[LLVMdev] Only use isel on A2 for PPC?

[Hal Finkel]

Olivier,

Also, we should leave it enabled for the Freescale cores.

 -Hal

----- Original Message -----
> From: "Olivier H Sallenave" <ohsallen at us.ibm.com>
> To: llvmdev at cs.uiuc.edu
> Cc: "Hal Finkel" <hfinkel at anl.gov>
> Sent: Tuesday, March 10, 2015 5:14:27 PM
> Subject: Only use isel on A2 for PPC?
> 
> 
> 
> Hi Hal,
> 
> I see that you worked on early if conversion and on the isel
> instruction generation. I found some hotspot in Graph500 where using
> isel generates a 10% slowdown compared to the traditional solution
> using branches (this is for POWER8). It appears that using branches
> is almost always better than using isel for P7 and P8, because of
> branch prediction — the decision outcome has to be almost completely
> random for isel to win. I also noticed that isel has a way higher
> latency for P8 than for A2.
> 
> I was wondering if you actually measured some improvements using isel
> on P7? Otherwise, we could only use isel for A2 and disable it for
> the other subtargets. Please tell me what you think.
> 
> Thanks,
> 
> Olivier

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory