[Mesa-dev] [PATCH] swr: Refactor checks for compiler feature flags

[Chuck Atkins]

So this seems to be different across versions as well.  It looks like
__AVX__ and __AVX2__ are the only ones we can really count on being there.
I can drop the second check to just __AVX2__.  I think it's redundant by
chance though that all CPUs that supported AVX2 also seem to support the
additional 2 instructions.

- Chuck

On Tue, Jun 28, 2016 at 1:52 PM, Rowley, Timothy O <
timothy.o.rowley at intel.com> wrote:

>
> > On Jun 28, 2016, at 8:24 AM, Chuck Atkins <chuck.atkins at kitware.com>
> wrote:
> >
> > Encapsulate the test for which flags are needed to get a compiler to
> > support certain features.  Along with this, give various options to try
> > for AVX and AVX2 support.  Ideally we want to use specific instruction
> > set feature flags, like -mavx2 for instance instead of -march=haswell,
> > but the flags required for certain compilers are different.  This
> > allows, for AVX2 for instance, GCC to use -mavx2 -mfma -mbmi2 -mf16c
> > while the Intel compiler which doesn't support those flags can fall
> > back to using -march=core-avx2.
> >
> > This addresses a bug where the Intel compiler will silently ignore the
> > AVX2 instruction feature flags and then potentially fail to build.
> >
> > Cc: Tim Rowley <timothy.o.rowley at intel.com>
> > Signed-off-by: Chuck Atkins <chuck.atkins at kitware.com>
> > ---
> > configure.ac | 86
> +++++++++++++++++++++++++++++++++++++++++++-----------------
> > 1 file changed, 62 insertions(+), 24 deletions(-)
> >
> > diff --git a/configure.ac b/configure.ac
> > index cc9bc47..806850e 100644
> > --- a/configure.ac
> > +++ b/configure.ac
> > @@ -2330,6 +2330,39 @@ swr_llvm_check() {
> >     fi
> > }
> >
> > +swr_cxx_feature_flags_check() {
> > +    ifndef_test=$1
> > +    option_list="$2"
> > +    unset SWR_CXX_FEATURE_FLAGS
> > +    AC_LANG_PUSH([C++])
> > +    save_CXXFLAGS="$CXXFLAGS"
> > +    save_IFS="$IFS"
> > +    IFS=","
> > +    found=0
> > +    for opts in $option_list
> > +    do
> > +        unset IFS
> > +        CXXFLAGS="$opts $save_CXXFLAGS"
> > +        AC_COMPILE_IFELSE(
> > +            [AC_LANG_PROGRAM(
> > +                [   $ifndef_test
> > +                    #error
> > +                    #endif
> > +                ])],
> > +            [found=1; break],
> > +            [])
> > +        IFS=","
> > +    done
> > +    IFS="$save_IFS"
> > +    CXXFLAGS="$save_CXXFLAGS"
> > +    AC_LANG_POP([C++])
> > +    if test $found -eq 1; then
> > +        SWR_CXX_FEATURE_FLAGS="$opts"
> > +        return 0
> > +    fi
> > +    return 1
> > +}
> > +
> > dnl Duplicates in GALLIUM_DRIVERS_DIRS are removed by sorting it after
> this block
> > if test -n "$with_gallium_drivers"; then
> >     gallium_drivers=`IFS=', '; echo $with_gallium_drivers`
> > @@ -2399,31 +2432,36 @@ if test -n "$with_gallium_drivers"; then
> >         xswr)
> >             swr_llvm_check "swr"
> >
> > -            AC_MSG_CHECKING([whether $CXX supports c++11/AVX/AVX2])
> > -            SWR_AVX_CXXFLAGS="-mavx"
> > -            SWR_AVX2_CXXFLAGS="-mavx2 -mfma -mbmi2 -mf16c"
> > -
> > -            AC_LANG_PUSH([C++])
> > -            save_CXXFLAGS="$CXXFLAGS"
> > -            CXXFLAGS="-std=c++11 $CXXFLAGS"
> > -            AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],[],
> > -                              [AC_MSG_ERROR([c++11 compiler support not
> detected])])
> > -            CXXFLAGS="$save_CXXFLAGS"
> > -
> > -            save_CXXFLAGS="$CXXFLAGS"
> > -            CXXFLAGS="$SWR_AVX_CXXFLAGS $CXXFLAGS"
> > -            AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],[],
> > -                              [AC_MSG_ERROR([AVX compiler support not
> detected])])
> > -            CXXFLAGS="$save_CXXFLAGS"
> > -
> > -            save_CFLAGS="$CXXFLAGS"
> > -            CXXFLAGS="$SWR_AVX2_CXXFLAGS $CXXFLAGS"
> > -            AC_COMPILE_IFELSE([AC_LANG_PROGRAM()],[],
> > -                              [AC_MSG_ERROR([AVX2 compiler support not
> detected])])
> > -            CXXFLAGS="$save_CXXFLAGS"
> > -            AC_LANG_POP([C++])
> > -
> > +            AC_MSG_CHECKING([whether $CXX supports c++11])
> > +            if ! swr_cxx_feature_flags_check \
> > +                "#if __cplusplus < 201103L" \
> > +                ",-std=c++11"; then
> > +                AC_MSG_RESULT([no])
> > +                AC_MSG_ERROR([swr requires C++11 support])
> > +            fi
> > +            AC_MSG_RESULT([$SWR_CXX_FEATURE_FLAGS])
> > +            CXXFLAGS="$SWR_CXX_FEATURE_FLAGS $CXXFLAGS”
>
> We don’t want to globally override CXXFLAGS; AC_SUBST on a SWR_CXXFLAGS
> and using that in swr’s Makefile.am would be better.
>
> > +
> > +            AC_MSG_CHECKING([whether $CXX supports AVX])
> > +            if ! swr_cxx_feature_flags_check \
> > +                "#ifndef __AVX__" \
> > +                ",-mavx,-march=core-avx"; then
> > +                AC_MSG_RESULT([no])
> > +                AC_MSG_ERROR([swr requires AVX compiler support])
> > +            fi
> > +            AC_MSG_RESULT([$SWR_CXX_FEATURE_FLAGS])
> > +            SWR_AVX_CXXFLAGS="$SWR_CXX_FEATURE_FLAGS"
> >             AC_SUBST([SWR_AVX_CXXFLAGS])
> > +
> > +            AC_MSG_CHECKING([whether $CXX supports AVX2])
> > +            if ! swr_cxx_feature_flags_check \
> > +                "#if
> !(defined(__AVX2__)&&defined(__FMA__)&&defined(__BMI2__)&&defined(__F16C__))”
> \
>
> Is there any standard that says these are defined if the compiler supports
> them?  With icc 16.0.3, the test falls into the #error path when it tries
> the fallback test of -march=core-avx2.
>
> > +                ",-mavx2 -mfma -mbmi2 -mf16c,-march=core-avx2"; then
> > +                AC_MSG_RESULT([no])
> > +                AC_MSG_ERROR([swr requires AVX2 compiler support])
> > +            fi
> > +            AC_MSG_RESULT([$SWR_CXX_FEATURE_FLAGS])
> > +            SWR_AVX2_CXXFLAGS="$SWR_CXX_FEATURE_FLAGS"
> >             AC_SUBST([SWR_AVX2_CXXFLAGS])
> >
> >             HAVE_GALLIUM_SWR=yes
> > --
> > 2.5.5
> >
>
>
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