Re: Possible missed optimization opportunity with const?

[David Brown <david dot brown at hesbynett dot no>]

On 17/08/16 02:21, Toshi Morita wrote:
> I was involved in a discussion over the semantics of "const" in C, and the following code was posted:
>
> #include <stdio.h>
> int foo = 0;
> const int *pfoo = &foo;
> void bar (void)
> {
>      foo +=3D;

I assume that's a typo?

> }
> int main(void)
> {
>     int a, b;
>     a = *pfoo;
>       bar();
>       b = *pfoo;
>     printf("a: %d, b: %d\n", a, b);
> }
>
>
> This code when compiled with gcc 4.8.2 using the optimization option -O3 produces:
>
> a: 0, b: 1
>
>
> So it appears even though pfoo is a const int *, the value *pfoo is read twice.
>
> Would it be valid for the code to print a:0, b: 0?
> If so, is this a missed optimization opportunity?

No, it would not be valid.  Declaring pfoo as a "const int*" tells the 
compiler "I will not change anything via this pointer - and you can 
optimise based on that promise".  It does /not/ tell the compiler "the 
thing that this points to will not change".

So the compiler is correct in reading *pfoo twice.