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

[Rackover, Zvi via llvm-dev]

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]
Sent: Thursday, January 12, 2017 20:19
To: Friedman, Eli <efriedma at codeaurora.org>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Rackover, Zvi <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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