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.

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}}