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List:       kde-bugs-dist
Subject:    [valgrind] [Bug 330617] ppc false positive conditional jump depends on uninitialised value
From:       Julian Seward <jseward () acm ! org>
Date:       2014-02-10 13:45:07
Message-ID: bug-330617-17878-vxRcaaIHXm () http ! bugs ! kde ! org/
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https://bugs.kde.org/show_bug.cgi?id=330617

--- Comment #3 from Julian Seward <jseward@acm.org> ---

> 2. I wrote a simple test in an attempt to reproduce the problem on x86:
> valgrind does not report a false positive on the following code when
> compiled with Clang trunk on x86 (even though the optimizers do the same
> optimization that proves problematic to valgrind on ppc):

On x86_64 it generates a test instruction (AND, set the flags, throw
away the result of the AND) which tests just the bits in question.
That's why.

0000000000400580 <foo>:
  400580:       f6 07 e0                testb  $0xe0,(%rdi)
  400583:       74 01                   je     400586 <foo+0x6>
  400585:       c3                      retq   
  400586:       bf 70 06 40 00          mov    $0x400670,%edi
  40058b:       e9 c0 fe ff ff          jmpq   400450 <puts@plt>

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