From: H.J. Lu <hjl.tools@gmail.com>
Date: Wed, 24 Jul 2019 21:48:33 +0000 (-0700)
Subject: x86-64: Compile branred.c with -mprefer-vector-width=128 [BZ #24603]
X-Git-Tag: glibc-2.30~9
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7e681561a3aea7aa8f21fb031a7c778147dfdf5b;p=thirdparty%2Fglibc.git

x86-64: Compile branred.c with -mprefer-vector-width=128 [BZ #24603]

When compiled with -O3 and AVX, GCC 8 and 9 optimize some loops in
sysdeps/ieee754/dbl-64/branred.c with 256-bit vector instructions,
which leads to store forward stall:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90579

There is no easy fix in compiler.  This patch limits vector width to
128 bits to work around this issue.  It improves performance of sin
and cos by more than 40% on Skylake compiled with -O3 -march=skylake.

Tested with GCC 7/8/9 on x86-64.

	[BZ #24603]
	* sysdeps/x86_64/configure.ac: Check if -mprefer-vector-width=128
	works.
	* sysdeps/x86_64/configure: Regenerated.
	* sysdeps/x86_64/fpu/Makefile (CFLAGS-branred.c): New.  Set
	to -mprefer-vector-width=128 if supported.
---

diff --git a/ChangeLog b/ChangeLog
index 88108d1e8be..31a6b38bd55 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+2019-07-24  H.J. Lu  <hongjiu.lu@intel.com>
+
+	[BZ #24603]
+	* sysdeps/x86_64/configure.ac: Check if -mprefer-vector-width=128
+	works.
+	* sysdeps/x86_64/configure: Regenerated.
+	* sysdeps/x86_64/fpu/Makefile (CFLAGS-branred.c): New.  Set
+	to -mprefer-vector-width=128 if supported.
+
 2019-07-24  Florian Weimer  <fweimer@redhat.com>
 
 	* scripts/build-many-glibcs.py (Context.checkout): Default to
diff --git a/sysdeps/x86_64/configure b/sysdeps/x86_64/configure
index 8674d145695..84f82c24068 100644
--- a/sysdeps/x86_64/configure
+++ b/sysdeps/x86_64/configure
@@ -54,6 +54,28 @@ fi
 config_vars="$config_vars
 config-cflags-avx512 = $libc_cv_cc_avx512"
 
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking -mprefer-vector-width=128" >&5
+$as_echo_n "checking -mprefer-vector-width=128... " >&6; }
+if ${libc_cv_cc_mprefer_vector_width+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  if { ac_try='${CC-cc} -mprefer-vector-width=128 -xc /dev/null -S -o /dev/null'
+  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
+  (eval $ac_try) 2>&5
+  ac_status=$?
+  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
+  test $ac_status = 0; }; }; then :
+  libc_cv_cc_mprefer_vector_width=yes
+else
+  libc_cv_cc_mprefer_vector_width=no
+fi
+
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_cc_mprefer_vector_width" >&5
+$as_echo "$libc_cv_cc_mprefer_vector_width" >&6; }
+config_vars="$config_vars
+config-cflags-mprefer-vector-width = $libc_cv_cc_mprefer_vector_width"
+
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking for Intel MPX support" >&5
 $as_echo_n "checking for Intel MPX support... " >&6; }
 if ${libc_cv_asm_mpx+:} false; then :
diff --git a/sysdeps/x86_64/configure.ac b/sysdeps/x86_64/configure.ac
index b7d2c0124fb..cdaba0c075a 100644
--- a/sysdeps/x86_64/configure.ac
+++ b/sysdeps/x86_64/configure.ac
@@ -25,6 +25,15 @@ if test $libc_cv_cc_avx512 = yes; then
 fi
 LIBC_CONFIG_VAR([config-cflags-avx512], [$libc_cv_cc_avx512])
 
+dnl Check if -mprefer-vector-width=128 works.
+AC_CACHE_CHECK(-mprefer-vector-width=128, libc_cv_cc_mprefer_vector_width, [dnl
+LIBC_TRY_CC_OPTION([-mprefer-vector-width=128],
+		   [libc_cv_cc_mprefer_vector_width=yes],
+		   [libc_cv_cc_mprefer_vector_width=no])
+])
+LIBC_CONFIG_VAR([config-cflags-mprefer-vector-width],
+		[$libc_cv_cc_mprefer_vector_width])
+
 dnl Check whether asm supports Intel MPX
 AC_CACHE_CHECK(for Intel MPX support, libc_cv_asm_mpx, [dnl
 cat > conftest.s <<\EOF
diff --git a/sysdeps/x86_64/fpu/Makefile b/sysdeps/x86_64/fpu/Makefile
index 2b7d69bb504..74b14ba0966 100644
--- a/sysdeps/x86_64/fpu/Makefile
+++ b/sysdeps/x86_64/fpu/Makefile
@@ -237,3 +237,15 @@ CFLAGS-test-float-libmvec-sincosf-avx512.c = -DREQUIRE_AVX512F
 CFLAGS-test-float-libmvec-sincosf-avx512-main.c = $(libmvec-sincos-cflags) $(float-vlen16-arch-ext-cflags)
 endif
 endif
+
+ifeq ($(subdir)$(config-cflags-mprefer-vector-width),mathyes)
+# When compiled with -O3 -march=skylake, GCC 8 and 9 optimize some loops
+# in branred.c with 256-bit vector instructions, which leads to store
+# forward stall:
+#
+# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90579
+#
+# Limit vector width to 128 bits to work around this issue.  It improves
+# performance of sin and cos by more than 40% on Skylake.
+CFLAGS-branred.c = -mprefer-vector-width=128
+endif