[LLVMdev] Why does Select have a higher speculation cost than other instructions?

[Hal Finkel]

----- Original Message -----
> From: "Andrew Trick" <atrick at apple.com>
> To: "Tom Stellard" <tom at stellard.net>
> Cc: llvmdev at cs.uiuc.edu
> Sent: Friday, May 30, 2014 2:15:29 PM
> Subject: Re: [LLVMdev] Why does Select have a higher speculation cost than other instructions?
> 
> 
> On May 28, 2014, at 11:52 AM, Tom Stellard <tom at stellard.net> wrote:
> 
> > Hi,
> > 
> > The ComputeSpeculationCost() function in
> > Transforms/Utils/SimplifyCFG.cpp uses a higher speculation cost for
> > Select than other instructions.  Does anyone know why this is?
> > 
> > I would like SimplifyCFG to be able to speculatively execute Select
> > instructions.  Which of these solutions makes the most sense:
> > 
> > 1. Change speculation cost of Select from 2 to 1.
> > 
> > 2. Add a TargetTransformInfo callback for Select speculation cost.
> > 
> > 3. Add a TargetTransformInfo callback for PhiNodeFoldingThreshold
> > to
> > enable speculation of more expensive instructions.
> > 
> > 4. Something else.
> 
> Hi Tom,
> 
> FWIW: I think any of the above could be justified given the current
> situation. Ideally, we shouldn’t use any TTI info in this pass. We
> should decide on a canonical form for CFG patterns based on
> facilitating downstream target-independent optimizations passes. We
> should have a target-specific CFG optimization pass that runs just
> before codegen and uses the subtarget’s cost model with as many
> hooks as we want to throw in.
> 
> Short of doing this, I can’t claim any of your workarounds makes more
> sense than the other. It looks like someone didn’t want to form
> chains of Selects, but I’m not sure why. If you could verify that it
> doesn’t regress x86, then changing the standard cost is fine with
> me.

Tom,

Out of curiosity, did you ever do anything with this?

 -Hal

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