llvm/llvm-project

[DAG] Add MVT:changeVectorElementCount and EVT::changeVectorElementCount

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#174,584 opened on Jan 6, 2026

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good first issuellvm:SelectionDAG

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Description

Similar to EVT::changeVectorElementType - but this will allow us to adjust the ElementCount on the fly.

We should assert that the original type was a vector already, but we will need to support Simple<->Extended transitions similar to EVT::changeVectorElementType.

There are a number of EVT::getVectorVT() calls in DAGCombine that would benefit from converting to changeVectorElementCount for clarity, but don't attempt to do too many in an initial patch

Contributor guide

Research direction
Study the existing implementation of EVT::changeVectorElementType as a pattern. Understand the MVT and EVT classes and the simple to extended transition logic. Add corresponding MVT::changeVectorElementCount and EVT::changeVectorElementCount functions. Also update relevant DAGCombine calls to use the new functions where appropriate, but limit initial patch to just adding the methods and one or two use cases.
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
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++Git
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
60