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? Richard. -- Richard Guenther <richard dot guenther at uni-tuebingen dot de> WWW: http://www.tat.physik.uni-tuebingen.de/~rguenth/
