-----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. Jeff -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQEcBAEBAgAGBQJOuHf+AAoJEBRtltQi2kC7QIEH/0/Agi9ClHIomtqYcCJzmJnk Agrg2ojaJp6KwEq8a//PpOk8Ja4ZuUvTpE71PuDQgDaUWWapd04GuPQMxLE65oho HZBW0CuIgN4IvrsQCnvzx6uMKRFtE4AZx+dmrWCtE0uKuXQFW2lcnPDWb0oEdT+D dsekkWb3BcRMDwVxBlbEUpzxOqVRSWnUuCTtLFCuGRdUOHqyjVp0eOtt+DkAPcXH CM3aVxZDGx9EfMISoZqCD3SkmAgAcYeTRiTsl7Mjkzo+XIivMxm6o9WgJrp1gW0E sdBN3XQcwYgpg9en7O0HGqyDqYBjDG2u3i4kh6uE549/m+e7vPFdAbxpyg8zJZI= =hRN5 -----END PGP SIGNATURE-----
