Re: [PATCH] Change fno-unit-at-a-time to fno-toplevel-reorder

[Fangrui Song <maskray@google.com>]

On 2022-05-04, Adhemerval Zanella wrote:
>
>
>On 16/04/2022 15:36, Fangrui Song wrote:
>> -fno-toplevel-reorder is a rough replacement for the legacy option
>> -fno-unit-at-a-time
>> (https://sourceware.org/pipermail/gcc-patches/2006-January/186801.html).
>> glibc currently requires GCC>=6.2 which assuredly has
>> -fno-toplevel-reorder -fno-section-anchors. Clang will use the else
>> branch as it does not implement -fno-toplevel-reorder.
>
>I don't see much improvement here, it just trades one option by another.
>Maybe a better approach would to reorganize the code to avoid requiring
>special compiler options.

The patch ensures the legacy -fno-unit-at-a-time option is no longer used.
With the GNU as patch https://sourceware.org/pipermail/binutils/2022-April/120311.html (gas: copy st_size only if unset)
and the LLVM integrated assembler patch https://reviews.llvm.org/D123283 ([MC][ELF] Improve st_size propagation rule),
the glibc code as is is robust enough.

Removing -fno-toplevel-reorder needs the unreleased binutils 2.39, so fno-toplevel-reorder can be kept for now.

>I did it for the gcc clang work [1], so check-abi is clean.
>
>[1] https://sourceware.org/git/?p=glibc.git;a=shortlog;h=refs/heads/azanella/fno-unit-at-fime-removal

This will be fine, too.

>> ---
>>  config.make.in        | 2 +-
>>  configure             | 6 +++---
>>  configure.ac          | 6 +++---
>>  stdio-common/Makefile | 4 ++--
>>  4 files changed, 9 insertions(+), 9 deletions(-)
>>
>> diff --git a/config.make.in b/config.make.in
>> index bf728c71c0..65e1f46da4 100644
>> --- a/config.make.in
>> +++ b/config.make.in
>> @@ -68,7 +68,7 @@ have-selinux = @have_selinux@
>>  have-libaudit = @have_libaudit@
>>  have-libcap = @have_libcap@
>>  have-cc-with-libunwind = @libc_cv_cc_with_libunwind@
>> -fno-unit-at-a-time = @fno_unit_at_a_time@
>> +fno-toplevel-reorder = @fno_toplevel_reorder@
>>  bind-now = @bindnow@
>>  have-hash-style = @libc_cv_hashstyle@
>>  use-default-link = @use_default_link@
>> diff --git a/configure b/configure
>> index d2f413d05d..4c7b159b13 100755
>> --- a/configure
>> +++ b/configure
>> @@ -620,7 +620,7 @@ libc_cv_cc_loop_to_function
>>  libc_cv_cc_submachine
>>  libc_cv_cc_nofma
>>  libc_cv_mtls_dialect_gnu2
>> -fno_unit_at_a_time
>> +fno_toplevel_reorder
>>  libc_cv_has_glob_dat
>>  libc_cv_hashstyle
>>  libc_cv_fpie
>> @@ -6305,9 +6305,9 @@ fi
>>  { $as_echo "$as_me:${as_lineno-$LINENO}: result: $libc_cv_fno_toplevel_reorder" >&5
>>  $as_echo "$libc_cv_fno_toplevel_reorder" >&6; }
>>  if test $libc_cv_fno_toplevel_reorder = yes; then
>> -  fno_unit_at_a_time="-fno-toplevel-reorder -fno-section-anchors"
>> +  fno_toplevel_reorder="-fno-toplevel-reorder -fno-section-anchors"
>>  else
>> -  fno_unit_at_a_time=-fno-unit-at-a-time
>> +  fno_toplevel_reorder=
>>  fi
>>
>>
>> diff --git a/configure.ac b/configure.ac
>> index b6a747dece..bdff5981c1 100644
>> --- a/configure.ac
>> +++ b/configure.ac
>> @@ -1460,11 +1460,11 @@ else
>>  fi
>>  rm -f conftest*])
>>  if test $libc_cv_fno_toplevel_reorder = yes; then
>> -  fno_unit_at_a_time="-fno-toplevel-reorder -fno-section-anchors"
>> +  fno_toplevel_reorder="-fno-toplevel-reorder -fno-section-anchors"
>>  else
>> -  fno_unit_at_a_time=-fno-unit-at-a-time
>> +  fno_toplevel_reorder=
>>  fi
>> -AC_SUBST(fno_unit_at_a_time)
>> +AC_SUBST(fno_toplevel_reorder)
>>
>>  AC_CACHE_CHECK([for -mtls-dialect=gnu2], libc_cv_mtls_dialect_gnu2,
>>  [dnl
>> diff --git a/stdio-common/Makefile b/stdio-common/Makefile
>> index 435cd8904f..6de2dd84ce 100644
>> --- a/stdio-common/Makefile
>> +++ b/stdio-common/Makefile
>> @@ -333,8 +333,8 @@ CFLAGS-isoc99_vfscanf.c += -fexceptions
>>  CFLAGS-isoc99_vscanf.c += -fexceptions
>>  CFLAGS-isoc99_fscanf.c += -fexceptions
>>  CFLAGS-isoc99_scanf.c += -fexceptions
>> -CFLAGS-errlist.c += $(fno-unit-at-a-time)
>> -CFLAGS-siglist.c += $(fno-unit-at-a-time)
>> +CFLAGS-errlist.c += $(fno-toplevel-reorder)
>> +CFLAGS-siglist.c += $(fno-toplevel-reorder)
>>
>>  # scanf14a.c and scanf16a.c test a deprecated extension which is no
>>  # longer visible under most conformance levels; see the source files