Disable optimizations on one function (was: 'pragma optimize' ...)

Jeffrey Walton noloader@gmail.com
Thu Jul 16 10:05:00 GMT 2015


On Thu, Jul 16, 2015 at 5:59 AM, Andrew Haley <aph@redhat.com> wrote:
> On 16/07/15 10:40, Jeffrey Walton wrote:
>>>> Many folks try and cast ptr to volatile, but that's an abuse because
>>>> GCC considers volatile something for memory mapped hardware. Volatile
>>>> should not be used in an attempt to tame the optimizer.
>>>
>>> GCC does not consider volatile to be something for memory mapped
>>> hardware.
>>
>> OK, this appears to be creating a moving definition (or the definition
>> has changed since I took note of it). I took the last definition from
>> Ian Lance Taylor. See http://www.airs.com/blog/archives/154 and, for
>> example, https://gcc.gnu.org/ml/gcc-help/2012-03/msg00257.html.
>
> Well, reluctant as I am to argue with Ian, the definition of volatile
> I used is a direct quote from the standard.  There are problems with
> volatile as defined, it's true: for example, nowhere is it specified
> exactly what constitutes a memory access.  And Ian is quite right to
> say that the standard doesn't guarantee that a pointer-to-volatile
> should be handled as though it pointed to a volatile object.  But in
> this case, with GCC, I think it's fine.
>
Yeah, it kind of crossed my wires for a moment. There's a handful of
folks I always listen to when it comes to GCC, and you and Ian are two
of them :o

If you tell me the memory barrier works to tame the optimizer for a
function, then that's what I will use.

Jeff



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