llvm/llvm-project

[Clang] Allow __builtin_fma/fmaf/fmal to be used in a constant expression

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#154,747 opened on Aug 21, 2025

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clang:bytecodeclang:frontendconstexprgood first issue

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Description

Building on #152455 - now that __builtin_elementwise_fma can be used with constexpr, the scalar __builtin_fma/fmaf/fmal/f16/f128 entries should be able to use the same codepaths.

Contributor guide

Research direction
Study how builtin elementwise fma was made constexpr in #152455 and replicate the logic for scalar builtin fma variants. Focus on clang/lib/AST/ExprConstant.cpp and clang/lib/Sema/SemaExpr.cpp to find the constant expression evaluation path for builtins.
Tech stack
ccpp
Domain
backendtooling
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++Clang internalsASTconstant expression evaluation
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20