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)?
