https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88854
--- Comment #5 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 15 Jan 2019, kretz at kde dot org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88854 > > --- Comment #4 from Matthias Kretz <kretz at kde dot org> --- > Another test case, which the patch doesn't optimize: > > short f(int *a, short *b) { > short y = *b; // 1 > int x = *a; // 2 > *b = 1; > *a = x; > return y; > } > > The loads in 1+2 are either UB or a and b must not alias. Consequently the > store to b won't change a and the store to a is dead. > > General rule: > Given two pointers a and b of different type, where b is not a pointer to > char, > unsigned char, or std::byte, if > - a load of a is followed by a load of b, or > - a store to a is followed by a load of b > then a and b are guaranteed to point to different addresses. Yeah, we do not perform this kind of "flow-sensitive" TBAA. So when trying to DSE *a = x; we only look at int x = *a; *b = 1; *a = x; and do not consider the earlier load from *b at all because it is not on the path from the load making the store possibly redundant.
