http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50557
--- Comment #5 from William J. Schmidt <wschmidt at gcc dot gnu.org> 2011-09-30 14:30:56 UTC --- Reassociation isn't doing anything untoward here that raises register pressure. The problem must be occurring downstream. Likely the scheduler is making a different decision that leads to more pressure. Block 9 contains the following prior to reassociation: D.3497_48 = D.3496_47 + D.3475_117; t_50 = D.3497_48 + D.3493_44; Reassociation changes this to: D.3497_48 = D.3493_44 + D.3496_47; t_50 = D.3497_48 + D.3475_117; This extends the lifetime of D.3475_117 but shortens the lifetime of D.3493_44, both of which go dead here. Register pressure is not raised at any point. There are no further changes to this code in the rest of the tree-SSA phases. Based on this, I don't see any reason to adjust the reassociation algorithm. Someone with some expertise in RTL phases could look into what happens later on to cause the additional pressure, but I don't see this as a tree-optimization issue.
