https://bugs.kde.org/show_bug.cgi?id=417993
Bug ID: 417993
Summary: vbit-test fail on s390x with Iop_Add32: spurious
dependency on uninitialised value
Product: valgrind
Version: unspecified
Platform: Other
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: memcheck
Assignee: jsew...@acm.org
Reporter: ar...@linux.ibm.com
Target Milestone: ---
This is seen on s390x with the test case in `memcheck/tests/vbit-test':
┌────
│ valgrind -q --expensive-definedness-checks=yes ./vbit-test
│ ==113353== Conditional jump or move depends on uninitialised value(s)
│ ==113353== at 0x10012A8: check_result_for_binary (binary.c:372)
│ ==113353== by 0x10025A7: test_binary_op (binary.c:683)
│ ==113353== by 0x1000B35: main (main.c:192)
└────
The uninitialised value results from the calculation in
int_add_or_sub_vbits(). In the failing case the function receives two
32-bit unsigned integers as arguments, the first of which equals zero,
but has its least significant bit undefined, and the second value equals
0xffffffff.
One step of the calculation determines the value `a_min' as
┌────
│ a_min = aa & ~vaa;
└────
where `aa' has the lowest bit undefined and `vaa' has exactly the lowest
bit set. However, the compiler transforms this to
┌────
│ a_min = (aa & vaa) ^ aa;
└────
After that, Valgrind's memcheck considers the lowest bit of `a_min'
undefined as well.
NOTE: In order to run the test case, I temporarily disabled Iop_Or1 and
Iop_And1 for s390x in the `irops' array in irops.c.
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