From c03b18934d413aec21ae9586ad012f7efd44e6c5 Mon Sep 17 00:00:00 2001 From: vangyzen Date: Wed, 5 Aug 2015 12:53:55 +0000 Subject: 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. --- libexec/rtld-elf/i386/Makefile.inc | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'libexec/rtld-elf/i386') diff --git a/libexec/rtld-elf/i386/Makefile.inc b/libexec/rtld-elf/i386/Makefile.inc index 7528dbe..a09db6f 100644 --- a/libexec/rtld-elf/i386/Makefile.inc +++ b/libexec/rtld-elf/i386/Makefile.inc @@ -1,6 +1,6 @@ # $FreeBSD$ -CFLAGS+= -mno-mmx -mno-3dnow -mno-sse -mno-sse2 -mno-sse3 -msoft-float +CFLAGS+= ${CFLAGS_NO_SIMD} -msoft-float # Uncomment this to build the dynamic linker as an executable instead # of a shared library: #LDSCRIPT= ${.CURDIR}/${MACHINE_CPUARCH}/elf_rtld.x -- cgit v1.1