http://gcc.gnu.org/bugzilla/show_bug.cgi?id=60272
Jakub Jelinek <jakub at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |jakub at gcc dot gnu.org,
| |rth at gcc dot gnu.org,
| |torvald at gcc dot gnu.org
--- Comment #1 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Even our __atomic_compare_exchange* documentation states that:
If they are not equal, the current contents of
@code{*@var{ptr}} is written into @code{*@var{expected}}.
But then expand_builtin_atomic_compare_exchange doesn't care:
oldval = expect;
if (!expand_atomic_compare_and_swap ((target == const0_rtx ? NULL : &target),
&oldval, mem, oldval, desired,
is_weak, success, failure))
return NULL_RTX;
if (oldval != expect)
emit_move_insn (expect, oldval);
That effectively means that expect will be stored unconditionally.
So, either we'd need to change this function, so that it sets oldval to
NULL_RTX
first, and passes ..., &oldval, mem, expected, ... and needs to also always ask
for target, then conditionally on target store to expected, or perhaps add
extra parameter to expand_atomic_compare_and_swap and do the store only
conditionally in that case. Richard/Torvald?
