git: 3b00222f156d - main - Avoid raising unexpected floating point exceptions in libm

[Dimitry Andric]

The branch main has been updated by dim:

URL: https://cgit.FreeBSD.org/src/commit/?id=3b00222f156dca5700c839d73e36daf479fa640c

commit 3b00222f156dca5700c839d73e36daf479fa640c
Author:     Dimitry Andric <dim at FreeBSD.org>
AuthorDate: 2021-04-09 23:07:54 +0000
Commit:     Dimitry Andric <dim at FreeBSD.org>
CommitDate: 2021-04-10 11:59:57 +0000

    Avoid raising unexpected floating point exceptions in libm
    
    When using clang with x86_64 CPUs that support AVX, some floating point
    transformations may raise exceptions that would not have been raised by
    the original code. To avoid this, use the -fp-exception-behavior=maytrap
    flag, introduced in clang 10.0.0.
    
    In particular, this fixes a number of test failures with ctanhf(3) and
    ctanf(3), when libm is compiled with -mavx. An unexpected FE_INVALID
    exception is then raised, because clang emits vdivps instructions to
    perform certain divides. (The vdivps instruction operates on multiple
    single-precision float operands simultaneously, but the exceptions may
    be influenced by unused parts of the XMM registers. In this particular
    case, it was calculating 0 / 0, which results in FE_INVALID.)
    
    If -fp-exception-behavior=maytrap is specified however, clang uses
    vdivss instructions instead, which work on one operand, and should not
    raise unexpected exceptions.
    
    Reported by:    olivier
    Reviewed by:    arichardson
    PR:             254911
    MFC after:      1 week
    Differential Revision: https://reviews.freebsd.org/D29686
---
 lib/msun/Makefile | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/lib/msun/Makefile b/lib/msun/Makefile
index 4491bc216a4c..959dbd113ec1 100644
--- a/lib/msun/Makefile
+++ b/lib/msun/Makefile
@@ -34,6 +34,16 @@ CFLAGS+=	-I${.CURDIR}/ld128
 
 CFLAGS+=	-I${.CURDIR}/${ARCH_SUBDIR}
 
+.include <bsd.compiler.mk>
+.if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 100000
+# When using clang with x86_64 CPUs that support AVX, some floating point
+# transformations may raise exceptions that would not have been raised by the
+# original code. To avoid this, use the -fp-exception-behavior=maytrap flag,
+# introduced in clang 10.0.0.
+# See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=254911
+CFLAGS+=	-ffp-exception-behavior=maytrap
+.endif
+
 .PATH:	${.CURDIR}/bsdsrc
 .PATH:	${.CURDIR}/src
 .PATH:	${.CURDIR}/man
@@ -113,7 +123,6 @@ COMMON_SRCS+=	catrigl.c \
 #     'long double' [-Werror=overflow]
 #     if( y >= LDBL_MAX )
 # See also: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=130067
-.include <bsd.compiler.mk>
 .if ${COMPILER_TYPE} == "gcc"
 CFLAGS.e_powl.c+= -Wno-error=overflow
 .endif