[LLVMdev] Only use isel on A2 for PPC?

Hal Finkel hfinkel at anl.gov
Tue Mar 10 15:22:43 PDT 2015


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


----- 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

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