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Re: [PATCH v2] Fix PR64242


Hi,

Jakub Jelinek wrote:
> On Fri, Dec 07, 2018 at 02:52:48PM +0000, Wilco Dijkstra wrote:
>> -  struct __attribute__((aligned (32))) S { int a[4]; } s;                                                                                        
>> -  bar (&s);                                                                                                                                      
>
> Any reason to remove the above?

The test case doesn't need an aligned object to fail, so why did you add it?

>> +  /* Compute expected next alloca offset - some targets don't align properly
>> +     and allocate too much.  */
>> +  p = q + (q - p);
>
> This is UB, pointer difference is only defined within the same object.
> So, you can only do such subtraction in some integral type rather than as
> pointer subtraction. 

__builtin_setjmp is already undefined behaviour, and the stack corruption is
even more undefined - trying to avoid harmless theoretical undefined behaviour
wouldn't be helpful.

> And I'm not sure you have a guarantee that every zero sized alloca is at the
> same offset from the previous one.

The above pointer adjustment handles the case where alloca overallocates.
It passes on x86-64 which always adds 8 unnecessary bytes.

Wilco

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