[PATCH] PowerPC: stpcpy optimization for PPC64/POWER7

[Richard Henderson]

On 09/18/2013 07:34 AM, Adhemerval Zanella wrote:
> +	extrdi.	rTMP, rALT, 8, 0
> +	stbu	rTMP, 8(rRTN)
> +	beqlr
> +	extrdi.	rTMP, rALT, 8, 8
> +	stbu	rTMP, 1(rRTN)
> +	beqlr
> +	extrdi.	rTMP, rALT, 8, 16
> +	stbu	rTMP, 1(rRTN)
> +	beqlr
> +	extrdi.	rTMP, rALT, 8, 24
> +	stbu	rTMP, 1(rRTN)
> +	beqlr
> +	extrdi.	rTMP, rALT, 8, 32
> +	stbu	rTMP, 1(rRTN)
> +	beqlr
> +	extrdi.	rTMP, rALT, 8, 40
> +	stbu	rTMP, 1(rRTN)
> +	beqlr
> +	extrdi.	rTMP, rALT, 8, 48
> +	stbu	rTMP, 1(rRTN)
> +	beqlr
> +	stbu	rALT, 1(rRTN)

I, like Ondrej, have trouble believing that 4 arithmetic insns + 1 unaligned
load + 1 unaligned store is slower than this compare-branch ladder.

However good Power7's branch predictor is, I bet its out-of-order insn
scheduler is better.  Issue the 6 insns, return from subroutine, surely.

You've got the location of the zero in rMASK from cmpb:

  cntlzd   rMASK, rMASK      // extract bit offset of nul byte
  srdi     rMASK, rMASK, 3   // convert bit offset to byte offset
  addi     rALT, rMASK, -7   // include the previous 7 bytes plus the nul
  ldx      rTMP, rSRC, rALT  // perform one last unaligned copy
  stdx     rTMP, rRTN, rALT
  add      rRTN, rRTN, rMASK // adjust the return value
  blr

For little-endian one needs 2-3 more insns, since there's no corresponding
count trailing zeros insn.


r~