On Tue, Nov 8, 2011 at 1:29 AM, Jeff Law <l...@redhat.com> wrote: > -----BEGIN PGP SIGNED MESSAGE----- > Hash: SHA1 > > On 11/07/11 15:53, Richard Guenther wrote: >> On Mon, Nov 7, 2011 at 10:25 PM, Jakub Jelinek <ja...@redhat.com> >> wrote: >>> Hi! >>> >>> This patch attempts to optimize VEC_BASE if we know that offsetof >>> of base is 0 (unless the compiler is doing something strange, it >>> is true). It doesn't have a clear code size effect, some .text >>> sections grew, supposedly because of more inlining, some .text >>> sections shrunk. >>> >>> Bootstrapped/regtested on x86_64-linux and i686-linux. >> >> I wonder why the compiler doesn't optimize this ... certainly it >> looks backward to, in >> >> <bb 2>: if (c_2(D) != 0B) goto <bb 3>; else goto <bb 4>; >> >> <bb 3>: D.2948_3 = &c_2(D)->fld; goto <bb 5>; >> >> <bb 4>: D.2948_4 = 0B; >> >> <bb 5>: # D.2948_1 = PHI <D.2948_3(3), 0B(4)> return D.2948_1; >> >> see that D.2948_4 is equal to D.2948_3 for c_2 == 0, so I'm not >> sure which pass would be able to detect this (but the optimziation >> opportunity would be on the PHI node, so maybe it should be done in >> phiopt). > ?!? When c2 == 0 the return value is supposed to be zero, that's one > of the fundamental problems with the way we've defined VEC_BASE. > > In fact cases where we immediately dereference VEC_BASE are precisely > what got me looking at the executable path optimization. > > Assuming this gets inlined and the result is used in a memory > dereference, the new pass will do exactly what we want. Namely it'll > determine that BB4 can never be executed at runtime and it's control > dependent on the edge 2->4. It zaps the edge 2->4, cleaning up the > conditional in the process. That makes BB4 unreachable and BB2, BB4 > and BB5 mergable and everything collapses into one simple assignment.
But there is no dereference in the code above - &c->base is an address computation. But we can still optimize if (c) return &c->base; return NULL; to return &c->base; if &c->base == NULL iff c == NULL. So I think this is orthogonal to any undefinedness of dereferencing. The above pattern occurs frequently so that the computed address is either a valid dereferencable address or NULL. Richard.
