[gentoo-dev] [PATCH] flag-o-matic.eclass: implement append-atomic-flags

[Sam James]

From: matoro <matoro@××××××××××××××××××××.com>

My take on implementing bug 820101 as conservatively as possible. This
will append -latomic only when absolutely necessary (even though it's
probably not required to be that conservative due to our use of
--as-needed).

This will take flags into account when testing as well. For example, if
you compile with -march=i386, this would require linking libatomic even
if the same toolchain with a higher -march level would not require it.
So rebuilding the same package with the same flags may change whether
-latomic is linked at build time if the CFLAGS change. Another instance
might be switching from GCC to clang - the former requires explicitly
linking -latomic, while the latter does not even HAVE libatomic (see bug
820095) as all atomic intrinsics are built-in internally. This function
would safely detect this and not append -latomic.

There is an optional parameter [bytes]. You can use this if you want to
be specific about what size atomic support is required. For example,
there are several platforms like MIPS where the 32-bit version has 1-,
2-, and 4-byte atomics builtin but requires libatomic linkage for 8-byte
atomics. If your program only requires, say, 4-byte atomics, you can use
append-atomic-flags 4 and this will then not attempt to link libatomic
on 32-bit MIPS.

I tested using this to solve bug 688574 on 32-bit SPARC.

Closes: https://bugs.gentoo.org/820101
Signed-off-by: matoro <matoro@××××××××××××××××××××.com>
Closes: https://github.com/gentoo/gentoo/pull/26334
Signed-off-by: Sam James <sam@g.o>
---
 eclass/flag-o-matic.eclass | 145 +++++++++++++++++++++++++++++++++++++
 1 file changed, 145 insertions(+)

diff --git a/eclass/flag-o-matic.eclass b/eclass/flag-o-matic.eclass
index 0dd2c1191273..503f23a141a1 100644
--- a/eclass/flag-o-matic.eclass
+++ b/eclass/flag-o-matic.eclass
@@ -875,4 +875,149 @@ no-as-needed() {
 	esac
 }
 
+# @FUNCTION: _test-compile-PROG
+# @USAGE: <language> <code>
+# @INTERNAL
+# @DESCRIPTION:
+# Attempts to compile (and possibly link) the given program.  The first
+# <language> parameter corresponds to the standard -x compiler argument.
+# If the program should additionally be attempted to be linked, the string
+# "+ld" should be added to the <language> parameter.
+_test-compile-PROG() {
+	local lang=$1
+	local code=$2
+	shift 2
+
+	[[ -z "${lang}" ]] && return 1
+	[[ -z "${code}" ]] && return 1
+
+	local compiler filename_in filename_out args=() libs=()
+	case "${lang}" in
+		c)
+			compiler="$(tc-getCC)"
+			filename_in="${T}/test.c"
+			filename_out="${T}/test.o"
+			args+=(${CFLAGS[@]} -xc -c)
+			;;
+		c++)
+			compiler="$(tc-getCXX)"
+			filename_in="${T}/test.cc"
+			filename_out="${T}/test.o"
+			args+=(${CXXFLAGS[@]} -xc++ -c)
+			;;
+		f77)
+			compiler="$(tc-getF77)"
+			filename_in="${T}/test.f"
+			filename_out="${T}/test.o"
+			args+=(${FFFLAGS[@]} -xf77 -c)
+			;;
+		f95)
+			compiler="$(tc-getFC)"
+			filename_in="${T}/test.f90"
+			filename_out="${T}/test.o"
+			args+=(${FCFLAGS[@]} -xf95 -c)
+			;;
+		c+ld)
+			compiler="$(tc-getCC)"
+			filename_in="${T}/test.c"
+			filename_out="${T}/test.exe"
+			args+=(${CFLAGS[@]} ${LDFLAGS[@]} -xc)
+			libs+=(${LIBS[@]})
+			;;
+		c+++ld)
+			compiler="$(tc-getCXX)"
+			filename_in="${T}/test.cc"
+			filename_out="${T}/test.exe"
+			args+=(${CXXFLAGS[@]} ${LDFLAGS[@]} -xc++)
+			libs+=(${LIBS[@]})
+			;;
+		f77+ld)
+			compiler="$(tc-getF77)"
+			filename_in="${T}/test.f"
+			filename_out="${T}/test.exe"
+			args+=(${FFLAGS[@]} ${LDFLAGS[@]} -xf77)
+			libs+=(${LIBS[@]})
+			;;
+		f95+ld)
+			compiler="$(tc-getFC)"
+			filename_in="${T}/test.f90"
+			filename_out="${T}/test.exe"
+			args+=(${FCFLAGS[@]} ${LDFLAGS[@]} -xf95)
+			libs+=(${LIBS[@]})
+			;;
+		*)
+			die "Unknown compiled language ${lang}"
+			;;
+	esac
+
+	printf "%s\n" "${code}" > "${filename_in}" || die "Failed to create '${test_in}'"
+
+	"${compiler}" ${args[@]} "${filename_in}" -o "${filename_out}" ${libs[@]} &>/dev/null
+}
+
+# @FUNCTION: append-atomic-flags
+# @USAGE: [bytes]
+# @DESCRIPTION: Attempts to detect if appending -latomic is required to use
+# a specific-sized atomic intrinsic, and if so, appends it.  If the bytesize
+# is not specified, then check the four most common byte sizes (1, 2, 4, 8).
+# >=16-byte atomics are not included in this default set and must be explicitly
+# passed if required.  This may require you to add a macro definition like
+# -Duint128_t=__uint128_t to your CFLAGS.
+append-atomic-flags() {
+	# this implementation is as described in bug #820101
+	local code
+
+	# first, ensure we can compile a trivial program
+	# this is because we can't distinguish if _test-compile-PROG
+	# fails because -latomic is actually needed or if we have a
+	# broken toolchain (like due to bad FLAGS)
+	read -r -d '' code <<- EOF
+		int main()
+		{
+			return 0;
+		}
+	EOF
+
+	# if toolchain is broken, just return silently.  it's better to
+	# let other pieces of the build fail later down the line than to
+	# make people think that something to do with atomic support is the
+	# cause of their problems.
+	_test-compile-PROG "c+ld" "${code}" || return
+
+	local bytesizes
+	[[ "${#}" == "0" ]] && bytesizes=( "1" "2" "4" "8" ) || bytesizes="${@}"
+
+	for bytesize in ${bytesizes[@]}
+	do
+		# this sample program is informed by the great testing from the buildroot project:
+		# https://github.com/buildroot/buildroot/commit/6856e417da4f3aa77e2a814db2a89429af072f7d
+		read -r -d '' code <<- EOF
+			#include <stdint.h>
+			int main()
+			{
+				uint$((${bytesize} * 8))_t a = 0;
+				__atomic_add_fetch(&a, 3, __ATOMIC_RELAXED);
+				__atomic_compare_exchange_n(&a, &a, 2, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED);
+				return 0;
+			}
+		EOF
+
+		# do nothing if test program links fine
+		_test-compile-PROG "c+ld" "${code}" && continue
+
+		# ensure that the toolchain supports -latomic
+		test-flags-CCLD "-latomic" &>/dev/null || die "-latomic is required but not supported by $(tc-getCC)"
+
+		append-libs "-latomic"
+
+		# verify that this did indeed fix the problem
+		_test-compile-PROG "c+ld" "${code}" || \
+			die "libatomic does not include an implementation of ${bytesize}-byte atomics for this toolchain"
+
+		# if any of the required bytesizes require -latomic, no need to continue
+		# checking the others
+		return
+	done
+}
+
 fi
-- 
2.37.1