llvm/llvm-project

[AArch64][GlobalISel] ABD operations and combines

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#118,085 opened on Nov 29, 2024

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backend:AArch64good first issuellvm:globaliselmissed-optimization

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Description

SDAG, via these nodes can pattern-recognise and optimise abd nodes. We should add the same operations for global isel.

  /// ABDS/ABDU - Absolute difference - Return the absolute difference between
  /// two numbers interpreted as signed/unsigned.
  /// i.e trunc(abs(sext(Op0) - sext(Op1))) becomes abds(Op0, Op1)
  ///  or trunc(abs(zext(Op0) - zext(Op1))) becomes abdu(Op0, Op1)
  ABDS,
  ABDU,

There are tests in, for example, llvm/test/CodeGen/AArch64/abd-combine.ll. https://godbolt.org/z/71PGjf486

Contributor guide

Research direction
Study the existing SDAG ABD implementation in SelectionDAG, review the test file llvm/test/CodeGen/AArch64/abd combine.ll, and implement similarly in GlobalISel using the new ABDS/ABDU G ops.
Tech stack
cpp
Domain
backend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
3-5 days
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Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++LLVMcompiler fundamentals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40