llvm/llvm-project

[Clang] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - allow SSE/AVX2 VMPSADBW intrinsics to be used in constexpr

Closed

#157,522 opened on Sep 8, 2025

View on GitHubContributor guide
 (10 comments) (0 reactions) (0 assignees)C++ (10,782 forks)batch import
backend:X86clang:bytecodeclang:frontendconstexprgood first issue

Repository metrics

Stars
 (26,378 stars)
PR merge metrics
 (Avg merge 1d 2h) (1,000 merged PRs in 30d)

Description

The horizontal sum of difference intrinsics can all be used in constexpr with suitable handling of the builtins inside VectorExprEvaluator::VisitCallExpr and InterBuiltin.cpp, and testing in the relevent *-builtins.c files.

_mm_mpsadbw_epu8 _mm256_mpsadbw_epu8

These intrinsics can be tricky to correctly evaluate, so I'd recommend previous knowledge of SSE/AVX instructions is required

Care needs to be taken with the signed/unsigned extension of the arguments - we might need to adjust the builtin signatures.

Contributor guide

Research direction
Examine VectorExprEvaluator::VisitCallExpr and InterpretBuiltin.cpp for existing SSE/AVX intrinsic handling, then implement similar handling for mm mpsadbw epu8 and mm256 mpsadbw epu8, ensuring correct signed/unsigned extension.
Tech stack
None
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.
1-2 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
SSE/AVX instruction knowledgeClang constexpr internals
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
30