Re: Use of __restrict__ in g++

[Andrew Haley <aph at redhat dot com>]

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quick@sparq.org wrote:
> 
> I have two questions regarding the use on __restrict__ qualifiers for function 
> arguments in C++:
> 
> 1) How does it interact with volatile?
> 
>   Example, given:
> 
>      void foo(volatile int* __restrict__ p1,
>               volatile int* __restrict__ p2)
>      {
>         *p1 = 3;
>         if (*p1 == 5) ...;
>         *p2 = 4;
>      }
> 
>   The most desireable for me would be that restrict would indicate that *p1 and 
> *p2 were disjoint and allow reordering statement 3 to execute before either 
> preceeding statement, but that volatile would indicate that the if expression 
> would need to re-fetch *p1 and not assume the results of the first statement.

It doesn't matter.  According to the definition of restrict in C99 [1],

During each execution of [Block] B, let L be any lvalue that has &L
based on P. If L is used to access the value of the object X that it
designates, and X is also modified (by any means), then the following
requirements apply: T shall not be const-qualified. Every other lvalue
used to access the value of X shall also have its address based on
P. Every access that modifies X shall be considered also to modify P,
for the purposes of this subclause."

So, if you access L other than via an address based on P, your program
is undefined, volatile notwithstanding.

[1] ISO/IEC 9899:1999  6.7.3.1 Formal definition of restrict

Andrew.