On 12/7/20 11:17 AM, Bernd Edlinger wrote:
On 12/7/20 4:04 PM, Jason Merrill wrote:
On 12/5/20 7:37 AM, Bernd Edlinger wrote:
On 12/2/20 7:57 PM, Jason Merrill wrote:
On 12/1/20 1:28 PM, Bernd Edlinger wrote:>>>> + 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.
I still don't think you need to load the whole word to check the guard bit.
Of course that is also possible. But I would not expect an
address offset and a byte access to be cheaper than a word access.
Fair point.
I just saw that get_guard_bits does the same thing:
access the first byte if !targetm.cxx.guard_mask_bit ()
and access the whole word otherwise, which is only
there for ARM EABI.
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.
I think the non-EABI code is always using bit 0 in the first byte,
by using the endian-neutral #define _GLIBCXX_GUARD_BIT __guard_test_bit (0, 1).
Except that set_guard sets the least-significant bit on all targets.
Hmm, as I said, get_guard_bits gets the guard as a word if
!targetm.cxx.guard_mask_bit (),
and as the first byte otherwise. Of course it could get the third byte,
if !targetm.cxx.guard_mask_bit () && BYTES_BIG_ENDIAN, but it would be more
complicated
this way, wouldn't it?
Ah, yes, I was overlooking that set_guard uses get_guard_bits.
The patch is OK.
Only ARM EABI uses bit 0 in byte 3 if big-endian and bit 0 in byte 0 otherwise.
For all other targets when _GLIBCXX_USE_FUTEX is defined,
__cxa_guard_XXX accesses the value as int* while the memory
is a 64-bit long, so I could imagine that is an aliasing violation.
But nothing that needs to be fixed immediately.
Agreed.
Attached is the corrected patch.
Tested again on arm-none-eabi with arm-sim.
Is it OK for trunk?
Thanks
Bernd.
Jason
