[LLVMbugs] [Bug 22172] New: wrong code (generated code hangs) at -O2 and -O3 on x86_64-linux-gnu

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22172

            Bug ID: 22172
           Summary: wrong code (generated code hangs) at -O2 and -O3 on
                    x86_64-linux-gnu
           Product: clang
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: su at cs.ucdavis.edu
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

The current clang trunk miscompiles the following code at -O2 and -O3 on
x86_64-linux-gnu in the 64-bit mode (but not in 32-bit mode).

This is a regression from 3.5.0.  


$ clang-trunk -v
clang version 3.6.0 (trunk 225511)
Target: x86_64-unknown-linux-gnu
Thread model: posix
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/4.8.1
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/4.8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Selected multilib: .;@m64
$ 
$ clang-trunk -m64 -Os small.c; a.out
$ clang-trunk -m32 -O2 small.c; a.out
$ clang-3.5.0 -m64 -O2 small.c; a.out
$ 
$ clang-trunk -m64 -O2 small.c
$ timeout -s 9 30 a.out
Killed
$ 


---------------------------------


struct
{
  char f1;
} a;

struct S
{
  int f2;
};

volatile struct S b;

int
main ()
{
  for (a.f1 = 0; a.f1 >= 0; a.f1++)
    b.f2;
  return 0;
}

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