-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 12/02/11 11:10, William J. Schmidt wrote: > >> >> I see a mention of creating equivalences for redundant PHIs? Are >> we just trying to determine that two PHIs are going to result in >> the same value? > > Jeff, see comment #37 in > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39976. The issue is > that we have two PHIs in the same block as follows: > > # prephitmp.35_322 = PHI <prephitmp.35_59(6), prephitmp.35_113(7)> > # prephitmp.35_225 = PHI <prephitmp.35_59(6), prephitmp.35_113(7)> OK. Make sure you has the phi argument & its edge... Simlarly the equality comparison needs to check args & edges. Or s/edge/edge->src/ if that's easier.
> > The coalescing algorithm in tree-outof-ssa doesn't handle this well > and ends up splitting the back arc of a simple loop as a result. > I'm creating an equivalence between prephitmp.35_322 and > prephitmp.35_225 in this case so that one PHI goes dead and is > removed. Right. I don't think I bothered with this because I didn't see it happening and the comparison of the PHIs with each other gets potentially expensive. As pointed out in > the comments, this needs to be restricted to occur only in the > same block since the PHIs aren't necessarily equivalent otherwise. I must be missing something -- why is the equivalence only valid in the same block? Conceptually this situation isn't any different replacing the second PHI with a copy blah_225 = blah_322 And the equivalence is valid for the entire dominator subtree. If you really need to create a temporary equiv, pushing the marker, create the equiv and pop the marker when equiv dies is simple and easy. Jeff -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Mozilla - http://enigmail.mozdev.org/ iQEcBAEBAgAGBQJO2RjmAAoJEBRtltQi2kC7I7YH/RTVZF6jzBpyg+68kMTZuHDH hETg03jnqhSXhWOzBoALZwtR9iCJ/UehCCcnjQ3eV7o1AqsImLMnap+VrS6lYfI3 hYQXtdb2F770H70yY/fgEX/VDAdGlyduXfMqHl4mLw7Apz3mpvyBucqHqECHJ+xj AtZf3duGXp3Fpvhm3PZ7wQa+Pl0qnJU3VxqU6xkGWn9A8et8U1IdYs/wnHPR/HPY KP7oY3JWxwR6kpzNkFJM1OZ+Nn9XWGgScAp1uBXJhK2RLgIuxaLiM5wl6VAJR1Fs EuMBnQbHELA1ugtqC26UsDCTkjDpLGgs02ID7ArySsVgmIYvXCVzEi0cdJA42V4= =NKrB -----END PGP SIGNATURE-----
