[PATCH] powerpc: Define USE_PPC64_NOTOC iff compiler and linker also supports it

[Adhemerval Zanella]

On 08/11/2021 19:48, Fangrui Song wrote:
> On 2021-11-08, Adhemerval Zanella wrote:
>> The @ntoc usage only yields any advantage on ISA 3.1+ machine (power10)
>> and for ld.bfd also when it see pcrel relocations used on the code
>> (generated if compiler targets ISA 3.1+).  On bfd case ISA 3.1+
>> instruction on stubs are used iff linker also sees the new pc-relative
>> relocations (for instance R_PPC64_D34), otherwise it generates default
>> stubs (ppc64_elf_check_relocs:4700).
>>
>> This patch also help on linkers that do not implement this optimization,
>> since building for older ISA (such as 3.0 / power9) will also trigger
>> power10 stubs generation in the assembly code uses the NOTOC imacro.
>>
>> Checked on powerpc64le-linux-gnu.
>> ---
>> sysdeps/powerpc/powerpc64/configure    | 42 ++++++++++++++++----------
>> sysdeps/powerpc/powerpc64/configure.ac | 25 ++++++++++-----
>> 2 files changed, 43 insertions(+), 24 deletions(-)
>>
>> diff --git a/sysdeps/powerpc/powerpc64/configure b/sysdeps/powerpc/powerpc64/configure
>> index 5ce77af631..db18791a55 100644
>> --- a/sysdeps/powerpc/powerpc64/configure
>> +++ b/sysdeps/powerpc/powerpc64/configure
>> @@ -32,26 +32,36 @@ if test x$libc_cv_overlapping_opd = xyes; then
>>
>> fi
>>
>> -# @notoc started to be supported in GNU Binutils 2.31.
>> -
>> -{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if the assembler supports @notoc" >&5
>> -$as_echo_n "checking if the assembler supports @notoc... " >&6; }
>> +# We check if compiler supports @notoc generation since there is no
>> +# gain by enabling it if it will be optimized away by the linker.
>> +# It also helps linkers that might not optimize it and end up
>> +# generating stubs with ISA 3.1 instruction even targetting older ISA.
>> +{ $as_echo "$as_me:${as_lineno-$LINENO}: checking if the compiler supports @notoc" >&5
>> +$as_echo_n "checking if the compiler supports @notoc... " >&6; }
>> if ${libc_cv_ppc64_notoc+:} false; then :
>>   $as_echo_n "(cached) " >&6
>> else
>> -
>> -           cat confdefs.h - <<_ACEOF >conftest.$ac_ext
>> -/* end confdefs.h.  */
>> -
>> -void foo (void) {asm("b foo@notoc");}
>> -
>> -_ACEOF
>> -if ac_fn_c_try_compile "$LINENO"; then :
>> -  libc_cv_ppc64_notoc=yes
>> -else
>> +    cat > conftest.c <<EOF
>> +int bar (void);
>> +int foo (void) { return bar () + 1; }
>> +EOF
>>   libc_cv_ppc64_notoc=no
>> -fi
>> -rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
>> +  if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -S -o conftest.s conftest.c'
>> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
>> +  (eval $ac_try) 2>&5
>> +  ac_status=$?
>> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
>> +  test $ac_status = 0; }; } \
>> +     && { ac_try='grep -q -E 'bar@notoc' conftest.s'
>> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
>> +  (eval $ac_try) 2>&5
>> +  ac_status=$?
>> +  $as_echo "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
>> +  test $ac_status = 0; }; }
>> +  then
>> +    libc_cv_ppc64_notoc=yes
>> +  fi
>> +  rm -rf conftest.*
>> fi
>> { $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_ppc64_notoc" >&5
>> $as_echo "$libc_cv_ppc64_notoc" >&6; }
>> diff --git a/sysdeps/powerpc/powerpc64/configure.ac b/sysdeps/powerpc/powerpc64/configure.ac
>> index b77156f696..05f0000807 100644
>> --- a/sysdeps/powerpc/powerpc64/configure.ac
>> +++ b/sysdeps/powerpc/powerpc64/configure.ac
>> @@ -22,13 +22,22 @@ if test x$libc_cv_overlapping_opd = xyes; then
>>   AC_DEFINE(USE_PPC64_OVERLAPPING_OPD)
>> fi
>>
>> -# @notoc started to be supported in GNU Binutils 2.31.
>> -AC_CACHE_CHECK([if the assembler supports @notoc],
>> -           libc_cv_ppc64_notoc, [
>> -           AC_COMPILE_IFELSE([AC_LANG_SOURCE([
>> -void foo (void) {asm("b foo@notoc");}
>> -          ])],
>> -          [libc_cv_ppc64_notoc=yes],
>> -          [libc_cv_ppc64_notoc=no])])
>> +# We check if compiler supports @notoc generation since there is no
>> +# gain by enabling it if it will be optimized away by the linker.
>> +# It also helps linkers that might not optimize it and end up
>> +# generating stubs with ISA 3.1 instruction even targetting older ISA.
>> +AC_CACHE_CHECK([if the compiler supports @notoc],
>> +           libc_cv_ppc64_notoc, [dnl
>> +  cat > conftest.c <<EOF
>> +int bar (void);
>> +int foo (void) { return bar () + 1; }
>> +EOF
>> +  libc_cv_ppc64_notoc=no
>> +  if AC_TRY_COMMAND([${CC-cc} $CFLAGS $CPPFLAGS -S -o conftest.s conftest.c]) \
>> +     && AC_TRY_COMMAND([grep -q -E 'bar@notoc' conftest.s])
>> +  then
>> +    libc_cv_ppc64_notoc=yes
>> +  fi
>> +  rm -rf conftest.*])
>> AS_IF([test x$libc_cv_ppc64_notoc = xyes],
>>       [AC_DEFINE(USE_PPC64_NOTOC)])
>> -- 
>> 2.32.0
>>
> 
> I think the original fix to https://sourceware.org/bugzilla/show_bug.cgi?id=26173
> complicated things. A simpler fix is to just check whether CC CFLAGS
> generates @notoc and use that to guide whether assembly code needs @notoc.

This is exactly what this patch does in fact: it check if $CC $CFLAGS
generates @notoc and defines USE_PPC64_NOTOC which is used internally
on NOTOC macro.

The title is misleading in fact, is should be "powerpc: Define USE_PPC64_NOTOC 
iff compiler supports it"