[PATCH 2/6] Andes nds32: machine description of nds32 porting (2).

[Chung-Lin Tang]

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...

Chung-Lin