flambda-backend: Add -mbranches-within-32B to major_gc.c compilation (where supported)

Author: stedolan

Makefile.config.in

@@ -243,6 +243,7 @@ PROBES=@probes@
 AWK=@AWK@
 STDLIB_MANPAGES=@stdlib_manpages@
 NAKED_POINTERS=@naked_pointers@
+INTEL_JCC_BUG_CFLAGS=@intel_jcc_bug_cflags@
 
 ### Native command to build ocamlrun.exe
 

aclocal.m4

@@ -30,6 +30,7 @@ m4_include([build-aux/lt~obsolete.m4])
 # Macros from the autoconf macro archive
 m4_include([build-aux/ax_func_which_gethostbyname_r.m4])
 m4_include([build-aux/ax_pthread.m4])
+m4_include([build-aux/ax_check_compile_flag.m4])
 
 # The following macro figures out which C compiler is used.
 # It does so by checking for compiler-specific predefined macros.

build-aux/ax_check_compile_flag.m4

@@ -0,0 +1,53 @@
+# ===========================================================================
+#  https://www.gnu.org/software/autoconf-archive/ax_check_compile_flag.html
+# ===========================================================================
+#
+# SYNOPSIS
+#
+#   AX_CHECK_COMPILE_FLAG(FLAG, [ACTION-SUCCESS], [ACTION-FAILURE], [EXTRA-FLAGS], [INPUT])
+#
+# DESCRIPTION
+#
+#   Check whether the given FLAG works with the current language's compiler
+#   or gives an error.  (Warnings, however, are ignored)
+#
+#   ACTION-SUCCESS/ACTION-FAILURE are shell commands to execute on
+#   success/failure.
+#
+#   If EXTRA-FLAGS is defined, it is added to the current language's default
+#   flags (e.g. CFLAGS) when the check is done.  The check is thus made with
+#   the flags: "CFLAGS EXTRA-FLAGS FLAG".  This can for example be used to
+#   force the compiler to issue an error when a bad flag is given.
+#
+#   INPUT gives an alternative input source to AC_COMPILE_IFELSE.
+#
+#   NOTE: Implementation based on AX_CFLAGS_GCC_OPTION. Please keep this
+#   macro in sync with AX_CHECK_{PREPROC,LINK}_FLAG.
+#
+# LICENSE
+#
+#   Copyright (c) 2008 Guido U. Draheim <[email protected]>
+#   Copyright (c) 2011 Maarten Bosmans <[email protected]>
+#
+#   Copying and distribution of this file, with or without modification, are
+#   permitted in any medium without royalty provided the copyright notice
+#   and this notice are preserved.  This file is offered as-is, without any
+#   warranty.
+
+#serial 6
+
+AC_DEFUN([AX_CHECK_COMPILE_FLAG],
+[AC_PREREQ(2.64)dnl for _AC_LANG_PREFIX and AS_VAR_IF
+AS_VAR_PUSHDEF([CACHEVAR],[ax_cv_check_[]_AC_LANG_ABBREV[]flags_$4_$1])dnl
+AC_CACHE_CHECK([whether _AC_LANG compiler accepts $1], CACHEVAR, [
+  ax_check_save_flags=$[]_AC_LANG_PREFIX[]FLAGS
+  _AC_LANG_PREFIX[]FLAGS="$[]_AC_LANG_PREFIX[]FLAGS $4 $1"
+  AC_COMPILE_IFELSE([m4_default([$5],[AC_LANG_PROGRAM()])],
+    [AS_VAR_SET(CACHEVAR,[yes])],
+    [AS_VAR_SET(CACHEVAR,[no])])
+  _AC_LANG_PREFIX[]FLAGS=$ax_check_save_flags])
+AS_VAR_IF(CACHEVAR,yes,
+  [m4_default([$2], :)],
+  [m4_default([$3], :)])
+AS_VAR_POPDEF([CACHEVAR])dnl
+])dnl AX_CHECK_COMPILE_FLAGS

configure

@@ -169,6 +169,7 @@ AC_SUBST([stdlib_manpages])
 AC_SUBST([compute_deps])
 AC_SUBST([naked_pointers])
 AC_SUBST([naked_pointers_checker])
+AC_SUBST([intel_jcc_bug_cflags])
 
 ## Generated files
 
@@ -1076,6 +1077,16 @@ AS_CASE([$arch],
        [common_cflags="-no-pie $common_cflags"],
     [])])
 
+# The major GC performs better with this flag on Intel processors
+# This is a workaround for an Intel CPU bug:
+# https://www.intel.co.uk/content/www/uk/en/support/articles/000055650/processors.html
+
+intel_jcc_bug_cflags=''
+AS_CASE([$arch],
+  [amd64],
+    [AX_CHECK_COMPILE_FLAG([-Wa,-mbranches-within-32B-boundaries],
+     [intel_jcc_bug_cflags=-Wa,-mbranches-within-32B-boundaries])])
+
 # Assembler
 
 AS_IF([test -n "$host_alias"], [toolpref="${host_alias}-"], [toolpref=""])

configure.ac

@@ -333,6 +333,11 @@ libasmrun_shared.$(SO): $(libasmrunpic_OBJECTS)
 %.npic.$(O): OC_CPPFLAGS += $(OC_NATIVE_CPPFLAGS)
 %.npic.$(D): OC_CPPFLAGS += $(OC_NATIVE_CPPFLAGS)
 
+# The major GC performs better with this flag on Intel processors
+# This is a workaround for an Intel CPU bug:
+# https://www.intel.co.uk/content/www/uk/en/support/articles/000055650/processors.html
+major_gc.%: OC_CFLAGS += $(INTEL_JCC_BUG_CFLAGS)
+
 # Compilation of C files
 
 # The COMPILE_C_FILE macro below receives as argument the pattern