bpo-28015: Support LTO build with clang (GH-9908)

.o generated by clang in LTO mode actually are LLVM bitcode files, which
leads to a few errors during configure/build step:

- add lto flags to the BASECFLAGS instead of CFLAGS, as CFLAGS are used
  to build autoconf test case, and some are not compatible with clang LTO
  (they assume binary in the .o, not bitcode)
- force llvm-ar instead of ar, as ar is not aware of .o files generated
  by clang -flto

1 files changed, 48 insertions, 19 deletions

diff --git a/configure.ac b/configure.ac
index f1b47db..d1502df 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1265,6 +1265,26 @@ else
   DEF_MAKE_RULE="all"
 fi
 
+# Make llvm-relatec checks work on systems where llvm tools are not installed with their
+# normal names in the default $PATH (ie: Ubuntu).  They exist under the
+# non-suffixed name in their versioned llvm directory.
+
+llvm_bin_dir=''
+llvm_path="${PATH}"
+if test "${CC}" = "clang"
+then
+  clang_bin=`which clang`
+  # Some systems install clang elsewhere as a symlink to the real path
+  # which is where the related llvm tools are located.
+  if test -L "${clang_bin}"
+  then
+    clang_dir=`dirname "${clang_bin}"`
+    clang_bin=`readlink "${clang_bin}"`
+    llvm_bin_dir="${clang_dir}/"`dirname "${clang_bin}"`
+    llvm_path="${llvm_path}${PATH_SEPARATOR}${llvm_bin_dir}"
+  fi
+fi
+
 # Enable LTO flags
 AC_MSG_CHECKING(for --with-lto)
 AC_ARG_WITH(lto, AS_HELP_STRING([--with-lto], [Enable Link Time Optimization in any build. Disabled by default.]),
@@ -1281,6 +1301,33 @@ fi],
 if test "$Py_LTO" = 'true' ; then
   case $CC in
     *clang*)
+      AC_SUBST(LLVM_AR)
+      AC_PATH_TARGET_TOOL(LLVM_AR, llvm-ar, '', ${llvm_path})
+      AC_SUBST(LLVM_AR_FOUND)
+      if test -n "${LLVM_AR}" -a -x "${LLVM_AR}"
+      then
+        LLVM_AR_FOUND="found"
+      else
+        LLVM_AR_FOUND="not-found"
+      fi
+      if test "$ac_sys_system" = "Darwin" -a "${LLVM_AR_FOUND}" = "not-found"
+      then
+        found_llvm_ar=`/usr/bin/xcrun -find llvm-ar 2>/dev/null`
+        if test -n "${found_llvm_ar}"
+        then
+          LLVM_AR='/usr/bin/xcrun llvm-ar'
+          LLVM_AR_FOUND=found
+          AC_MSG_NOTICE([llvm-ar found via xcrun: ${LLVM_AR}])
+        fi
+      fi
+      if test $LLVM_AR_FOUND = not-found
+      then
+        LLVM_PROFR_ERR=yes
+        AC_MSG_ERROR([llvm-ar is required for a --with-lto build with clang but could not be found.])
+      else
+        LLVM_AR_ERR=no
+      fi
+      AR="${LLVM_AR}"
       case $ac_sys_system in
         Darwin*)
           # Any changes made here should be reflected in the GCC+Darwin case below
@@ -1310,7 +1357,7 @@ if test "$Py_LTO" = 'true' ; then
       LTOFLAGS="$LTOFLAGS -g"
   fi
 
-  CFLAGS="$CFLAGS $LTOFLAGS"
+  BASECFLAGS="$BASECFLAGS $LTOFLAGS"
   LDFLAGS="$LDFLAGS $LTOFLAGS"
 fi
 
@@ -1320,24 +1367,6 @@ AC_SUBST(PGO_PROF_USE_FLAG)
 AC_SUBST(LLVM_PROF_MERGER)
 AC_SUBST(LLVM_PROF_FILE)
 AC_SUBST(LLVM_PROF_ERR)
-# Make this work on systems where llvm tools are not installed with their
-# normal names in the default $PATH (ie: Ubuntu).  They exist under the
-# non-suffixed name in their versioned llvm directory.
-llvm_bin_dir=''
-llvm_path="${PATH}"
-if test "${CC}" = "clang"
-then
-  clang_bin=`which clang`
-  # Some systems install clang elsewhere as a symlink to the real path
-  # which is where the related llvm tools are located.
-  if test -L "${clang_bin}"
-  then
-    clang_dir=`dirname "${clang_bin}"`
-    clang_bin=`readlink "${clang_bin}"`
-    llvm_bin_dir="${clang_dir}/"`dirname "${clang_bin}"`
-    llvm_path="${llvm_path}${PATH_SEPARATOR}${llvm_bin_dir}"
-  fi
-fi
 AC_SUBST(LLVM_PROFDATA)
 AC_PATH_TARGET_TOOL(LLVM_PROFDATA, llvm-profdata, '', ${llvm_path})
 AC_SUBST(LLVM_PROF_FOUND)