[PATCH] Allow explicitly specifying the thread model for runtime libs

John Ericson John.Ericson@Obsidian.Systems
Wed Aug 18 20:38:40 GMT 2021

From: John Ericson <git@JohnEricson.me>

Previously, they always scraped the thread mode from `$CC -v', now, that
is the default but one may pass `--with-threads=MODEL` to be explicit

One use-case is bootstraping with a shorter critical path. The
traditionally route was to build an entire "static stage" GCC, build
libc, and then build GCC again supporting dynamic linking,
multithreading, etc. But this is wasteful in that GCC itself is built

With this change, rather than having to mess with spec files we can just
configure the runtime libraries the way we want directly. In turn, that
opens to just building libgcc twice rather than all of GCC.

Frankly, specs were always a rather indirect approach to coordinate this
during GCC's bootstrap, since GCC itself really doesn't care what the
threading model is, just that the runtime libraries agree among
themselves. Relying on a hard-coded spec for this also keeps us one step
further from the long-term goal of multi-target GCC, for what it's

For the record, "single stage" builds of GCC have some precedent in
downstream packaging, for example with [1]. That one, as far as I can
tell, builds libgcc once, but with the headers of the libc
implementation provided and then cyclic linking down later. They are
both fine approaches, but I don't think one should have to be forced
into cyclic dependencies if they don't want to. That opens the door to
non-terminating programs due to, e.g., atomics used in a threads
implementation being lowered to threads absent hardware support.

Finally, I understand that such custom bootstrapping is not officially
supported. I don't mean to imply it should be --- a lot more cleanup
work to the build system would be necessary before supporting it
wouldn't be a huge additional maintainer burden --- I just hope to add a
reasonable knob for those comfortable with doing unsupported things

[1]: https://github.com/richfelker/musl-cross-make
 config/gthr.m4              | 32 ++++++++++++++++++++++++++++++++
 libatomic/configure.ac      |  4 +---
 libgcc/configure.ac         |  4 +---
 libphobos/m4/druntime/os.m4 |  2 +-
 libstdc++-v3/acinclude.m4   |  8 +++-----
 5 files changed, 38 insertions(+), 12 deletions(-)

diff --git a/config/gthr.m4 b/config/gthr.m4
index 4b937306ad0..c585b618e40 100644
--- a/config/gthr.m4
+++ b/config/gthr.m4
@@ -5,6 +5,35 @@ 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
+# With threads
+# 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.
+		[specify thread model for this GCC
+		 runtime library])],,
+if test x"$with_threads" = x'yes'; then
+    AC_MSG_ERROR([Cannot pass bare --with-threads, must pass explicit --with-threads=MODEL])
+elif test x"$with_threads" = x'no'; then
+    target_thread_file=single
+elif test x"$with_threads" = ''; then
+    AC_MSG_CHECKING([for thread model used by GCC])
+    target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
+    AC_MSG_RESULT([$target_thread_file])
+    target_thread_file=$with_threads
 dnl Define header location by thread model
 dnl usage: GCC_AC_THREAD_HEADER([thread_model])
@@ -22,6 +51,9 @@ case $1 in
     tpf)	thread_header=config/s390/gthr-tpf.h ;;
     vxworks)	thread_header=config/gthr-vxworks.h ;;
     win32)	thread_header=config/i386/gthr-win32.h ;;
+    *)
+        AC_MSG_ERROR([No known header for threading model '$1'.])
+        ;;
diff --git a/libatomic/configure.ac b/libatomic/configure.ac
index 2a371870c2f..42d2016b7a2 100644
--- a/libatomic/configure.ac
+++ b/libatomic/configure.ac
@@ -161,9 +161,7 @@ libtool_VERSION=3:0:2
 # Check for used threading-model
-AC_MSG_CHECKING([for thread model used by GCC])
-target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
 case "$target" in
diff --git a/libgcc/configure.ac b/libgcc/configure.ac
index 13a80b2551b..1209a0986e0 100644
--- a/libgcc/configure.ac
+++ b/libgcc/configure.ac
@@ -298,9 +298,7 @@ AC_SUBST([use_tm_clone_registry])
-AC_MSG_CHECKING([for thread model used by GCC])
-target_thread_file=`$CC -v 2>&1 | sed -n 's/^Thread model: //p'`
 # 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 351558dbcda..2b44fbca8fc 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.
 	    DCFG_THREAD_MODEL="Single" ;;
-    *)	    as_fn_error "Thread implementation '$1' not recognised" "$LINENO" 5 ;;
+    *)	    AC_MSG_ERROR([Thread implementation '$1' not recognised]) ;;
diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4
index 90ecc4a87a2..83204176c05 100644
--- a/libstdc++-v3/acinclude.m4
+++ b/libstdc++-v3/acinclude.m4
@@ -3910,9 +3910,7 @@ dnl Substs:
 dnl  thread_header
-  AC_MSG_CHECKING([for thread model used by GCC])
-  target_thread_file=`$CXX -v 2>&1 | sed -n 's/^Thread model: //p'`
-  AC_MSG_RESULT([$target_thread_file])
@@ -3922,7 +3920,8 @@ dnl Check if gthread implementation defines the types and functions
 dnl required by the c++0x thread library.  Conforming gthread
 dnl implementations can define __GTHREADS_CXX0X to enable use with c++0x.
-dnl GLIBCXX_ENABLE_SYMVERS must be done before this.
+dnl before this.
   GLIBCXX_ENABLE(libstdcxx-threads,auto,,[enable C++11 threads support])
@@ -3937,7 +3936,6 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [
   CXXFLAGS="$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

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