llvm/llvm-project

[X86] Add support for X86ISD float2int constant folding

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#37,146 opened on Jun 14, 2018

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Description
Bugzilla Link 37798
Version trunk
OS Windows NT
Depends On llvm/llvm-project#37099
CC @adibiagio,@topperc,@filcab,@gregbedwell,@LebedevRI,@rotateright

Extended Description

Split off from [Bug #​37751].

Now that we've standardised to using X86ISD opcodes for float2int conversions:

  // Vector float/double to signed/unsigned integer.
  CVTP2SI, CVTP2UI, CVTP2SI_RND, CVTP2UI_RND,
  // Scalar float/double to signed/unsigned integer.
  CVTS2SI_RND, CVTS2UI_RND,

  // Vector float/double to signed/unsigned integer with truncation.
  CVTTP2SI, CVTTP2UI, CVTTP2SI_RND, CVTTP2UI_RND,
  // Scalar float/double to signed/unsigned integer with truncation.
  CVTTS2SI_RND, CVTTS2UI_RND,

We should investigate adding safe constant folding support to these. We could at least support exact conversions (these should be rounding mode independent for non-truncation instructions?).

For truncation instructions we can probably accept inexact conversions as well, as long as no underflow/overflow occurs - this would need to be tested to confirm.

Contributor guide

Research direction
Study the existing X86ISD float2int opcodes and implement constant folding by handling exact conversions and truncation with overflow checks.
Tech stack
cpp
Domain
compilersbackend
Issue type
Feature
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
4
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
Over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
C++LLVMcompiler backend
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20