On 2021-06-06, Andrew Pinski wrote:
On Sun, Jun 6, 2021 at 4:13 PM Fangrui Song via Gcc-patches
<gcc-patches@gcc.gnu.org> wrote:
From: Fangrui Song <i...@maskray.me>
--enable-default-semantic-interposition=no makes -fPIC default to
-fno-semantic-interposition which enables interprocedural optimizations
for default visibility non-vague-linkage function definitions.
The suppression of interprocedural optimizations and inlining for such
functions is the biggest difference between -fPIE/-fPIC.
Distributions may want to enable default -fno-semantic-interposition to
reclaim the lost performance (e.g. CPython is said to be 27% faster;
Clang is 3% faster).
This breaks assumptions across the board. If software packages want
to use -fno-semantic-interposition that is one thing. But distros
should not be changing the default. This is just like using
-ffast-math :).
Thanks,
Andrew Pinski
What assumption?
Vague linkage function definitions already support IPO/inlining.
For a large non-vague linkage function, presumably GCC will not inline
it into the call sites in the same TU, users can keep using LD_PRELOAD
to interpose that single symbol.
Even if the non-vague function is small and inlined, users can use LD_PRELOAD to
interpose both the callee and callers.
You cannot do such interposition for Windows.
macOS two-level namespace requires explicit `ld -flat_interpose` for such
definition interposition.
Users are free to add noinline or -fsemantic-interposition.
gcc/
PR 100937
* common.opt (fsemantic-interposition): Initialize to -1.
* configure.ac: Add --enable-default-semantic-interposition. Default
to yes.
* defaults.h (DEFAULT_FLAG_SEMANTIC_INTERPOSITION): New.
* opts.c (finish_options): Update
opts->x_flag_semantic_interposition if it is -1.
* doc/install.texi: Document --enable-default-semantic-interposition.
* config.in: Add template.
* configure: Regenerate.
---
gcc/common.opt | 2 +-
gcc/config.in | 6 ++++++
gcc/configure | 22 ++++++++++++++++++++--
gcc/configure.ac | 12 ++++++++++++
gcc/defaults.h | 6 ++++++
gcc/doc/install.texi | 3 +++
gcc/opts.c | 3 +++
7 files changed, 51 insertions(+), 3 deletions(-)
diff --git a/gcc/common.opt b/gcc/common.opt
index ffb968d90f8..68fcbac96b1 100644
--- a/gcc/common.opt
+++ b/gcc/common.opt
@@ -2480,7 +2480,7 @@ Common Var(flag_sel_sched_reschedule_pipelined) Init(0)
Optimization
Reschedule pipelined regions without pipelining.
fsemantic-interposition
-Common Var(flag_semantic_interposition) Init(1)
+Common Var(flag_semantic_interposition) Init(-1)
Allow interposing function (or variables) by ones with different semantics (or
initializer) respectively by dynamic linker.
; sched_stalled_insns means that insns can be moved prematurely from the queue
diff --git a/gcc/config.in b/gcc/config.in
index e54f59ce0c3..7f1d56c1903 100644
--- a/gcc/config.in
+++ b/gcc/config.in
@@ -125,6 +125,12 @@
#endif
+/* Define if -fPIC defaults to -fsemantic-interposition */
+#ifndef USED_FOR_TARGET
+#undef ENABLE_DEFAULT_SEMANTIC_INTERPOSITION
+#endif
+
+
/* Define if your target supports default stack protector and it is enabled.
*/
#ifndef USED_FOR_TARGET
diff --git a/gcc/configure b/gcc/configure
index 4a9e4fa08ab..3835b22f6a5 100755
--- a/gcc/configure
+++ b/gcc/configure
@@ -632,6 +632,7 @@ ac_includes_default="\
ac_subst_vars='LTLIBOBJS
LIBOBJS
CET_HOST_FLAGS
+enable_default_semantic_interposition
NO_PIE_FLAG
NO_PIE_CFLAGS
enable_default_pie
@@ -1027,6 +1028,7 @@ with_linker_hash_style
with_diagnostics_color
with_diagnostics_urls
enable_default_pie
+enable_default_semantic_interposition
enable_cet
enable_s390_excess_float_precision
'
@@ -1787,6 +1789,8 @@ Optional Features:
--disable-libquadmath-support
disable libquadmath support for Fortran
--enable-default-pie enable Position Independent Executable as default
+ --enable-default-semantic-interposition
+ enable -fsemantic-interposition as -fPIC default
--enable-cet enable Intel CET in host libraries [default=auto]
--enable-s390-excess-float-precision
on s390 targets, evaluate float with double
@@ -19435,7 +19439,7 @@ else
lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
lt_status=$lt_dlunknown
cat > conftest.$ac_ext <<_LT_EOF
-#line 19438 "configure"
+#line 19454 "configure"
#include "confdefs.h"
#if HAVE_DLFCN_H
@@ -19541,7 +19545,7 @@ else
lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
lt_status=$lt_dlunknown
cat > conftest.$ac_ext <<_LT_EOF
-#line 19544 "configure"
+#line 19560 "configure"
#include "confdefs.h"
#if HAVE_DLFCN_H
@@ -31893,6 +31897,20 @@ if test "$gcc_cv_no_pie" = "yes"; then
fi
+# Check whether --enable-default-semantic-interposition=no was given.
+if test "${enable_default_semantic_interposition+set}" = set; then :
+ enableval=$enable_default_semantic_interposition;
enable_default_semantic_interposition=$enableval
+else
+ enable_default_semantic_interposition=yes
+fi
+
+if test x$enable_default_semantic_interposition != xno ; then
+
+$as_echo "#define ENABLE_DEFAULT_SEMANTIC_INTERPOSITION 1" >>confdefs.h
+
+fi
+
+
# Enable Intel CET on Intel CET enabled host if jit is enabled.
# Check whether --enable-cet was given.
if test "${enable_cet+set}" = set; then :
diff --git a/gcc/configure.ac b/gcc/configure.ac
index d9fc3c219e8..1cdf8b4ce00 100644
--- a/gcc/configure.ac
+++ b/gcc/configure.ac
@@ -7419,6 +7419,18 @@ if test "$gcc_cv_no_pie" = "yes"; then
fi
AC_SUBST([NO_PIE_FLAG])
+# Check whether --enable-default-semantic-interposition=no was given.
+AC_ARG_ENABLE(default-semantic-interposition,
+[AS_HELP_STRING([--enable-default-semantic-interposition],
+ [enable -fsemantic-interposition as -fPIC default])],
+enable_default_semantic_interposition=$enableval,
+enable_default_semantic_interposition=yes)
+if test x$enable_default_semantic_interposition != xno ; then
+ AC_DEFINE(ENABLE_DEFAULT_SEMANTIC_INTERPOSITION, 1,
+ [Define if -fPIC defaults to -fsemantic-interposition.])
+fi
+AC_SUBST([enable_default_semantic_interposition])
+
# Enable Intel CET on Intel CET enabled host if jit is enabled.
GCC_CET_HOST_FLAGS(CET_HOST_FLAGS)
case x$enable_languages in
diff --git a/gcc/defaults.h b/gcc/defaults.h
index 91216593e75..548b1f64f17 100644
--- a/gcc/defaults.h
+++ b/gcc/defaults.h
@@ -1249,6 +1249,12 @@ see the files COPYING3 and COPYING.RUNTIME respectively.
If not, see
# define DEFAULT_FLAG_PIE 0
#endif
+#ifdef ENABLE_DEFAULT_SEMANTIC_INTERPOSITION
+# define DEFAULT_FLAG_SEMANTIC_INTERPOSITION 1
+#else
+# define DEFAULT_FLAG_SEMANTIC_INTERPOSITION 0
+#endif
+
#ifndef SWITCHABLE_TARGET
#define SWITCHABLE_TARGET 0
#endif
diff --git a/gcc/doc/install.texi b/gcc/doc/install.texi
index 591ccaacbc1..17e756c7e7a 100644
--- a/gcc/doc/install.texi
+++ b/gcc/doc/install.texi
@@ -1867,6 +1867,9 @@ mips-linux and s390-linux.
@item --enable-default-pie
Turn on @option{-fPIE} and @option{-pie} by default.
+@item --enable-default-semantic-interposition=no
+Make @option{-fPIC} default to @option{-fno-semantic-interposition}.
+
@item --enable-secureplt
This option enables @option{-msecure-plt} by default for powerpc-linux.
@ifnothtml
diff --git a/gcc/opts.c b/gcc/opts.c
index a751d95d32b..c6f49af2583 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -1068,6 +1068,9 @@ finish_options (struct gcc_options *opts, struct
gcc_options *opts_set,
if (opts->x_flag_stack_protect == -1)
opts->x_flag_stack_protect = DEFAULT_FLAG_SSP;
+ if (opts->x_flag_semantic_interposition == -1)
+ opts->x_flag_semantic_interposition = DEFAULT_FLAG_SEMANTIC_INTERPOSITION;
+
if (opts->x_optimize == 0)
{
/* Inlining does not work if not optimizing,
--
2.32.0.rc1.229.g3e70b5a671-goog
