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

[Rackover, Zvi via llvm-dev]

Suppose we prefer the ‘trunc’ form, then what about cases such as:
define <2 x i16> @shuffle(<16 x i16> %x) {
  %shuf = shufflevector <16 x i16> %x, <16 x i16> undef, <2 x i32> <i32 0, i32 8>
  ret <2 x i16> %shuf
}

Will the ‘shufflevector’ be canonicalized to a ‘trunc’ of a vector of i128?
define <2 x i16> @trunc(<16 x i16> %x) {
  %bc = bitcast <16 x i16> %x to <2 x i128>
  %tr = trunc <2 x i128> %bc to <2 x i16>
  ret <2 x i16> %tr
}
This may challenge the Legalizer downstream.

--Zvi

From: Sanjay Patel [mailto:spatel at rotateright.com]
Sent: Friday, January 13, 2017 18:19
To: Rackover, Zvi <zvi.rackover at intel.com>
Cc: Friedman, Eli <efriedma at codeaurora.org>; llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] IR canonicalization: shufflevector or vector trunc?

Right - I think that case looks like this for little endian:

define <2 x i32> @zextshuffle(<2 x i16> %x) {
  %zext_shuffle = shufflevector <2 x i16> %x, <2 x i16> zeroinitializer, <4 x i32> <i32 0, i32 2, i32 1, i32 2>
  %bc = bitcast <4 x i16> %zext_shuffle to <2 x i32>
  ret <2 x i32> %bc
}

define <2 x i32> @zextvec(<2 x i16> %x) {
  %zext = zext <2 x i16> %x to <2 x i32>
  ret <2 x i32> %zext
}
IMO, the fact that we have to take endianness into account with the shuffles makes the trunc/zext forms the better choice. That way, we limit the endian dependency to one place in InstCombine, and other transforms don't have to worry about it. We also have lots of existing folds for trunc/zext and hardly any for shuffles.


On Thu, Jan 12, 2017 at 1:14 PM, Rackover, Zvi <zvi.rackover at intel.com<mailto:zvi.rackover at intel.com>> wrote:
Just to add, there is also the ‘zext’ – ‘shuffle with zero’ duality which can broaden the discussion.

--Zvi

From: Sanjay Patel [mailto:spatel at rotateright.com<mailto:spatel at rotateright.com>]
Sent: Thursday, January 12, 2017 20:19
To: Friedman, Eli <efriedma at codeaurora.org<mailto:efriedma at codeaurora.org>>
Cc: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>; Rackover, Zvi <zvi.rackover at intel.com<mailto:zvi.rackover at intel.com>>
Subject: Re: [llvm-dev] IR canonicalization: shufflevector or vector trunc?



On Thu, Jan 12, 2017 at 11:06 AM, Friedman, Eli <efriedma at codeaurora.org<mailto:efriedma at codeaurora.org>> wrote:
On 1/12/2017 9:04 AM, Sanjay Patel via llvm-dev wrote:
It's time for another round of "What is the canonical IR?"

Credit for this episode to Zvi and PR31551. :)
https://llvm.org/bugs/show_bug.cgi?id=31551

define <4 x i16> @shuffle(<16 x i16> %x) {

  %shuf = shufflevector <16 x i16> %x, <16 x i16> undef, <4 x i32> <i32 0, i32 4, i32 8, i32 12>

  ret <4 x i16> %shuf

}



define <4 x i16> @trunc(<16 x i16> %x) {

  %bc = bitcast <16 x i16> %x to <4 x i64>

  %tr = trunc <4 x i64> %bc to <4 x i16>

  ret <4 x i16> %tr

}


Potential reasons to prefer one or the other:
1. Shuffle is the most compact.
2. Trunc is easier to read.
3. One of these is easier for value tracking.
4. Compatibility with existing IR transforms (eg, InterleavedAccess recognizes the shuffle form).
5. We don't create arbitrary shuffle masks in IR because that's bad for a lot of targets (but maybe this mask pattern should always be recognized as special?).

Hmm... not sure what the right answer is, but a couple more observations:
1. If we're going to canonicalize, we should probably canonicalize the same way independent of the original argument type (so we would introduce bitcasts either way).

Ah, right - kill #1 in my list.

2. Those two functions are only equivalent on little-endian platforms.

I was wondering about that. So yes, if we do want to canonicalize (until the recent compile-time complaints, I always thought this was the objective of InstCombine...maybe it still is), then the masks we're matching or generating will differ based on endianness.


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