meson: disable libatomic with GCC >= 16

Historically it was required to ask for libatomic explicitly with
-latomic, but with GCC >= 16 apps will get linked to libatomic
whether they ask for it or not.

This invalidates QEMU's check for atomic op support for int128
which explicitly does NOT want to use the libatomic impl. As a
result with GCC >= 16, QEMU is now getting linked to libatomic
and is activating CONFIG_ATOMIC128. This in turn exposes a bug
in GCC's libatomic.a static buld which is incompatible with the
use of -static-pie leading to build failures like:

    /usr/bin/ld: /usr/lib/gcc/x86_64-redhat-linux/16/libatomic.a(cas_16_.o): relocation R_X86_64_32 against hidden symbol `libat_compare_exchange_16_i1' can not be used when making a PIE object
    /usr/bin/ld: failed to set dynamic section sizes: bad value
collect2: error: ld returned 1 exit status

The newly introduced -fno-link-libatomic flag can be used to
disable the new automatic linking of libatomic. Setting this in
qemu_isa_flags early on ensures that the check for CONFIG_ATOMIC128
still works correctly.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20260108141407.2151817-1-berrange@redhat.com>

diff --git a/meson.build b/meson.build
index db87358d62d34607d41b28e4ff3912f1915afa0a..56df08c10eda63a4ed0bd6816a8cde96f85db73d 100644 (file)
--- a/meson.build
+++ b/meson.build
@@ -445,6 +445,15 @@ if host_arch in ['i386', 'x86_64']
   endif
 endif
 
+# GCC >= 16 automatically tries to link libatomic for all programs.
+#
+# QEMU explicitly does NOT want to use libatomic for int128 types.
+#
+# Later checks assume we won't get atomic ops for int128 without
+# explicitly asking for -latomic, so we must disable GCC's new
+# automatic linking with the new -fno-link-libatomic flag
+qemu_isa_flags += cc.get_supported_arguments('-fno-link-libatomic')
+
 qemu_common_flags = qemu_isa_flags + qemu_common_flags
 
 if get_option('prefer_static')