[llvm-dev] Does middle-end pass need to consider some special type when doing optimization? Or letting back-end to revert the optimization accordingly?

[Luo, Yuanke via llvm-dev]

Hi,

I discussed with Florian for a solution at https://reviews.llvm.org/D99152. Florian suggest introducing a specific intrinsic to replace bitcast in front-end, and backend need extra effort to optimize or eliminate the intrinsic. This idea looks good to me. Here is my plan.

  1.  specify x86_amx in LangRef and verify the IR. Patches were uploaded at https://reviews.llvm.org/D100032 and https://reviews.llvm.org/D100472.
  2.  Add llvm.x86.tile.cast intrinsic in LLVM.
  3.  Optimize some of llvm.x86.tile.cast code as bitcast does, and transform llvm.x86.tile.cast to amx intrinsic if it can't be eliminated.
  4.  After the above 3 items are finished, replace bitcast with llvm.x86.tile.cast in front-end when generate IR for amx builtin.
  5.  After some time for stabilization, remove bitcast transform code from LLVM.

Thanks
Yuanke

From: Florian Hahn <florian_hahn at apple.com>
Sent: Tuesday, March 23, 2021 6:16 PM
To: Luo, Yuanke <yuanke.luo at intel.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Zhang, Xiang1 <xiang1.zhang at intel.com>; James Y Knight <jyknight at google.com>
Subject: Re: [llvm-dev] Does middle-end pass need to consider some special type when doing optimization? Or letting back-end to revert the optimization accordingly?




On Mar 23, 2021, at 08:21, Luo, Yuanke <yuanke.luo at intel.com<mailto:yuanke.luo at intel.com>> wrote:

I prototyped the approach 1 at https://reviews.llvm.org/D99152 and I realized that sometimes bitcast optimization in middle-end is helpful. For the test case of inner_product(), we need extra effort eliminate llvm.x86.vector.amx.cast.x86amx.v256i32 by ourselves.


I think that’s expected, you might need to add some optimizations for the conversion intrinsic. But that can easily be limited to the AMX specific passes and all existing LLVM transformations should remain correct without changes.

Cheers,
Florian
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