[LLVMbugs] [Bug 10881] New: Compiler optimizer generates incorrect i386 code

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=10881

           Summary: Compiler optimizer generates incorrect i386 code
           Product: clang
           Version: unspecified
          Platform: Macintosh
        OS/Version: MacOS X
            Status: NEW
          Severity: normal
          Priority: P
         Component: -New Bugs
        AssignedTo: unassignedclangbugs at nondot.org
        ReportedBy: eric.trepanier at gmail.com
                CC: llvmbugs at cs.uiuc.edu


Created an attachment (id=7234)
 --> (http://llvm.org/bugs/attachment.cgi?id=7234)
Small Foundation Obj-C program that demonstrates the issue

The attached code sample demonstrates a situation in which the 32-bit compiler
optimizer generates incorrect code for a given while loop.

Compile the attached source file using the following command:

clang -arch i386 -framework Foundation -Os main.m

The run the resulting executable, you should see the following result:

2011-09-07 07:19:42.458 a.out[27557:507] Calculating r using while loop =
{{287, 733}, {33.000004, 5.0509523e-40}}
2011-09-07 07:19:42.460 a.out[27557:507] Calculating r without using while loop
= {{287, 733}, {33.000004, 15.999994}}

If you disable compiler optimizations, (-O0), you will instead see that the
calculated "r" results are the same in both cases:

2011-09-07 07:23:20.042 a.out[27564:507] Calculating r using while loop =
{{287, 733}, {33.000004, 15.999994}}
2011-09-07 07:23:20.044 a.out[27564:507] Calculating r without using while loop
= {{287, 733}, {33.000004, 15.999994}}

Our code was initially using the while-loop approach and generating some
incorrect results. Fortunately, unrolling the loop works around the problem for
us so it is not a showstopper, but it's probably something you want to look at
nonetheless.

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