https://bugs.kde.org/show_bug.cgi?id=462398
Bug ID: 462398
Summary: x86 FMA - wrong rounding mode
Classification: Developer tools
Product: valgrind
Version: 3.19.0
Platform: unspecified
OS: Linux
Status: REPORTED
Severity: normal
Priority: NOR
Component: general
Assignee: jsew...@acm.org
Reporter: jl_kdeb...@conductive.de
Target Milestone: ---
Created attachment 154139
--> https://bugs.kde.org/attachment.cgi?id=154139&action=edit
fmatest.cpp reproducer
SUMMARY
It seems the valgrind VM will always round down for the FMA instructions.
So assuming the program didn't change the rounding mode, valgrind computes the
wrong result:
0.0 + (- (0.0 * 1.0)) = -0.0
which should be 0.0 in all cases except _MM_ROUND_DOWN, which is not the
default.
STEPS TO REPRODUCE
1. Compile attached cpp with "g++ -O2 -mfma fmatest.cpp"
2. Run natively, observe no negative numbers
3. Run through valgrind with "valgrind --tool=none ./a.out" and observe the
wrong results
OBSERVED RESULT
negative zero
EXPECTED RESULT
positive zero
Even if not implementing switching rounding modes, the most common rounding
mode should be chosen
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