On Fri, Jul 24, 2015 at 10:04 AM, Kyrill Tkachov <kyrylo.tkac...@arm.com> wrote: >> It arrives as SSA_NAME == N and you can use get_gimple_for_ssa_name >> or get_def_for_expr to get at the defining stmt if that is possible >> (it's still unexpanded and thus TERed) and expand a different >> expression. > > > Thanks, so it's where we expand compares... (what's TERed?)
Temporary Expression Replacement - IIRC something done when you just come out of ssa . tree-ssa-ter.[hc]. Ramana > >> >> But why can't simplify-rtx via combine handle this - it should have >> access to target costs. > > > That would require for the target to expand to an SMOD rtx > which, if the target has no direct instruction for would be somewhat > awkward. > > Thanks, > Kyrill > > >> >>>> Because, ((1 << (size - 1)) | (C - 1))) constant might be very >>>> expensive, >>>> while C cheap, and % might not be that expensive compared to & to offset >>>> that. >>>> >>>> E.g. on x86_64, for 32-bit and smaller X the constant is cheap as any >>>> other >>>> (well, if we don't take instruction size into account), but 64-bit >>>> constant >>>> is at least 3 times more expensive (movabsq is needed with its latency). >>>> In the x86_64 case supposedly the divmod is still more expensive, but >>>> there >>>> are many other targets. On sparc64 for 64-bit constants, you might need >>>> many instructions to create the constants, etc. >>> >>> Ok, I am not familiar with sparc64. The constant is just a 1 >>> in the sign bit orred with a continuous string of ones. >>> That's usually cheap on aarch64 but may not be so on other targets. >> >> On GIMPLE we might still want to canonicalize to one form. I'd >> canonicalize to the form with "smaller" constants if the number >> of operations is the same. >> >> Richard. >> >>> Thanks, >>> Kyrill >>> >>>> Jakub >>>> >>> >
