Infer libatomic version info using libstdc++ at configure time.

Author: insertinterestingnamehere

config/qthread_check_128_bit_atomics_fallback.m4

@@ -0,0 +1,65 @@
+AC_DEFUN([QT_NEEDS_128_BIT_ATOMIC_FALLBACK],
+[
+AC_CACHE_CHECK([whether Qthreads may need to use the fallback implementation for 128 bit atomics],
+               [qt_cv_128b_atomic_fallback],
+[
+AC_LANG_PUSH([C++])
+AC_TRY_COMPILE(
+[
+// This file compiles only if clang or similar can safely use 128 bit atomics at runtime even in debug builds.
+// For that to be the case it needs to be using a sufficiently recent libatomic from gcc or it needs
+// to be using compiler_rt for its atomics.
+#if __cplusplus >= 202002L
+#include <version>
+#else
+#include <ciso646>
+#endif
+
+// Only have a fallback implementation of 128 bit atomics for these architectures,
+// so let the others use the standard implementation and hope for the best.
+// On OSX and FreeBSD, compiler-rt is used by default instead of libatomic so we don't need to worry about this.
+#if defined(__aarch64__) || defined(__arm__) || !(defined(__clang__) && (defined(__APPLE__) || defined(__FreeBSD__)))
+// gcc version info as obtained from libstdc++ since it's surprisingly difficult to figure out which gcc clang is using for its support libraries.
+// Note: if no C++ compiler is available, this compilation will fail and the fallback will be used since we couldn't confirm
+// that the underlying libatomic is safe to use.
+// If we're seeing libstdc++ here, that means there's an underlying gcc install also providing libatomic and that libatomic is actually
+// what we're using because libstdc++ depends on it.
+#if !defined(__clang__) && defined(__GNUC__) && __GNUC__ < 7
+#error Too early of a version to have _GLIBCXX_RELEASE, let alone 128 bit atomics.
+#endif
+#if !defined(_LIBCPP_VERSION) && !defined(__GLIBCXX__)
+#error Too early of a libstdc++ version to even include its version info in <ciso646>.
+#endif
+#if defined(__GLIBCXX__)
+#ifdef __amd64__
+#if !(_GLIBCXX_RELEASE == 13 || (_GLIBCXX_RELEASE == 12 && __GLIBCXX__ >= 20230508) || (_GLIBCXX_RELEASE == 11 && __GLIBCXX__ >= 20230529))
+#error Only more recent versions of gcc have lock-free 128 bit atomics.
+#endif
+#elif defined(__aarch64__)
+#if !(_GLIBCXX_RELEASE == 13)
+#error Only more recent versions of gcc have lock-free 128 bit atomics.
+#endif
+#endif
+#endif
+
+// If we're using libc++ we can't get the underlying gcc version from it here so we can't guarantee that
+// 128 bit atomics will actually be lock-free. We only have clang use the fallback implementation for
+// 128 bit atomics in debug builds anyway and it's extremely unusual to build qthreads with libc++,
+// so this case is not super important.
+// It's also technically possible for libc++ and clang to just get their atomics from
+// compiler-rt instead, but again we don't have an easy way to check that here.
+#if defined(_LIBCPP_VERSION)
+#error can't guarantee lock-free 128 bit atomics without gcc version info.
+#endif
+
+#endif
+],
+[return 0;],
+[qt_cv_128b_atomic_fallback=no],
+[qt_cv_128b_atomic_fallback=yes])
+AC_LANG_POP([C++])
+])
+AS_IF([test x$qt_cv_128b_atomic_fallback = xyes],
+       [AC_DEFINE([QTHREADS_NEEDS_128_BIT_ATOMIC_FALLBACK], [1],
+       [Whether a fallback is needed because the underlying libatomic may not provide lock-free 128 bit atomics])])
+])

configure.ac

@@ -588,6 +588,8 @@ AS_IF([test "x$enable_eurekas" = "xyes"],
 
 AC_CACHE_SAVE
 
+QT_NEEDS_128_BIT_ATOMIC_FALLBACK
+
 ## ----------------------- ##
 ## Checks for header files ##
 ## ----------------------- ##

include/qt_128b_atomics.h

@@ -25,13 +25,6 @@
 // and pipe it through to here via our own preprocessor define.
 // icc exactly mimics gcc in this case, and icx and acfl behave the same as clang but do
 // not require their own detection logic here.
-#if defined(__clang__)
-#define QTHREADS_GCC_LIB_MAJOR_VERSION QTHREADS_CLANG_UNDERLYING_GCC_MAJOR_VERSION
-#define QTHREADS_GCC_LIB_MINOR_VERSION QTHREADS_CLANG_UNDERLYING_GCC_MINOR_VERSION
-#else
-#define QTHREADS_GCC_LIB_MAJOR_VERSION __GNUC__
-#define QTHREADS_GCC_LIB_MINOR_VERSION __GNUC_MINOR__
-#endif
 #ifdef __x86_64__
 #ifdef __AVX__
 // Intel and AMD both specify that 128 bit loads and stores are atomic (with reasonable alignment constraints)
@@ -52,7 +45,7 @@
 // This all works assuming that qthreads is never compiled with a newer libatomic than is available at runtime.
 #if defined(__clang__) && defined(__OPTIMIZE__)
 #define QTHREADS_USE_STANDARD_128_BIT_ATOMICS
-#elif QTHREADS_GCC_LIB_MAJOR_VERSION >= 13 || (QTHREADS_GCC_LIB_MAJOR_VERSION == 12 && QTHREADS_GCC_LIB_MINOR_VERSION >= 3) || (QTHREADS_GCC_LIB_MAJOR_VERSION == 11 && QTHREADS_GCC_LIB_MINOR_VERSION >= 4)
+#elif __GNUC__ >= 13 || (__GNUC__ == 12 && __GNUC_MINOR__ >= 3) || (__GNUC__ == 11 && __GNUC_MINOR__ >= 4)
 #define QTHREADS_USE_STANDARD_128_BIT_ATOMICS
 #endif
 #endif // #ifdef __AVX__
@@ -61,12 +54,12 @@
 #if __ARM_ARCH < 8
 #error "Qthreads is not compatible with arm versions earlier than 8."
 #endif
-#if defined(__clang) && defined(__OPTIMIZE__)
+#if defined(__clang) && (defined(__OPTIMIZE__) || !defined(QTHREADS_NEEDS_128_BIT_ATOMIC_FALLBACK))
 // clang inlines the 128 bit atomic loads on arm as long as optimizations are on, but falls back to
 // the gcc libatomic implementation (which isn't always equivalent) when optimizations aren't on.
 // Again, this all works assuming that qthreads is never compiled with a newer libatomic than is available at runtime.
 #define QTHREADS_USE_STANDARD_128_BIT_ATOMICS
-#elif QTHREADS_GCC_LIB_MAJOR_VERSION >= 13
+#elif __GNUC__ >= 13
 #if __ARM_ARCH > 8 && __ARM_ARCH != 801 && __ARM_ARCH != 802 && __ARM_ARCH != 803
 // gcc only provides lock-free 128 bit atomics in libatomic for armv8.4 and later.
 // We want arm 8.4 or later, but there's inconsistency with how to detect arm versions.

src/Makefile.am

@@ -3,12 +3,7 @@
 # Copyright (c)      2008  Sandia Corporation
 #
 
-# Get the version of gcc that clang (or one of its derivatives) is using for its supporting libraries.
-# This will just be empty for gcc and icc.
-clang_underlying_gcc_major_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\1/g'
-clang_underlying_gcc_minor_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\2/g'
-
-AM_CPPFLAGS = -I$(top_srcdir)/include -DQTHREADS_CLANG_UNDERLYING_GCC_MAJOR_VERSION=$(clang_underlying_gcc_major_version) -DQTHREADS_CLANG_UNDERLYING_GCC_MINOR_VERSION=$(clang_underlying_gcc_minor_version)
+AM_CPPFLAGS = -I$(top_srcdir)/include
 AM_CCASFLAGS = -I$(top_srcdir)/include -I$(top_builddir)/include -DHAVE_CONFIG_H
 
 lib_LTLIBRARIES = libqthread.la

test/Makefile.am

@@ -37,12 +37,7 @@ buildall: buildtests buildextra
 
 noinst_HEADERS = argparsing.h
 
-# Get the version of gcc that clang (or one of its derivatives) is using for its supporting libraries.
-# This will just be empty for gcc and icc.
-clang_underlying_gcc_major_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\1/g'
-clang_underlying_gcc_minor_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\2/g'
-
-AM_CPPFLAGS = -I$(top_srcdir)/include -DQTHREADS_CLANG_UNDERLYING_GCC_MAJOR_VERSION=$(clang_underlying_gcc_major_version) -DQTHREADS_CLANG_UNDERLYING_GCC_MINOR_VERSION=$(clang_underlying_gcc_minor_version)
+AM_CPPFLAGS = -I$(top_srcdir)/include
 outputdir = $(top_builddir)/src
 qthreadlib = $(outputdir)/libqthread.la
 

test/basics/Makefile.am

@@ -59,12 +59,7 @@ if WANT_SINGLE_WORKER_SCHEDULER
 TESTS_ENVIRONMENT += env QT_NUM_SHEPHERDS=2 QT_NUM_WORKERS_PER_SHEPHERD=1
 endif
 
-# Get the version of gcc that clang (or one of its derivatives) is using for its supporting libraries.
-# This will just be empty for gcc and icc.
-clang_underlying_gcc_major_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\1/g'
-clang_underlying_gcc_minor_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\2/g'
-
-AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/test/ -DQTHREADS_CLANG_UNDERLYING_GCC_MAJOR_VERSION=$(clang_underlying_gcc_major_version) -DQTHREADS_CLANG_UNDERLYING_GCC_MINOR_VERSION=$(clang_underlying_gcc_minor_version)
+AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/test/
 qthreadlib = $(top_builddir)/src/libqthread.la
 
 buildall: $(TESTS)

test/features/Makefile.am

@@ -54,12 +54,7 @@ if WANT_SINGLE_WORKER_SCHEDULER
 TESTS_ENVIRONMENT += env QT_NUM_SHEPHERDS=2 QT_NUM_WORKERS_PER_SHEPHERD=1
 endif
 
-# Get the version of gcc that clang (or one of its derivatives) is using for its supporting libraries.
-# This will just be empty for gcc and icc.
-clang_underlying_gcc_major_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\1/g'
-clang_underlying_gcc_minor_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\2/g'
-
-AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/test/ -DQTHREADS_CLANG_UNDERLYING_GCC_MAJOR_VERSION=$(clang_underlying_gcc_major_version) -DQTHREADS_CLANG_UNDERLYING_GCC_MINOR_VERSION=$(clang_underlying_gcc_minor_version)
+AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/test/
 qthreadlib = $(top_builddir)/src/libqthread.la
 
 buildall: $(TESTS)

test/stress/Makefile.am

@@ -32,12 +32,7 @@ if WANT_SINGLE_WORKER_SCHEDULER
 TESTS_ENVIRONMENT += env QT_NUM_SHEPHERDS=2 QT_NUM_WORKERS_PER_SHEPHERD=1
 endif
 
-# Get the version of gcc that clang (or one of its derivatives) is using for its supporting libraries.
-# This will just be empty for gcc and icc.
-clang_underlying_gcc_major_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\1/g'
-clang_underlying_gcc_minor_version !=  clang -v 2>&1 | sed '/Selected GCC installation/!d;s/^.*\/\([0-9]*\)\.\([0-9]*\)\.\([0-9]*\)$$/\2/g'
-
-AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/test/ -DQTHREADS_CLANG_UNDERLYING_GCC_MAJOR_VERSION=$(clang_underlying_gcc_major_version) -DQTHREADS_CLANG_UNDERLYING_GCC_MINOR_VERSION=$(clang_underlying_gcc_minor_version)
+AM_CPPFLAGS = -I$(top_srcdir)/include -I$(top_srcdir)/test/
 qthreadlib = $(top_builddir)/src/libqthread.la
 utils_rnglib = $(top_builddir)/test/utils/rng/librng.la