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

[Friedman, Eli via llvm-dev]

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

-Eli

-- 
Employee of Qualcomm Innovation Center, Inc.
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project

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