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?