On 02/20/2014 01:22 PM, Richard Henderson wrote:
> On 02/20/2014 12:09 PM, Jakub Jelinek wrote:
>> On Thu, Feb 20, 2014 at 11:49:30AM -0600, Richard Henderson wrote:
>>> Tested on x86_64 and i686, and manually inspecting the generated code.
>>> Any ideas how to regression test this?
>>
>> No idea about how to test this.
>>
>>> @@ -5330,14 +5330,23 @@ expand_builtin_atomic_compare_exchange (enum
>>> machine_mode mode, tree exp,
>>> if (tree_fits_shwi_p (weak) && tree_to_shwi (weak) != 0)
>>> is_weak = true;
>>>
>>> + if (target == const0_rtx)
>>> + target = NULL;
>>> oldval = expect;
>>> - if (!expand_atomic_compare_and_swap ((target == const0_rtx ? NULL :
>>> &target),
>>> - &oldval, mem, oldval, desired,
>>> +
>>> + if (!expand_atomic_compare_and_swap (&target, &oldval, mem, oldval,
>>> desired,
>>
>> I'm wondering if this shouldn't be instead:
>> oldval = NULL;
>> if (!expand_atomic_compare_and_swap (&target, &oldval, mem, expected,
>> desired,
>> is_weak, success, failure))
>>
>> because otherwise expand_atomic_compare_and_swap could in theory already
>> store into expect MEM, couldn't it? I mean, it does:
>> /* Load expected into a register for the compare and swap. */
>> if (MEM_P (expected))
>> expected = copy_to_reg (expected);
>>
>> /* Make sure we always have some place to put the return oldval.
>> Further, make sure that place is distinct from the input expected,
>> just in case we need that path down below. */
>> if (ptarget_oval == NULL
>> || (target_oval = *ptarget_oval) == NULL
>> || reg_overlap_mentioned_p (expected, target_oval))
>> target_oval = gen_reg_rtx (mode);
>> so with NULL *ptarget_oval it will surely allocate a pseudo, but if it is
>> the expected MEM, as expected has been forced into register earlier,
>> I don't think it overlaps with that REG and thus it can be already stored
>> and have oldval == expect after the call.
>
> I don't know any target that actually accepts a MEM for oldval, and since the
> current definition of __atomic_compare_and_swap_n takes an address for
> expected, we'll always have a MEM. So at present we'll always allocate a new
> pseudo just as if we zero out oldval.
>
> But, fair enough. It does seem generally safer your way.
Like so.
r~
diff --git a/gcc/builtins.c b/gcc/builtins.c
index 09fefe50..35969ad 100644
--- a/gcc/builtins.c
+++ b/gcc/builtins.c
@@ -5332,9 +5332,12 @@ expand_builtin_atomic_compare_exchange (enum
machine_mode mode, tree exp,
if (target == const0_rtx)
target = NULL;
- oldval = expect;
- if (!expand_atomic_compare_and_swap (&target, &oldval, mem, oldval, desired,
+ /* Lest the rtl backend create a race condition with an imporoper store
+ to memory, always create a new pseudo for OLDVAL. */
+ oldval = NULL;
+
+ if (!expand_atomic_compare_and_swap (&target, &oldval, mem, expect, desired,
is_weak, success, failure))
return NULL_RTX;
