[PATCH] InstCombine: Transform select with and/or cond into select sequence

[Hal Finkel]

----- Original Message -----
> From: "Matt Arsenault" <Matthew.Arsenault at amd.com>
> To: "Matthias Braun" <matze at braunis.de>
> Cc: reviews+D7399+public+7f94a38def591e0c at reviews.llvm.org, hfinkel at anl.gov, llvm-commits at cs.uiuc.edu
> Sent: Wednesday, February 4, 2015 7:56:11 PM
> Subject: Re: [PATCH] InstCombine: Transform select with and/or cond into select sequence
> 
> On 02/04/2015 05:45 PM, Matthias Braun wrote:
> > I'm currently working on a new version that uses
> > TLI->isOperationLegal(ISD::And, i1) /
> > TLI->isOperationLegal(ISD::Or, i1) to decide whether to do the
> > transform or not, is that sufficient for your targets?
> >
> > - Matthias
> 
> It is not. Checking if the select is legal would be better, but I
> don't
> think quite enough in this case.

Agreed. I think you need a separate target hook. IMHO, A reasonable default for the target hook would be to check whether the select is 'legal or custom'.

 -Hal

> 
> 
> >> On Feb 4, 2015, at 5:41 PM, Matt Arsenault
> >> <Matthew.Arsenault at amd.com> wrote:
> >>
> >> On 02/03/2015 08:14 PM, Matthias Braun wrote:
> >>> This transforms:
> >>> select((C0 & C1), a, b) -> select(C0, select(C1, a, b), b)
> >>> select((C0 | C1), a, b) -> select(C0, a, select(C1, a, b))
> >>>
> >>> The result is better on most targets as C0 and C1 do not need to
> >>> be
> >>> materialized into an integer register but can stay flags.
> >> I think this should be controllable by a target hook depending on
> >> the type. On R600, where a 64-bit select needs to be split into 2
> >> instructions, this would be beneficial for 32-bit selects but not
> >> 64-bit (unless the select can be proven uniform in which case the
> >> second form is better for two reasons, but that's a much bigger
> >> problem to solve)
> >>
> >> -Matt
> >> _______________________________________________
> >> llvm-commits mailing list
> >> llvm-commits at cs.uiuc.edu
> >> http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits
> 
> 

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