https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82405
--- Comment #10 from rguenther at suse dot de <rguenther at suse dot de> --- On Tue, 3 Oct 2017, jakub at gcc dot gnu.org wrote: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82405 > > --- Comment #9 from Jakub Jelinek <jakub at gcc dot gnu.org> --- > So it means maybe llvm performs more advanced switchconv in this case, at > least > judging from the #c0 assembly snippet. We look solely for PHIs which have > arguments SSA_NAMEs initialized in the cases to constants, while in order to > optimize this without -ffast-math, it would need to handle at least a couple > of > stmts where the operands match except for some constant argument that is > changing. > > <L6> [20.00%]: > _5 = r_4(D) * 4.0e+0; > _6 = r_4(D) * _5; > goto <bb 8>; [100.00%] > > <L7> [20.00%]: > _7 = r_4(D) * 3.141500000000000181188397618825547397136688232421875e+0; > _8 = r_4(D) * _7; > goto <bb 8>; [100.00%] > > So, in the above case we'd look from the PHI with _6 and _8 arguments, and see > that the because the def stmt isn't assignment from constant, we'd notice it > is > a binary (or unary or ternary) assign where one of the operands is identical > (r_4(D), while the other one is another SSA_NAME defined in the case, and we'd > loop to that, seeing another assign where one operand is the same and another > one is a constant. Thus, we'd build a table with the 4.0e+0 and > 3.141500000000000181188397618825547397136688232421875e+0 constants, and after > the load from the table did _21 = r_4(D) * value_loaded_from_table_20; _22 = > r_4(D) * _21; > The question is if we'd require all operands matching except for one which > could be a constant eventually, or something different (allow some small > number > of constant arguments to a computation). > > Or should we have a separate pass that performs such an optimization (noticing > similar code blocks with just changing constant parameters and merge the > blocks > except for computing the parameters)? Looks like sth for tail-merging / code hoisting? Of course we need to reassoc first (if valid). If the whole thing were if()s it may be PRE would already catch it.
