Chung-Lin Tang <clt...@codesourcery.com> writes: > On 2013/10/6 05:57 PM, Richard Sandiford wrote: >>> > But case 16 is different. >>> > This case is only produced at prologue/epilogue phase, using a temporary >>> > register $r15 to hold a large constant for adjusting stack pointer. >>> > Since prologue/epilogue is after split1/split2 phase, we can only >>> > output "sethi" + "ori" directly. >>> > (The "addi" instruction with $r15 is a 32-bit instruction.) >> But this code is in the output template of the define_insn. That code >> is only executed during final, after all passes have been run. If the >> template returns "#", final will split the instruction itself, which is >> possible even at that late stage. "#" doesn't have any effect on the >> passes themselves. >> >> (FWIW, there's also a split3 pass that runs after prologue/epilogue >> generation but before sched2.) >> >> However, ISTR there is/was a rule that prologue instructions shouldn't >> be split, since they'd lose their RTX_FRAME_RELATED_P bit or something. >> Maybe you hit an ICE because of that? >> >> Another way to handle this would be to have the movsi expander split >> large constant moves. When can_create_pseudo_p (), the intermediate >> results can be stored in new registers, otherwise they should reuse >> operands[0]. Two advantages to doing it that way are that high parts >> can be shared before RA, and that calls to emit_move_insn from the >> prologue code will split the move automatically. I think many ports >> do it that way (including MIPS FWIW). > > FWIW, most ports usually just handle such "large adjustment" cases in > the prologue/epilogue code manually; either multiple SP-adjustments, or > use of a temp register (better control of RTX_FRAME_RELATED_P anyways). > You might be able to get it to work, but trying to rely on the splitter > does not seem like best practice...
To be clear, I wasn't talking about relying on the splitter in the define_split sense. I was saying that the move expanders could split large constants. MIPS prologue code does use emit_move_insn to move large constants, which automatically produces a split form from the outset. I don't really agree that it's bad practice. Thanks, Richard
