Building Skia inside of mozilla-central with GCC 4.9.4 causes problems:
[...]c++/4.9.4/bits/atomic_base.h:581:70: error: failure memory model cannot be stronger than success memory model for '__atomic_compare_exchange'
The error stack accompanying this message points at SkEventTracer::GetInstance:
SkEventTracer* SkEventTracer::GetInstance() {
if (SkEventTracer* tracer = sk_atomic_load(&gUserTracer, sk_memory_order_acquire)) {
return tracer;
}
static SkOnce once;
static SkDefaultEventTracer* defaultTracer;
once([] { defaultTracer = new SkDefaultEventTracer; });
return defaultTracer;
}
The only place that compare_exchange_strong could be called here is from SkOnce::operator():
template <typename Fn, typename... Args>
void operator()(Fn&& fn, Args&&... args) {
auto state = fState.load(std::memory_order_acquire);
if (state == Done) {
return;
}
// If it looks like no one has started calling fn(), try to claim that job.
if (state == NotStarted && fState.compare_exchange_strong(state, Claimed,
std::memory_order_relaxed)) {
// Great! We'll run fn() then notify the other threads by releasing Done into fState.
fn(std::forward<Args>(args)...);
return fState.store(Done, std::memory_order_release);
}
[...code elided...]
where |fState| is an atomic<uint8_t>.
The three-argument form of atomic<uint8_t>::compare_exchange_strong is defined as:
_GLIBCXX_ALWAYS_INLINE bool
compare_exchange_strong(__int_type& __i1, __int_type __i2,
memory_order __m = memory_order_seq_cst) noexcept
{
return compare_exchange_strong(__i1, __i2, __m,
__cmpexch_failure_order(__m));
}
__cmpexch_failure_order relaxes the given memory_order:
// Drop release ordering as per [atomics.types.operations.req]/21
constexpr memory_order
__cmpexch_failure_order2(memory_order __m) noexcept
{
return __m == memory_order_acq_rel ? memory_order_acquire
: __m == memory_order_release ? memory_order_relaxed : __m;
}
constexpr memory_order
__cmpexch_failure_order(memory_order __m) noexcept
{
return memory_order(__cmpexch_failure_order2(__m & __memory_order_mask)
| (__m & __memory_order_modifier_mask));
}
which then gets us to the four-argument version of compare_exchange_strong:
_GLIBCXX_ALWAYS_INLINE bool
compare_exchange_strong(__int_type& __i1, __int_type __i2,
memory_order __m1, memory_order __m2) noexcept
{
memory_order __b2 = __m2 & __memory_order_mask;
memory_order __b1 = __m1 & __memory_order_mask;
__glibcxx_assert(__b2 != memory_order_release);
__glibcxx_assert(__b2 != memory_order_acq_rel);
__glibcxx_assert(__b2 <= __b1);
return __atomic_compare_exchange_n(&_M_i, &__i1, __i2, 0, __m1, __m2);
}
Despite the constexpr annotation on __cmpexch_failure_order and friends,
which ought to imply that they get constant-folded, I think what is
happening is that GCC doesn't see |memory_order_relaxed| when it
examines __m2. Instead, it seems some internal tree representation for
the call to __cmpexch_failure_order. Since this is not an integer
constant, GCC treats __m2 as being equivalent to memory_order_seq_cst
(see gcc/builtins.c:get_memmodel). And since memory_order_seq_cst is
stronger than memory_order_relaxed, we get the above error.
In any event, the easiest fix is to simply use the four-argument form of
compare_exchange_strong directly, explicitly specifying the failure
memory order.
Use CONFIG['NEON_FLAGS'] on moz.build instead.
MozReview-Commit-ID: F6R532Hi5mg
--HG--
extra : rebase_source : 7243f316de3138c702f09b336f6d430e6c9c15b5
warning: reference cannot be bound to dereferenced null pointer in
well-defined C++ code; pointer may be assumed to always convert to
true [-Wundefined-bool-conversion]
--HG--
extra : rebase_source : 2de925356078f191b953cc8f6eec546686940c63
Lee Salzman
Mike Hommey
Jonathan Kew
Mason Chang
Ryan VanderMeulen
Nathan Froyd
Kevin Chen
Jonathan Watt
Matt Woodrow
Wes Kocher
Phil Ringnalda
Daniel Holbert
Makoto Kato
Alexandre Lissy
Mats Palmgren
vincentliu