llvm/llvm-project

[Clang] VectorExprEvaluator::VisitCallExpr / InterpretBuiltin - add MMX/SSE/AVX/AVX512 VPSADBW intrinsics to be used in constexpr

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#188062 opened on Mar 23, 2026

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backend:X86clang:bytecodeclang:frontendconstexprgood first issue

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_sad_epu8 _mm256_sad_epu8 _mm512_sad_epu8 _mm_sad_pu8

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

Review other sub-issues from #30794 on implementation

Split off from #157522

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.
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.
mostly clear
Prerequisites
LLVM VectorExprEvaluatorSSE/AVX intrinsicsC++ constexprLLVM testing
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
15
Research direction
Study the existing handling of similar intrinsics in VectorExprEvaluator::VisitCallExpr and InterpretBuiltin.cpp. Review the patterns used in sub issues from #30794 for implementing constexpr intrinsics. Pay attention to sign/zero extension handling in builtin signatures. Implement mm sad epu8, mm256 sad epu8, mm512 sad epu8, and mm sad pu8 with corresponding tests in * builtins.c files.