[llvm-dev] Cost model is missing in InstCombiner

[Hal Finkel via llvm-dev]

----- Original Message -----

> From: "Mehdi Amini via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Shixiong Xu" <shixiong at cadence.com>
> Cc: llvm-dev at lists.llvm.org
> Sent: Thursday, August 18, 2016 11:05:35 AM
> Subject: Re: [llvm-dev] Cost model is missing in InstCombiner

> +David M.

> > On Aug 17, 2016, at 3:48 AM, Shixiong Xu via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> 

> > Hi,
> 

> > I think canEvaluateTruncated() in InstCombiner needs use cost model
> > to decide whether perform optimization or not.
> 
> I’ve always seen InstCombine as doing “canonicalization” of the IR
> and not “optimization”. So the output of InstCombine should be in a
> form that is the most suitable for further analyses and
> transformations.
This is exactly our traditional view. Why can the backend not be fixed to generate better code for mul <32 x i8>? It looks like the widening in the IR is something natural to get from legalization (if you set up the correct promotion preferences in *ISelLowering). 

-Hal 

> I’m not sure how this view fits with TTI, I may not be incompatible
> if used within some limits I guess?

> —
> Mehdi

> > Without cost model from TargetTransformInfo, aggressively
> > optimizing
> > IR in vector types according to the number of bits demanded may
> > lead
> > to scalarization of vector operations. For example, if the input IR
> > is:
> 

> > %wide.load25 = load <32 x i8>, <32 x i8>* %231, align 1
> 
> > %232 = zext <32 x i8> %wide.load25 to <32 x i16>
> 
> > %233 = mul nuw nsw <32 x i16> %232, %164
> 
> > …
> 
> > %237 = trunc <32 x i16> %233 to <32 x i8>
> 
> > store <32 x i8> %237, <32 x i8>* %236, align 1
> 

> > ICE: EvaluateInDifferentType converting expression type to avoid
> > cast: %9 = trunc <32 x i16> %6 to <32 x i8>
> 
> > IC: ADD: %6 = mul <32 x i8> %wide.load25, %wide.load
> 
> > IC: Replacing %10 = trunc <32 x i16> %7 to <32 x i8>
> 
> > with %6 = mul <32 x i8> %wide.load25, %wide.load
> 

> > If the target doesn’t have support for mul <32 x i8>, the inst
> > combiner will yield less profitable code.
> 

> > Cheers,
> 

> > Shixiong (Jason) Xu
> 
> > _______________________________________________
> 
> > LLVM Developers mailing list
> 
> > llvm-dev at lists.llvm.org
> 
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> 
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20160818/68cc311c/attachment-0001.html>