[LLVMbugs] [Bug 15607] New: Uninitialized register on Shift operation

bugzilla-daemon at llvm.org bugzilla-daemon at llvm.org
Tue Mar 26 22:55:38 PDT 2013


            Bug ID: 15607
           Summary: Uninitialized register on Shift operation
           Product: libraries
           Version: 3.1
          Hardware: PC
                OS: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: Backend: X86
          Assignee: unassignedbugs at nondot.org
          Reporter: ishiura-compiler at ml.kwansei.ac.jp
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

Apple clang 4.1 (based on LLVM 3.1svn) seems to miscompile the
following C program with optimize option -O1.

  $ cat test.c

  int x = 0;
  int main (void)
    return ( 1 >> ( x << 8 ) );  /* should return 1 */

  $ clang -O1 test.c
  $ ./a.out
  $ echo $?

Note that a.out may yield correct output (1) depending on
environments.  This seems to happen because register "%cl" on the
"shrl" instruction is not initialized in the generated code.

     pushq  %rbp
     .cfi_def_cfa_offset 16
     .cfi_offset %rbp, -16
     movq  %rsp, %rbp
     .cfi_def_cfa_register %rbp
     movl  $1, %eax
     shrl  %cl, %eax
     popq  %rbp

The code may be valid for x != 0 (due to undefined behavior), but not
for x == 0.

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