On Fri, 23 Oct 2009, Eric Botcazou wrote: > Hi Richard, > > I just (re-)discovered that the new TBAA machinery is quite aggressive and > breaks cases that used to work in Ada (-O2 testcase for SPARC64 attached). > > The problem boils down to this: > > D.1416_1 = (struct p__rec &) &r.F; > r.F = ... > ... = D.1416_1->d; > > DSE computes that the store to r.F is dead and eliminates it at -O2 because > ultimately nonaliasing_component_refs_p returns false: > > /* If we have two type access paths B1.path1 and B2.path2 they may > only alias if either B1 is in B2.path2 or B2 is in B1.path1. */ > return false; > > [Shouldn't nonaliasing_component_refs_p be named aliasing_component_refs_p or > component_refs_may_alias_p instead]?
Err, yes ;) I named it after the RTL variant in alias.c. > Yes, it's a blatant type-punning case but all the structure types are given > the same alias set (struct p__rec, type of r, type of F) and 'd' is not > addressable so all the memory accesses are done with the same alias set. > > The root of the problem is that same_type_for_tbaa never returns true since > the types don't have the same TYPE_CANONICAL (rightfully so, they are not > equivalent) so we fall back to the final return of nonaliasing_c_r_p. > > Shouldn't this final return be 'true' instead of 'false', like the final > return in indirect_ref_may_alias_decl_p, so that the ultimate fallback is the > comparison of alias sets like it used to be? I changed this default to false somewhen in the past. There was a big fat comment there on the alias-improvements branch (where I wondered if returning false would be a safe thing to do). I didn't find (or could construct) a single C or C++ testcase that wasn't fine with the new default, so I switched it (IIRC the default is disambiguating the most cases). I'm fine with switching it back though, this time with a comment explaining why it is not safe (instead of just speculating) and a testcase (I guess you now indeed have one). Care to prepare a patch? Thanks, Richard.
