Re: [PATCH] Silence clang -Wignored-attributes when building with clang

On 27/07/26 11:54, Matt Turner wrote:
> Building glibc with clang trips -Wignored-attributes on the alias
> redirections glibc uses intentionally, e.g. the long_double_symbol
> machinery where a public name aliases a weak alias of an internal
> symbol.
> 
> These warnings are harmless: the alias attribute still generates
> the correct ELF symbol table entries, and the GNU linker resolves
> indirect alias chains at link time.  Clang warns that attributes
> on the intermediate alias target won't be inherited by the outer
> alias, but that attribute propagation is neither expected nor
> needed here -- the aliasing semantics themselves are unaffected.
> 
> Detect whether CC is clang (cc-clang) and add -Wno-ignored-attributes
> to the warning flags.

I could avoid add this flag by reworking how the weak alias were done
for some routines (check 6b7067460f0ad8eb591735d21c60bcf3b52023df). It
does work for x86_64 and aarch64 for clang 18 through 22 (I haven't
tested 23 yet).

Which ABI are you targeting?

> ---
>  Makeconfig   |  4 ++++
>  configure    | 29 +++++++++++++++++++++++++++++
>  configure.ac |  9 +++++++++
>  3 files changed, 42 insertions(+)
> 
> diff --git ./Makeconfig ./Makeconfig
> index 8fe7217dd7..4c34e59aed 100644
> --- ./Makeconfig
> +++ ./Makeconfig
> @@ -908,6 +908,10 @@ endif
>  ifeq ($(enable-werror),yes)
>  +gccwarn += -Werror
>  endif
> +# Clang warns on the intentional alias redirections (e.g. long_double_symbol).
> +ifeq ($(cc-clang),yes)
> ++gccwarn += -Wno-ignored-attributes
> +endif
>  +gccwarn-c = -Wstrict-prototypes -Wold-style-definition \
>    $(cc-option-wfree-labels) $(cc-option-wmissing-parameter-name)
>  
> diff --git ./configure ./configure
> index 90c4b4f858..fbe8e4ac30 100755
> --- ./configure
> +++ ./configure
> @@ -6031,6 +6031,35 @@ config_vars="$config_vars
>  have-test-clang = $libc_cv_test_clang"
>  
>  
> +cat > conftest.c <<EOF
> +$conftest_code
> +EOF
> +{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether CC is clang" >&5
> +printf %s "checking whether CC is clang... " >&6; }
> +if test ${libc_cv_cc_clang+y}
> +then :
> +  printf %s "(cached) " >&6
> +else case e in #(
> +  e)   if { ac_try='${CC-cc} $CFLAGS $CPPFLAGS -c conftest.c -o conftest 1>&5'
> +  { { eval echo "\"\$as_me\":${as_lineno-$LINENO}: \"$ac_try\""; } >&5
> +  (eval $ac_try) 2>&5
> +  ac_status=$?
> +  printf "%s\n" "$as_me:${as_lineno-$LINENO}: \$? = $ac_status" >&5
> +  test $ac_status = 0; }; }
> +  then
> +    libc_cv_cc_clang=yes
> +  else
> +    libc_cv_cc_clang=no
> +  fi ;;
> +esac
> +fi
> +{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: $libc_cv_cc_clang" >&5
> +printf "%s\n" "$libc_cv_cc_clang" >&6; }
> +rm -f conftest*
> +config_vars="$config_vars
> +cc-clang = $libc_cv_cc_clang"
> +
> +
>  cat > conftest.cc <<EOF
>  $conftest_code
>  EOF
> diff --git ./configure.ac ./configure.ac
> index 2a788c13b7..0d81a6e7a0 100644
> --- ./configure.ac
> +++ ./configure.ac
> @@ -651,6 +651,15 @@ LIBC_TRY_TEST_CC_COMMAND([for clang],
>  )
>  LIBC_CONFIG_VAR([have-test-clang], [$libc_cv_test_clang])
>  
> +dnl Check whether the library itself is built with clang (CC).
> +LIBC_TRY_CC_COMMAND([whether CC is clang],
> +  [$conftest_code],
> +  [-c],
> +  libc_cv_cc_clang,
> +  [libc_cv_cc_clang=yes], [libc_cv_cc_clang=no],
> +)
> +LIBC_CONFIG_VAR([cc-clang], [$libc_cv_cc_clang])
> +
>  dnl Check if clang++ is used to test glibc.
>  LIBC_TRY_TEST_CXX_COMMAND([for clang++],
>    [$conftest_code],