Re: [PATCH-1, rs6000] Put constant into pseudo at expand when it needs two insns [PR86106]

[HAO CHEN GUI <guihaoc@linux.ibm.com>]

Hi Richard,

在 2023/3/16 15:57, Richard Biener 写道:
> So this is one way around the lack of CSE/PRE of constant operands.  I'd
> argue that a better spot for this _might_ be LRA (split the constant out if
> there's a free register available), postreload-[g]cse (CSE the constants) and
> then maybe cprop_hardreg to combine back single-use constants?
> 
> I'm not sure if careful constraints massaging like adding magic letters to
> alternatives with constants to pessimize them for LRA, making them
> more expensive than spilling the constant to a register but avoid
> secondary reloads with spilling a register to the stack to make room
> for the constant, is possible - but in theory a special constraint modifier
> for this purpose could be invented.

Thanks so much for your advice.

cse/gcse doesn't take cost of constant set (the def insn of the constant) into
consideration. So it won't replace the register with a constant as it costs 1
insn with the register and costs 2 insn with the constant. Finally, the single-
use constants can't be back to 2 insn. Not sure if I understand it correctly.
Looking forward to your advice.

Thanks
Gui Haochen