[llvm-dev] IR canonicalization: vector select or shufflevector?

[Hal Finkel via llvm-dev]

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

> From: "Sanjay Patel via llvm-dev" <llvm-dev at lists.llvm.org>
> To: "Martin ORiordan" <Martin.ORiordan at movidius.com>
> Cc: "LLVM Developers" <llvm-dev at lists.llvm.org>
> Sent: Monday, August 29, 2016 5:45:51 PM
> Subject: Re: [llvm-dev] IR canonicalization: vector select or
> shufflevector?

> x86 has also put a lot of effort into shuffle lowering...so much so
> that it is its own life-form and brings most online codeviewer apps
> to their knees when you try to open X86ISelLowering.cpp. :)

> Given that:
> 1. There are at least 2 targets that lean towards shuffle (Martin's
> comment + x86 uses lowerVSELECTtoVectorShuffle() for all cases like
> the example posted here)

This is irrelevant, as such. We can always transform these into shuffle SDAG nodes regardless of how they look in the IR. 

That having been said, I'm fine with choosing shuffles as the canonical form, over selects with constant vector conditions - If we don't, we'd need some utility to abstract away the difference regardless. 

-Hal 

> 2. Size-changing shuffles are easier to reason about with other
> shuffles (Michael's comment)
> 3. Insert/extract are easier to reason about with shuffles (Eli's
> comment in D22114)

> ...we should probably go with shuffle as the canonical encoding. Like
> Philip, I think the select is easier to read in IR (and mentally
> translate to an x86 'blend'), but there's no other advantage for
> select?

> I'll give this thread some more time before posting a patch...in case
> we've missed something.

> On Mon, Aug 29, 2016 at 1:34 PM, Martin J. O'Riordan via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:

> > I must admit, I prefer the shuffle canonicalization, but mainly
> > because we have put a lot of effort into finding optimal
> > instruction
> > sequences for obscure shuffle patterns. But we could refactor
> > easily
> > enough to use either.
> 

> > I don’t know which makes the most logical sense in this case
> > though.
> > Certainly choosing the select pattern better matches OpenCL’s
> > native
> > select interface.
> 

> > MartinO
> 

> > From: llvm-dev [mailto: llvm-dev-bounces at lists.llvm.org ] On Behalf
> > Of Michael Kuperstein via llvm-dev
> 
> > Sent: 29 August 2016 19:28
> 
> > To: Philip Reames < listmail at philipreames.com >
> 
> > Cc: llvm-dev < llvm-dev at lists.llvm.org >
> 
> > Subject: Re: [llvm-dev] IR canonicalization: vector select or
> > shufflevector?
> 

> > I have a slight preference towards shufflevector, because it makes
> > sequences of shuffles, where only some of the shuffles can be
> > converted into selects (because the input and output vector sizes
> > of
> > the others don't match) simpler to reason about.
> 

> > I'm not sure this is a particularly good reason, though.
> 

> > On Mon, Aug 29, 2016 at 8:19 AM, Philip Reames via llvm-dev <
> > llvm-dev at lists.llvm.org > wrote:
> 
> > > I don't have a strong preference, though it is clear we should
> > > pick
> > > one. I'd mildly prefer the select form for readability. From an
> > > optimization standpoint, I see reasonable arguments for either.
> > 
> 
> > > Philip
> > 
> 

> > > On 08/28/2016 12:37 PM, Sanjay Patel via llvm-dev wrote:
> > 
> 
> > > > A vector select with a constant vector condition operand:
> > > 
> > 
> 

> > > > define <4 x i32> @foo(<4 x i32> %a, <4 x i32> %b) {
> > > 
> > 
> 
> > > > %sel = select <4 x i1> <i1 true, i1 false, i1 false, i1 true>,
> > > > <4
> > > > x
> > > > i32> %a, <4 x i32> %b
> > > 
> > 
> 
> > > > ret <4 x i32> %sel
> > > 
> > 
> 
> > > > }
> > > 
> > 
> 

> > > > ...is equivalent to a shufflevector:
> > > 
> > 
> 

> > > > define <4 x i32> @goo(<4 x i32> %a, <4 x i32> %b) {
> > > 
> > 
> 
> > > > %shuf = shufflevector <4 x i32> %a, <4 x i32> %b, <4 x i32>
> > > > <i32
> > > > 0,
> > > > i32 5, i32 6, i32 3>
> > > 
> > 
> 
> > > > ret <4 x i32> %shuf
> > > 
> > 
> 
> > > > }
> > > 
> > 
> 

> > > > For the goal of canonicalization in IR, which of these should
> > > > we
> > > > prefer? Some backend / lowering differences for AArch64 and PPC
> > > > are
> > > > noted in:
> > > 
> > 
> 
> > > > https://llvm.org/bugs/show_bug.cgi?id=28530
> > > 
> > 
> 
> > > > https://llvm.org/bugs/show_bug.cgi?id=28531
> > > 
> > 
> 
> > > > x86 converts either form optimally in all cases I've looked at.
> > > 
> > 
> 

> > > > This question first came up in D22114 (
> > > > https://reviews.llvm.org/D22114 ) and is extended in D23886 (
> > > > https://reviews.llvm.org/D23886 ) with a constant value
> > > > example.
> > > 
> > 
> 
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> > > 
> > 
> 
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-- 

Hal Finkel 
Assistant Computational Scientist 
Leadership Computing Facility 
Argonne National Laboratory 
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