1 and above: Wrong branch taken
           Product: clang
           Version: trunk
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: jonathan.sauer at gmx.de
                CC: dgregor at apple.com, llvmbugs at cs.uiuc.edu
    Classification: Unclassified


(this bug was reported by Salvatore 'antirez' Sanfilippo on cfe-dev and
reproduced by myself)

The following code (from <https://gist.github.com/1876875>) miscompiles on
x86_64-apple-darwin10.8.0 with clang r151042 with -O1, -O2, -O3 and -Os:

#include <stdio.h>

int getValue(long long *l) {
    *l = -9223372036854775484LL;
    return 1;
}

void bug(void) {
    long long value, incr, oldvalue;

    incr = -1000;
    if (getValue(&value) != 1) return;

    oldvalue = value;
    value += incr;
    if ((incr < 0 && value > oldvalue) || (incr > 0 && value < oldvalue)) {
        printf("Correct behavior\n");
    } else {
        printf("Wrong behavior\n");
    }
}

int main(void) {
    bug();
}

When compiled with -O0 and run, this outputs:

$ clang++ -O0 clang.cpp && ./a.out 
Correct behavior


When compiled with -O1 or higher and run, this outputs:

$ clang++ -O1 clang.cpp && ./a.out 
Wrong behavior


When compiled with Apple's gcc 4.2.1 and run, this outputs:

g++ -O1 clang.cpp && ./a.out 
Correct behavior


clang identifies as:

clang version 3.1 (trunk 151042)
Target: x86_64-apple-darwin10.8.0
Thread model: posix

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