https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120811
--- Comment #8 from Vineet Gupta <vineetg at gcc dot gnu.org> --- (In reply to Jeffrey A. Law from comment #5) > So a bit of clarification here from the patchwork meeting today. > > Shreya's work isn't meant to directly fix this particular issue, though it > is very closely related and likely a prerequisite to fix this issue. > > What Shreya is doing is adjusting the addsi3/adddi3 code. First she's going > to create an addsi3/adddi3 expander. Those expanders will widen the set of > valid operands to allow more constants for the 2nd input operand. > > The existing addsi3/adddi3 define_insns will need a name change to *addsi3 > and *adddi3 to indicate the names are internal. > > In the new expanders for a case like x+2048, rather than construct the 2048 > constant into a register and using the add instruction, instead generate > addi+addi during initial RTL generation. There's also cases we can handle > with shNadd. So we effectively avoid constant synthesis and generate > efficient code at initial RTL generation. > > We would then remove the special define_insn_and_split for the sum of two > simm12 cases. > > Once that's done I think we define add_ptr3 (or whatever it's called). That > expander/pattern is special for LRA's use in generating more efficient code > in some cases. I'm pretty sure we can define it as an expander and use the > same code as we're using for addsi3/adddi3. > > That should result in getting reasonable code out of LRA for this case and > in a form that fold-mem-offsets can likely handle rather than waiting for > sched2 to do the height reduction transformation and leaving the useless add > instruction in the RTL stream. > > Anyway, that's the basic plan. All this makes sense. But one question What's to stop combine/late-combine from merging back the optimal expanded sequences, given the presence of mvconstant_internal still. AFAIKR this has been a fundamental question: should we do this early or late in the pipeline, with the concern/caution that if done early interim passes can undo this.
