Richard Guenther wrote:
On Wed, 16 Feb 2005, Paul Schlie wrote:
Richard Guenther wrote: While trying to implement folding of &a +- cst to &a[c] I came along the C frontend, which for
int a[4]; int *p = &a[-1];
produces
p = &a + (int *)-4;
Would guess it should be:
p = &a - (int *)4;
or even:
p = &a + - (int *)4;
Yes, of course, but it is the C frontent that is producing &a + (int *)-4, not me. I'm just trying to work around this...
In fact, it is c-common.c:2289 that does -4 -> (int *)-4 conversion, but pointer_int_sum is already called with PLUS_EXPR. build_unary_op unconditionally expands &x[y] to x+y, regardless of the sign of y. Of course the standard says that they are equal. But is &x[-1] == x + (int *)4*(int *)-1 ? From this follows that we have no way to convert this back to &x[-1], as we loose the sign information by the (int *) cast.
How do the loop optimizers handle this - negative offsets by relying
on unsigned pointer wrap-around?
Out of curiosity, how can a cast to int* ever be correct here? The result of adding (or subtracting) two pointers is a ptrdiff_t, so it seems to me that the original transformation should be
&a[-1] -> &a + (ptrdiff_t)-4
-- Brane
