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author | vangyzen <vangyzen@FreeBSD.org> | 2015-10-26 16:21:56 +0000 |
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committer | vangyzen <vangyzen@FreeBSD.org> | 2015-10-26 16:21:56 +0000 |
commit | e22b77611897ae7d2d5eb4de12f21d2a5fc1ccb6 (patch) | |
tree | eddbd2bd9d74250a0f1379938c2e3424071536d9 /share/mk | |
parent | 582d35681a4d2ceee539e894e0183d3b4552aecf (diff) | |
download | FreeBSD-src-e22b77611897ae7d2d5eb4de12f21d2a5fc1ccb6.zip FreeBSD-src-e22b77611897ae7d2d5eb4de12f21d2a5fc1ccb6.tar.gz |
Disable SSE in libthr
Clang emits SSE instructions on amd64 in the common path of
pthread_mutex_unlock. If the thread does not otherwise use SSE,
this usage incurs a context-switch of the FPU/SSE state, which
reduces the performance of multiple real-world applications by a
non-trivial amount (3-5% in one application).
Instead of this change, I experimented with eagerly switching the
FPU state at context-switch time. This did not help. Most of the
cost seems to be in the read/write of memory--as kib@ stated--and
not in the #NM handling. I tested on machines with and without
XSAVEOPT.
One counter-argument to this change is that most applications already
use SIMD, and the number of applications and amount of SIMD usage
are only increasing. This is absolutely true. I agree that--in
general and in principle--this change is in the wrong direction.
However, there are applications that do not use enough SSE to offset
the extra context-switch cost. SSE does not provide a clear benefit
in the current libthr code with the current compiler, but it does
provide a clear loss in some cases. Therefore, disabling SSE in
libthr is a non-loss for most, and a gain for some.
I refrained from disabling SSE in libc--as was suggested--because
I can't make the above argument for libc. It provides a wide variety
of code; each case should be analyzed separately.
https://lists.freebsd.org/pipermail/freebsd-current/2015-March/055193.html
Suggestions from: dim, jmg, rpaulo
Sponsored by: Dell Inc.
Diffstat (limited to 'share/mk')
-rw-r--r-- | share/mk/bsd.cpu.mk | 21 |
1 files changed, 21 insertions, 0 deletions
diff --git a/share/mk/bsd.cpu.mk b/share/mk/bsd.cpu.mk index 5453bd4..f0da4ae 100644 --- a/share/mk/bsd.cpu.mk +++ b/share/mk/bsd.cpu.mk @@ -267,6 +267,27 @@ _CPUCFLAGS += -mfloat-abi=softfp CFLAGS += ${_CPUCFLAGS} .endif +# +# Prohibit the compiler from emitting SIMD instructions. +# These flags are added to CFLAGS in areas where the extra context-switch +# cost outweighs the advantages of SIMD instructions. +# +# gcc: +# Setting -mno-mmx implies -mno-3dnow +# Setting -mno-sse implies -mno-sse2, -mno-sse3, -mno-ssse3 and -mfpmath=387 +# +# clang: +# Setting -mno-mmx implies -mno-3dnow and -mno-3dnowa +# Setting -mno-sse implies -mno-sse2, -mno-sse3, -mno-ssse3, -mno-sse41 and +# -mno-sse42 +# (-mfpmath= is not supported) +# +.if ${MACHINE_CPUARCH} == "i386" || ${MACHINE_CPUARCH} == "amd64" +CFLAGS_NO_SIMD.clang= -mno-avx +CFLAGS_NO_SIMD= -mno-mmx -mno-sse +.endif +CFLAGS_NO_SIMD += ${CFLAGS_NO_SIMD.${COMPILER_TYPE}} + # Add in any architecture-specific CFLAGS. # These come from make.conf or the command line or the environment. CFLAGS += ${CFLAGS.${MACHINE_ARCH}} |