llvm/llvm-project

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

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#174584 opened on Jan 6, 2026

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good first issuellvm:SelectionDAG

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

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.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
blocked
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
mostly clear
Prerequisites
Basic understanding of LLVMFamiliarity with EVT and SelectionDAG
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
25
Research direction
Examine the existing implementation of EVT::changeVectorElementType in llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp. Understand the Simple< >Extended transition logic. Implement EVT::changeVectorElementCount similarly, ensuring assertion that original type is a vector. Then add MVT::changeVectorElementCount, referencing the same file. Review existing getVectorVT() calls in DAGCombine to optionally convert them later, but keep initial patch minimal.