http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52889
Bug #: 52889
Summary: incorrect sign of _mm_nmsub_XX intrinsics in
fma4intrin.h
Classification: Unclassified
Product: gcc
Version: 4.6.2
Status: UNCONFIRMED
Severity: critical
Priority: P3
Component: c
AssignedTo: unassig...@gcc.gnu.org
ReportedBy: mathiaspu...@gmx.de
Created attachment 27106
--> http://gcc.gnu.org/bugzilla/attachment.cgi?id=27106
corrected fma4intrin.h include file
All _mm_nmsub_xx and _mm256_nmsub_xx intrinsic definitions for FMA4
instructions in the fma4intrin.h header file are incorrect.
nmsub(a,b,c) should compute the equivalent of -( a*b - c) = c - a*b.
However the fma4intrin.h file maps
nmsub(a,b,c) -> madd(-(a),b,-(c)) -> -a*b - c
i.e. the sign in front of the c operand is erroneous.
The impact of this bug is, that code which actively uses the _mm_nmsub_xx
intrinsics gives incorrect results.
The attached fma4intrin.h file has all signs properly corrected
and can be used as drop-in replacement for fma4intrin.h in GCC 4.6.2.
The bug is also present in 4.6.1.
I have not checked correctness of prior GCC version 4.5.x nor newer GCC 4.7.
However the fma4intrin.h mapping has changed from 4.5 -> 4.6,
which might likely have introduced the error.
Best regards,
Mathias Puetz / Cray Inc.
