在 2020/12/3 下午7:47, Andrea Corallo via Gcc-help 写道:
> 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?
How about:
```c
void
f (void)
{
int i;
for (__auto_type p = ( // warning: use of GCC extension
(i = 0), // set `i` to zero
y->fun_ptr); // warning: abuse of comma operator
i < 1000; ++i)
p ();
}
```
You would only ever need one `i` in each function, so it'd not be very complicated. The scope of `p`
is limited to the enclosing `for` statement.
>
> 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.
>
`const` may be used for optimization only if it causes an object to be declared `const`. Something
such as `const int x = 42;` is known by the compiler to be immutable, but the result of an
indirection through a pointer might not be.
--
Best regards,
LH_Mouse
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