[gcc r12-5412] options: Make -Ofast switch off -fsemantic-interposition

[gcc r12-5412] options: Make -Ofast switch off -fsemantic-interposition

[Martin Jambor]

https://gcc.gnu.org/g:458d2c689963d8461d84670a3d8988cd6ecbfd81

commit r12-5412-g458d2c689963d8461d84670a3d8988cd6ecbfd81
Author: Martin Jambor <mjambor@suse.cz>
Date:   Fri Nov 19 18:46:00 2021 +0100

    options: Make -Ofast switch off -fsemantic-interposition
    
    Using -fno-semantic-interposition has been reported by various people
    to bring about considerable speed up at the cost of strict compliance
    to the ELF symbol interposition rules  See for example
    https://fedoraproject.org/wiki/Changes/PythonNoSemanticInterpositionSpeedup
    
    As such I believe it should be implied by our -Ofast optimization
    level, not only so that benchmarks that can benefit run faster, but
    also so that people looking at -Ofast documentation for options that
    could speed their programs find it.
    
    gcc/ChangeLog:
    
    2021-11-12  Martin Jambor  <mjambor@suse.cz>
    
            * opts.c (default_options_table): Switch off
            flag_semantic_interposition at Ofast.
            * doc/invoke.texi (Optimize Options): Document that Ofast switches off
            -fsemantic-interposition.

Diff:
---
 gcc/doc/invoke.texi | 1 +
 gcc/opts.c          | 1 +
 2 files changed, 2 insertions(+)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 11ef7970378..1cfb70291f6 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -10677,6 +10677,7 @@ valid for all standard-compliant programs.
 It turns on @option{-ffast-math}, @option{-fallow-store-data-races}
 and the Fortran-specific @option{-fstack-arrays}, unless
 @option{-fmax-stack-var-size} is specified, and @option{-fno-protect-parens}.
+It turns off @option {-fsemantic-interposition}.
 
 @item -Og
 @opindex Og
diff --git a/gcc/opts.c b/gcc/opts.c
index 175b4635bb4..b16497e45b3 100644
--- a/gcc/opts.c
+++ b/gcc/opts.c
@@ -682,6 +682,7 @@ static const struct default_options default_options_table[] =
     /* -Ofast adds optimizations to -O3.  */
     { OPT_LEVELS_FAST, OPT_ffast_math, NULL, 1 },
     { OPT_LEVELS_FAST, OPT_fallow_store_data_races, NULL, 1 },
+    { OPT_LEVELS_FAST, OPT_fsemantic_interposition, NULL, 0 },
 
     { OPT_LEVELS_NONE, 0, NULL, 0 }
   };