[all-commits] [llvm/llvm-project] 62fc58: [AArch64] Improve codegen for "trunc <4 x i64> to ...

[Sheng via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 62fc58a61d153daa015b9e22b0c386c74a5f7076
      https://github.com/llvm/llvm-project/commit/62fc58a61d153daa015b9e22b0c386c74a5f7076
  Author: Sheng <ox59616e at gmail.com>
  Date:   2022-10-13 (Thu, 13 Oct 2022)

  Changed paths:
    M llvm/lib/Target/AArch64/AArch64ISelLowering.cpp
    M llvm/test/CodeGen/AArch64/aarch64-uzp1-combine.ll
    M llvm/test/CodeGen/AArch64/fptosi-sat-vector.ll
    M llvm/test/CodeGen/AArch64/fptoui-sat-vector.ll

  Log Message:
  -----------
  [AArch64] Improve codegen for "trunc <4 x i64> to <4 x i8>" for all cases

To achieve this, we need this observation:

`uzp1` is just a `xtn` that operates on two registers

For example, given the following register with type v2i64:

LSB_______MSB

x0 x1	x2 x3

Applying xtn on it we get:

x0	x2

This is equivalent to bitcast it to v4i32, and then applying uzp1 on it:

x0	x1	x2	x3
   |
  uzp1
   v
x0	x2	<value from other register>

We can transform xtn to uzp1 by this observation, and vice versa.

This observation only works on little endian target. Big endian target has
a problem: the uzp1 cannot be replaced by xtn since there is a discrepancy
in the behavior of uzp1 between the little endian and big endian.

To illustrate, take the following for example:

LSB____________________MSB

x0	x1	x2	x3

On little endian, uzp1 grabs x0 and x2, which is right; on big endian, it
grabs x3 and x1, which doesn't match what I saw on the document. But, since
I'm new to AArch64, take my word with a pinch of salt. This bevavior is
observed on gdb, maybe there's issue in the order of the value printed by it ?

Whatever the reason is, the execution result given by qemu just doesn't match.
So I disable this on big endian target temporarily until we find the crux.

Fixes #57502

Reviewed By: dmgreen, mingmingl

Co-authored-by: Mingming Liu <mingmingl at google.com>

Differential Revision: https://reviews.llvm.org/D133850