Re: [PATCH v3 02/13] tcg/s390x: Remove TCG_REG_TB

[Richard Henderson]

On 12/6/22 16:22, Richard Henderson wrote:
> > Wouldn't it be worth keeping XILF/XIFH here?
>
> I don't know.  It's difficult for me to guess whether a dependency chain like
>
>      val -> xor -> xor
>
> (3 insns with serial dependencies) is better than
>
>      val   --> xor
>      load  -/
>
> (3 insns, but only one serial dependency) is better.  But there may also be instruction 
> fusion going on at the micro-architectural level, so that there's really only one xor.
>
> If you have suggestions, I'm all ears.

Related microarchitectural question:

If a 32-bit insn and a 64-bit insn have a similar size encoding (and perhaps even if they 
don't), is it better to produce a 64-bit output so that the hw doesn't have a false 
dependency on the upper 32-bits of the register?

Just wondering whether most of the distinction between 32-bit and 64-bit opcodes ought to 
be discarded, simplifying code generation.  The only items that seem most likely to have 
real execution time differences are multiply and divide.


r~