[LLVMbugs] [Bug 12133] New: if overflow is required to exit a loop clang will emit an infinite loop

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12133

             Bug #: 12133
           Summary: if overflow is required to exit a loop clang will emit
                    an infinite loop
           Product: clang
           Version: 3.0
          Platform: PC
        OS/Version: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: kretz at kde.org
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified


Created attachment 8119
  --> http://llvm.org/bugs/attachment.cgi?id=8119
testcase

The following loop (see also the attached source file) only ends because i
overflows and thus finally reaches the required 0x7fffffff:
for (int i = -0x80000000; i < 0x7fffffff; i += 0x1fffff)

clang translates this into an unconditional infinite loop. If the loop body is
empty, though, clang removes the infinite loop, which is inconsistent. :)

Here's what GCC makes of it:
   1:   bb 01 00 20 00          mov    $0x200001,%ebx
   6:   66 2e 0f 1f 84 00 00    nopw   %cs:0x0(%rax,%rax,1)
   d:   00 00 00 
  10:   e8 00 00 00 00          callq  15 <bar()+0x15>
  15:   83 eb 01                sub    $0x1,%ebx
  18:   75 f6                   jne    10 <bar()+0x10>


And clang's infinite loop:
   0:   55                      push   %rbp
   1:   48 89 e5                mov    %rsp,%rbp
   4:   66 66 66 2e 0f 1f 84    data32 data32 nopw %cs:0x0(%rax,%rax,1)
   b:   00 00 00 00 00 
  10:   e8 00 00 00 00          callq  15 <bar()+0x15>
  15:   eb f9                   jmp    10 <bar()+0x10>

I would appreciate a compiler that translates the overflow correctly, but also
warns me about it. This was really a bug in my code, and thanks to this bug in
clang I found it... :)

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