[PATCH] Fix c++/60272

[Richard Henderson]

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.


r~