[llvm-dev] Bug in ComputeSpeculationCost in SimplifyCFG?

[Friedman, Eli via llvm-dev]

On 9/10/2018 4:02 PM, Gautam Chakrabarti via llvm-dev wrote:
>
> I am trying to understand the cost returned by ComputeSpeculationCost 
> function in SimplifyCFG. It appears even when it returns TCC_Expensive 
> (which means “prohibitively expensive”), the caller may proceed with a 
> transform, because TCC_Expensive is not really a high cost, but its 
> value is 4.
>
> For example when DominatesMergePoint calls this, it may still move 
> ahead because TCC_Expensive cost may be less than CostRemaining. 
> Looking thru the history, it appears this behavior changed with 
> r228826 
> (http://llvm.org/viewvc/llvm-project?view=revision&revision=228826), 
> when this API was moved to use TargetTransformInfo for cost analysis 
> instead of the hard-coded cost estimates.
>
> Is this a bug?
>

It looks dubious, yes... although I'm not sure it's actually possible to 
cause issues in practice with the default thresholds. Patch welcome.

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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