Eli Schwartz <eschwartz93@gmail.com> writes:

It curently uses some magic test to decide whether handcrafted code
works with or without -latomic. But it can claim that -latomic is not
needed for that case, while it is still needed for other cases.

okay so append-atomic-flags does not work for me in this case
noise-suppression-for-voice is doing `struct RnNoiseStats { uint32_t a, b, c, d; }; std::atomic<RnNoiseStats> m_stats;`
not just a single large integer

It is simplest to always add -latomic when an ebuild gets that deep
feeling that yeah, it would like some atomics please. The downsides to listing a linker library are exactly:

- it might be unavailable
- it might be unneeded

And the former case is trivial to solve -- this function already does so
-- while the latter case has a sanctioned approach that is already used
for other intrinsic compiler libraries, but not for atomic "because the
build system would have a hard time if we had to build atomic early on"
which isn't a very good reason to break ebuilds which aren't building sys-devel/gcc.

As a side benefit, we now handle -latomic such that a package which
requires it, but only for parts of the installed package, does not
overlink to libatomic in *all* binaries/libraries, even if the default LDFLAGS are overridden and the global -Wl,--as-needed disappears.

Bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81358
Bug: https://bugs.gentoo.org/820101
Bug: https://bugs.gentoo.org/925672
Signed-off-by: Eli Schwartz <eschwartz93@gmail.com>

LGTM. Thanks.

---
eclass/flag-o-matic.eclass | 80 +++++++++-----------------------------
1 file changed, 19 insertions(+), 61 deletions(-)

diff --git a/eclass/flag-o-matic.eclass b/eclass/flag-o-matic.eclass
index 5ce7601fdde2..0e5271c7824f 100644
--- a/eclass/flag-o-matic.eclass
+++ b/eclass/flag-o-matic.eclass
@@ -1015,69 +1015,27 @@ test-compile() {
}

# @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.
+# Attempts to detect if appending -latomic works, and does so.
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
- # 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(void)
- {
- 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 "c+ld" "${code}" || return
-
- local bytesizes
- [[ "${#}" == "0" ]] && bytesizes=( "1" "2" "4" "8" ) || bytesizes="${@}"
-
- for bytesize in ${bytesizes[@]}
- do
- # this sample program is informed by the great testing from the buildroot project:
- # https://github.com/buildroot/buildroot/commit/6856e417da4f3aa77e2a814db2a89429af072f7d
- read -r -d '' code <<- EOF
- #include <stdint.h>
- int main(void)
- {
- uint$((${bytesize} * 8))_t a = 0;
- __atomic_add_fetch(&a, 3, __ATOMIC_RELAXED);
- __atomic_compare_exchange_n(&a, &a, 2, 1, __ATOMIC_RELAXED, __ATOMIC_RELAXED);
- return 0;
- }
- EOF
-
- # do nothing if test program links fine
- test-compile "c+ld" "${code}" && continue
-
- # ensure that the toolchain supports -latomic
- test-flags-CCLD "-latomic" &>/dev/null || die "-latomic is required but not supported by $(tc-getCC)"
-
- append-libs "-latomic"
-
- # verify that this did indeed fix the problem
- test-compile "c+ld" "${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
+ # Make sure that the flag is actually valid. If it isn't, then maybe the
+ # library both doesn't exist and is redundant, or maybe the toolchain is
+ # broken, but let the build succeed or fail on its own.
+ test-flags-CCLD "-latomic" &>/dev/null || return
+
+ # We unconditionally append this flag. In the case that it's needed, the
+ # flag is, well, needed. In the case that it's not needed, it causes no + # harm, because we ensure that this specific library is definitely
+ # certainly linked with as-needed.
+ #
+ # Really, this should be implemented directly in the compiler, including
+ # the use of push/pop for as-needed. It's exactly what the gcc spec file
+ # does for e.g. -lgcc_s, but gcc is concerned about doing so due to build
+ # system internals and as a result all users have to deal with this mess
+ # instead.
+ #
+ # See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81358
+ append-libs "-Wl,--push-state,--as-needed,-latomic,--pop-state"
}

fi

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