Re: [PATCH 1/2] Using pli(paddi) and rotate to build 64bit constants

[Peter Bergner <bergner@linux.ibm.com>]

On 9/1/22 4:52 PM, Segher Boessenkool wrote:
> On Thu, Sep 01, 2022 at 11:24:00AM +0800, Jiufu Guo wrote:
>> As mentioned in PR106550, since pli could support 34bits immediate, we could
>> use less instructions(3insn would be ok) to build 64bits constant with pli.
> 
>> For example, for constant 0x020805006106003, we could generate it with:
>> asm code1:
>> pli 9,101736451 (0x6106003)
>> sldi 9,9,32
>> paddi 9,9, 2130000 (0x0208050)
> 
> 3 insns, 2 insns dependent on the previous, each.
> 
>> or asm code2:
>> pli 10, 2130000
>> pli 9, 101736451
>> rldimi 9, 10, 32, 0
> 
> 3 insns, 1 insn dependent on both others.

Yeah, the improvement here is the fewer dependent instructions, since
2 64-bit + 1 32-bit instructions is the same size as 5 32-bit insns.
Those 5 32-bit insns are all dependent on the previous insn, so not ideal.

It's too bad we don't have a paddis or poris insns where we could specify
in the prefix a shift of 32-bits rather than the normal 16-bits.
If we had those, we could generate the constant with just 2 64-bit insns.

Peter