On 12/1/20 1:28 PM, Bernd Edlinger wrote:
On 11/24/20 11:10 PM, Jason Merrill wrote:
On 11/22/20 3:05 AM, Bernd Edlinger wrote:
Hi,
this avoids the need to use -fno-threadsafe-statics on
arm-none-eabi or working around that problem by supplying
a dummy __sync_synchronize function which might
just lead to silent code failure of the worst kind
(non-reproducable, racy) at runtime, as was pointed out
on previous discussions here.
When the atomic access involves a call to __sync_synchronize
it is better to call __cxa_guard_acquire unconditionally,
since it handles the atomics too, or is a non-threaded
implementation when there is no gthread support for this target.
This fixes also a bug for the ARM EABI big-endian target,
that is, previously the wrong bit was checked.
Instead of a new target macro, can't you follow
fold_builtin_atomic_always_lock_free/can_atomic_load_p?
Yes, thanks, that should work too.
Would you like this better?
+is_atomic_expensive_p (machine_mode mode)
+{
+ if (!flag_inline_atomics)
+ return false;
Why not true?
+ if (!can_compare_and_swap_p (mode, false) || !can_atomic_load_p (mode))
+ return false;
This also seems backwards; I'd think we want to return false if either
of those tests are true. Or maybe just can_atomic_load_p, and not
bother about compare-and-swap.
+ tree type = targetm.cxx.guard_mask_bit ()
+ ? TREE_TYPE (guard) : char_type_node;
+
+ if (is_atomic_expensive_p (TYPE_MODE (type)))
+ guard = integer_zero_node;
+ else
+ guard = build_atomic_load_type (guard, MEMMODEL_ACQUIRE, type);
It should still work to load a single byte, it just needs to be the
least-significant byte. And this isn't an EABI issue; it looks like the
non-EABI code is also broken for big-endian targets, both the atomic
load and the normal load in get_guard_bits.
Jason
