gccNGPackages: Rework threading model patch based on feedback on IRC (#424918)

From 1eaf7ce77bb4eb73e5565ede220557c2ef0290b0 Mon Sep 17 00:00:00 2001

From 46280d27a5c96b2efa7a61f49ee8dc14f7e10d0c Mon Sep 17 00:00:00 2001

From: John Ericson <git@JohnEricson.me>

Date: Sun, 22 Aug 2021 01:14:22 -0400

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

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

instead.

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

twice.

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

worth.

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

already.

[1]: https://github.com/richfelker/musl-cross-make

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

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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              | 32 ++++++++++++++++++++++++++++++++

 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, 38 insertions(+), 12 deletions(-)

 5 files changed, 29 insertions(+), 12 deletions(-)

diff --git a/config/gthr.m4 b/config/gthr.m4

index 11996247f15..7aed5d3df0d 100644

index 11996247f15..e8fac4a5721 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

@@ -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 usage: GCC_AC_THREAD_MODEL

+AC_DEFUN([GCC_AC_THREAD_MODEL],

+[

+# With threads

+# 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_ARG_WITH(threads,

+[AS_HELP_STRING([[--with-threads=MODEL]],

+		[specify thread model for this GCC

+		 runtime library])],,

+[with_threads=''])

+

+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" = x''; 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])

+else

+    target_thread_file=$with_threads

+fi

+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 +52,9 @@ case $1 in

@@ -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_SUBST(thread_header)

 ])

diff --git a/libatomic/configure.ac b/libatomic/configure.ac

index 85824fa7614..691f4948f6c 100644

index aafae71028d..a1aa3bdf69f 100644

--- a/libatomic/configure.ac

+++ b/libatomic/configure.ac

@@ -162,9 +162,7 @@ libtool_VERSION=3:0:2

 case "$target" in

  *aarch64*)

diff --git a/libgcc/configure.ac b/libgcc/configure.ac

index 4e8c036990f..acf54e24c79 100644

index 85e4f1bc48b..d44493f9653 100644

--- a/libgcc/configure.ac

+++ b/libgcc/configure.ac

@@ -305,9 +305,7 @@ AC_SUBST([use_tm_clone_registry])

 # 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 15cde3b04b8..28bd4d3d2bd 100644

index ef8ca434407..7bb91362dbe 100644

--- a/libphobos/m4/druntime/os.m4

+++ b/libphobos/m4/druntime/os.m4

@@ -32,7 +32,7 @@ case $1 in

 AC_SUBST(DCFG_THREAD_MODEL)

 ])

diff --git a/libstdc++-v3/acinclude.m4 b/libstdc++-v3/acinclude.m4

index 51a08bcc8b1..71697f59d6d 100644

index a0094c2dd95..66fc3abe4fd 100644

--- a/libstdc++-v3/acinclude.m4

+++ b/libstdc++-v3/acinclude.m4

@@ -4342,9 +4342,7 @@ dnl Substs:

@@ -4345,9 +4345,7 @@ dnl Substs:

 dnl  thread_header

 dnl

 AC_DEFUN([GLIBCXX_ENABLE_THREADS], [

   GCC_AC_THREAD_HEADER([$target_thread_file])

 ])

@@ -4354,7 +4352,8 @@ dnl Check if gthread implementation defines the types and functions

@@ -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

 AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [

   GLIBCXX_ENABLE(libstdcxx-threads,auto,,[enable C++11 threads support])

@@ -4369,7 +4368,6 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [

@@ -4372,7 +4371,6 @@ AC_DEFUN([GLIBCXX_CHECK_GTHREADS], [

   CXXFLAGS="$CXXFLAGS -fno-exceptions \

 	-I${toplevel_srcdir}/libgcc -I${toplevel_builddir}/libgcc"

     posix)

       CXXFLAGS="$CXXFLAGS -DSUPPORTS_WEAK -DGTHREAD_USE_WEAK -D_PTHREADS"

-- 

2.44.1

2.47.2

  ];

  patches = [

    # Submitted:

    #  - https://gcc.gnu.org/pipermail/gcc-patches/2021-August/577639.html

    #  - https://gcc.gnu.org/pipermail/gcc-patches/2021-August/577640.html

    #  - https://gcc.gnu.org/pipermail/gcc-patches/2021-August/577638.html

    #

    # In Git: https://github.com/Ericson2314/gcc/tree/prog-target-15

    (getVersionFile "gcc/0001-find_a_program-First-search-with-machine-prefix.patch")

    (getVersionFile "gcc/0002-driver-for_each_pass-Pass-to-callback-whether-dir-is.patch")

    (getVersionFile "gcc/0003-find_a_program-Only-search-for-prefixed-paths-in-und.patch")

    # Submitted: https://inbox.sourceware.org/gcc-patches/20210818203840.1550133-1-John.Ericson@Obsidian.Systems/T/#t

    # In Git: https://github.com/Ericson2314/gcc/tree/libgcc-custom-threading-model-15

    (getVersionFile "gcc/custom-threading-model.patch")

    # TODO: fix up and send to upstream

    (getVersionFile "gcc/fix-collect2-paths.diff")

  ];

  configureFlags = [

    "--disable-dependency-tracking"

    "--with-threads=single"

    "gcc_cv_target_thread_file=single"

    # $CC cannot link binaries, let alone run then

    "cross_compiling=true"

    # Do not have dynamic linker without libc

  configureFlags = [

    "--disable-dependency-tracking"

    "--with-threads=single"

    "gcc_cv_target_thread_file=single"

    # $CC cannot link binaries, let alone run then

    "cross_compiling=true"

  ];

  configureFlags = [

    "--disable-dependency-tracking"

    "--with-toolexeclibdir=${builtins.placeholder "out"}/lib"

    "--with-threads=posix"

    "gcc_cv_target_thread_file=posix"

    "cross_compiling=true"

    "--disable-multilib"