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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 Approved by: kib (mentor) MFC after: 2 weeks Sponsored by: Dell Inc.
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svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=286317
@ -1,3 +1,9 @@
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#$FreeBSD$
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SRCS+= _umtx_op_err.S
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# With the current compiler and libthr code, using SSE in libthr
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# does not provide enough performance improvement to outweigh
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# the extra context switch cost. This can measurably impact
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# performance when the application also does not use enough SSE.
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CFLAGS+=${CFLAGS_NO_SIMD}
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@ -1,3 +1,9 @@
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# $FreeBSD$
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SRCS+= _umtx_op_err.S
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# With the current compiler and libthr code, using SSE in libthr
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# does not provide enough performance improvement to outweigh
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# the extra context switch cost. This can measurably impact
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# performance when the application also does not use enough SSE.
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CFLAGS+=${CFLAGS_NO_SIMD}
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@ -1,6 +1,6 @@
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# $FreeBSD$
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CFLAGS+= -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -msoft-float
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CFLAGS+= ${CFLAGS_NO_SIMD} -msoft-float
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# Uncomment this to build the dynamic linker as an executable instead
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# of a shared library:
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#LDSCRIPT= ${.CURDIR}/${MACHINE_CPUARCH}/elf_rtld.x
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@ -1,6 +1,6 @@
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# $FreeBSD$
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CFLAGS+= -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -msoft-float
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CFLAGS+= ${CFLAGS_NO_SIMD} -msoft-float
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# Uncomment this to build the dynamic linker as an executable instead
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# of a shared library:
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#LDSCRIPT= ${.CURDIR}/${MACHINE_CPUARCH}/elf_rtld.x
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@ -282,6 +282,27 @@ _CPUCFLAGS += -mfloat-abi=softfp
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CFLAGS += ${_CPUCFLAGS}
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.endif
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#
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# Prohibit the compiler from emitting SIMD instructions.
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# These flags are added to CFLAGS in areas where the extra context-switch
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# cost outweighs the advantages of SIMD instructions.
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#
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# gcc:
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# Setting -mno-mmx implies -mno-3dnow
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# Setting -mno-sse implies -mno-sse2, -mno-sse3, -mno-ssse3 and -mfpmath=387
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#
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# clang:
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# Setting -mno-mmx implies -mno-3dnow and -mno-3dnowa
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# Setting -mno-sse implies -mno-sse2, -mno-sse3, -mno-ssse3, -mno-sse41 and
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# -mno-sse42
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# (-mfpmath= is not supported)
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#
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.if ${MACHINE_CPUARCH} == "i386" || ${MACHINE_CPUARCH} == "amd64"
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CFLAGS_NO_SIMD.clang= -mno-avx
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CFLAGS_NO_SIMD= -mno-mmx -mno-sse
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.endif
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CFLAGS_NO_SIMD += ${CFLAGS_NO_SIMD.${COMPILER_TYPE}}
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# Add in any architecture-specific CFLAGS.
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# These come from make.conf or the command line or the environment.
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CFLAGS += ${CFLAGS.${MACHINE_ARCH}}
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