[PATCH] Factor out thread model detection with `GCC_AC_THREAD_MODEL`

[John Ericson]

From: John Ericson <git@JohnEricson.me>

This macro deduplicates the

    $CC -v 2>&1 | sed -n 's/^Thread model: //p'

check that was occurring in various runtime libs.

Additionally, as a bit of an Easter egg, this also allows overriding
what the compiler would return by setting the
`gcc_cv_target_thread_file` cache variable first. I admit that it is in
fact this Easter egg that led me to write the patch. The use-case for it
is for making multilib builds where the library sets do not all share
the same thread model easier. See also `THREAD_MODEL_SPEC` for more
about the varying thread models use-case.

Arguably one could could try to define on `THREAD_MODEL_SPEC` on more
platforms (besides e.g. AIX) but the ramifications of this are a bit
unclear. Setting `gcc_cv_target_thread_file` directly is a "low tech"
solution that will work for now for sure. Of course, since setting a
cache variable like this a hacky trick, I will not expect this to be at
all stable/guaranteed to work, going forward.

Thanks to Arsen who on IRC discussed these things with me, including in
particular making it a cache var not `--with-model` flag, to not
prematurely foster expectations that this is stable.

Suggested-by: Arsen Arsenović <arsen@aarsen.me>
---
 config/gthr.m4              | 23 +++++++++++++++++++++++
 libatomic/configure.ac      |  4 +---
 libgcc/configure.ac         |  4 +---
 libphobos/m4/druntime/os.m4 |  2 +-
 libstdc++-v3/acinclude.m4   |  8 +++-----
 5 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/config/gthr.m4 b/config/gthr.m4
index 11996247f15..e8fac4a5721 100644
--- a/config/gthr.m4
+++ b/config/gthr.m4
@@ -5,6 +5,26 @@ dnl Public License, this file may be distributed as part of a program
 dnl that contains a configuration script generated by Autoconf, under
 dnl the same distribution terms as the rest of that program.
 
+dnl Define thread model
+
+dnl usage: GCC_AC_THREAD_MODEL
+AC_DEFUN([GCC_AC_THREAD_MODEL],
+[
+# Specify the threading model for this GCC runtime library
+# Pass with no value to take from compiler's metadata
+# Pass with a value to specify a thread package
+# 'single' means single threaded -- without threads.
+AC_CACHE_CHECK([for the threading model used by GCC], [gcc_cv_target_thread_file], [
+  # Set new cache variable
+  gcc_cv_target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
+])
+# Set variable name (not prefixed enough to be a good cache variable
+# name) traditionally used for this purpose, to avoid having to change
+# a bunch of configure scripts.
+target_thread_file="$gcc_cv_target_thread_file"
+])
+
+
 dnl Define header location by thread model
 
 dnl usage: GCC_AC_THREAD_HEADER([thread_model])
@@ -23,6 +43,9 @@ case $1 in
     vxworks)	thread_header=config/gthr-vxworks.h ;;
     win32)	thread_header=config/i386/gthr-win32.h ;;
     mcf)	thread_header=config/i386/gthr-mcf.h ;;
+    *)
+        AC_MSG_ERROR([No known header for threading model '$1'.])
+        ;;
 esac
 AC_SUBST(thread_header)
 ])
diff --git a/libatomic/configure.ac b/libatomic/configure.ac
index aafae71028d..a1aa3bdf69f 100644
--- a/libatomic/configure.ac
+++ b/libatomic/configure.ac
@@ -162,9 +162,7 @@ libtool_VERSION=3:0:2
 AC_SUBST(libtool_VERSION)
 
 # Check for used threading-model
-AC_MSG_CHECKING([for thread model used by GCC])
-target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
-AC_MSG_RESULT([$target_thread_file])
+GCC_AC_THREAD_MODEL
 
 case "$target" in
  *aarch64*)
diff --git a/libgcc/configure.ac b/libgcc/configure.ac
index 85e4f1bc48b..d44493f9653 100644
--- a/libgcc/configure.ac
+++ b/libgcc/configure.ac
@@ -305,9 +305,7 @@ AC_SUBST([use_tm_clone_registry])
 
 AC_LIB_PROG_LD_GNU
 
-AC_MSG_CHECKING([for thread model used by GCC])
-target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
-AC_MSG_RESULT([$target_thread_file]) 
+GCC_AC_THREAD_MODEL
 
 # Check for assembler CFI support.
 AC_CACHE_CHECK([whether assembler supports CFI directives], [libgcc_cv_cfi],
diff --git a/libphobos/m4/druntime/os.m4 b/libphobos/m4/druntime/os.m4
index ef8ca434407..7bb91362dbe 100644
--- a/libphobos/m4/druntime/os.m4
+++ b/libphobos/m4/druntime/os.m4
@@ -32,7 +32,7 @@ case $1 in
     # TODO: These targets need porting.
     dce|mipssde|rtems|tpf|vxworks)
 	    DCFG_THREAD_MODEL="Single" ;;
-    *)	    as_fn_error "Thread implementation '$1' not recognised" "$LINENO" 5 ;;
+    *)	    AC_MSG_ERROR([Thread implementation '$1' not recognised]) ;;
 esac
 AC_SUBST(DCFG_THREAD_MODEL)
 ])
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index a0094c2dd95..66fc3abe4fd 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -4345,9 +4345,7 @@ dnl Substs:
 dnl  thread_header
 dnl
 AC_DEFUN([GLIBCXX_ENABLE_THREADS], [
-  AC_MSG_CHECKING([for thread model used by GCC])
-  target_thread_file=`$CXX -v 2>&1 | sed -n 's/^Thread model: //p'`
-  AC_MSG_RESULT([$target_thread_file])
+  GCC_AC_THREAD_MODEL
   GCC_AC_THREAD_HEADER([$target_thread_file])
 ])
 
@@ -4357,7 +4355,8 @@ dnl Check if gthread implementation defines the types and functions
 dnl required by the c++0x thread library.  Conforming gthread
 dnl implementations can define __GTHREADS_CXX0X to enable use with c++0x.
 dnl
-dnl GLIBCXX_ENABLE_SYMVERS must be done before this.
+dnl GLIBCXX_ENABLE_SYMVERS and GLIBCXX_ENABLE_THREADS must be done
+dnl before this.
 dnl
 AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [
   GLIBCXX_ENABLE(libstdcxx-threads,auto,,[enable C++11 threads support])
@@ -4372,7 +4371,6 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [
   CXXFLAGS="$CXXFLAGS -fno-exceptions \
 	-I${toplevel_srcdir}/libgcc -I${toplevel_builddir}/libgcc"
 
-  target_thread_file=`$CXX -v 2>&1 | sed -n 's/^Thread model: //p'`
   case $target_thread_file in
     posix)
       CXXFLAGS="$CXXFLAGS -DSUPPORTS_WEAK -DGTHREAD_USE_WEAK -D_PTHREADS"
-- 
2.47.2