Re: [PATCH] Add --enable-default-semantic-interposition to GCC configure

[Fangrui Song <i@maskray.me>]

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
>>