[llvm-bugs] [Bug 27392] New: wrong code bug

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=27392

            Bug ID: 27392
           Summary: wrong code bug
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: regehr at cs.utah.edu
                CC: llvm-bugs at lists.llvm.org,
                    sanjoy at playingwithpointers.com
    Classification: Unclassified

regehr at hawking:~/test/reduce_wrong_333190$ clang small.c ; ./a.out 
-65536
regehr at hawking:~/test/reduce_wrong_333190$ gcc small.c ; ./a.out 
-65536
regehr at hawking:~/test/reduce_wrong_333190$ clang small.c -O ; ./a.out 
0
regehr at hawking:~/test/reduce_wrong_333190$ cat small.c
int printf(const char *, ...);
short a;
volatile unsigned b;
int main() {
  short *c = &a;
lbl:
  b--;
  (*c)--;
  if (a)
    goto lbl;
  printf("%d\n", b);
  return 0;
}
regehr at hawking:~/test/reduce_wrong_333190$ clang -v
clang version 3.9.0 (trunk 266564)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/regehr/llvm-install/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.6.4
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.7.3
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8.5
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/5.3.0
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64

Sanjoy this looks like it might be an instance of the SCEV issue you already
know about, but I wanted to make sure, since this example came up
independently.

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