llvm/llvm-project

[clang] Bitcasts from vectors to integers not supported at compile time

Open

#159,905 opened on Sep 20, 2025

View on GitHub
 (14 comments) (0 reactions) (1 assignee)C++ (10,782 forks)batch import
clang:frontendconstexprgood first issue

Repository metrics

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

Description

I have test case that seems to regressed for latest constexpr changes.

#include<immintrin.h>
constexpr
__m64 foo(__m64 a, int b){
    return (__m64)(long long)a << b;
}
int main(){
    foo((__m64){5},1);
    return 0;
}

gcc-trunk works fine. clang-trunk errors out.

<source>:3:7: error: constexpr function never produces a constant expression [-Winvalid-constexpr]
    3 | __m64 foo(__m64 a, int b){
      |       ^~~
<source>:4:19: note: subexpression not valid in a constant expression
    4 |     return (__m64)(long long)a << b;

Following change passed

#include<immintrin.h>
constexpr
__m64 foo(__m64 a, int b){
    if(b > 63)
        return (__m64){0};
    return (__m64)(long long)a << b;
}
int main(){
    foo((__m64){5},1);
    return 0;
}

here is repro https://godbolt.org/z/d7obEcb57 Does anyone know what may be trouble here?

Contributor guide

Research direction
Investigate the constant expression evaluator in clang to support bitcasts from vector types to integer types.
Tech stack
cpp
Domain
backendtoolingdeveloper experience
Issue type
Bug
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.
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
None
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
20