powerpc: Define USE_PPC64_NOTOC iff compiler supports it

The @notoc usage only yields an advantage on ISA 3.1+ machine (power10)
and for ld.bfd also when it sees 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.

Reviewed-by: Fangrui Song <maskray@google.com>
Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>

2 files changed, 43 insertions, 24 deletions

diff --git a/sysdeps/powerpc/powerpc64/configure b/sysdeps/powerpc/powerpc64/configure
index 5ce77af631..fddea0355a 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} $libc_cv_cc_submachine $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..1f3d54414c 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} $libc_cv_cc_submachine $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)])