Re: Correct way to express to the compiler "this does not get clobbered"?

[Andrea Corallo <andrea.corallo@arm.com>]

Richard Earnshaw <Richard.Earnshaw@foss.arm.com> writes:

> On 03/12/2020 11:47, Andrea Corallo via Gcc-help wrote:
>> Hi all,
>> 
>> I've a piece of code that reduced looks like this:
>> 
>> #+begin_src C
>> typedef struct {
>>   void (*fun_ptr)(void);
>> } x_t;
>> 
>> x_t *x;
>> 
>> void
>> f (void)
>> {
>>   const x_t const *y = x;
>>   for (int i = 0; i < 1000; ++i)
>>     y->fun_ptr ();
>> }
>> #+end_src
>> 
>> What is the correct way (if any) to express to the compiler that the
>> value of y->fun_ptr does not get clobbered by the function call itself
>> so the corresponding load to obtain its value can be moved out of the
>> loop?
>> 
>> My understanding is that the const qualifier is more for diagnostic
>> reasons and is not sufficient for GCC to make this assumption.  OTOH I
>> cannot give 'fun_ptr' the attribute pure as it's not.
>> 
>> Thanks
>> 
>>   Andrea
>> 
>
> Why not just put the function pointer in a local variable?  Then the
> compiler will know that the value can't change.

Hi Richard,

yeah that would do the job.

This is generated code and changing the code generator that way might
not be completely trivial (e.g. where is the best position to perform
the assignments to each local variable?).  Therefore before going for
this solution I'd like to be convinced there's no way to express this
directly to GCC.

  Andrea