llvm/llvm-project

[OpenMP] Assertion `llvm::isUIntN(BitWidth, val) && "Value is not an N-bit unsigned value"' failed.

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#139,268 opened on May 9, 2025

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clang:frontendclang:openmpconfirmedcrashgood first issue

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Description

Reproducible on Godbolt: https://godbolt.org/z/xzM7oEf33

void f(void) {
#pragma omp unroll partial(0xFFFFFFFFF)
  for (int i = 0; i < 10; i++)
    ;
}

Contributor guide

Research direction
Reproduce the assertion failure with the provided godbolt link. Examine the OpenMP unroll implementation in Clang/LLVM to understand how the 'partial' argument is handled. The value 0xFFFFFFFFF is 40 bits, which exceeds the typical immediate field size. Trace the code path from parsing in Clang to code generation, and identify where an assertion `isUIntN` is triggered. Determine if the input should be validated earlier or if the representation needs adjustment.
Tech stack
None
Domain
backend
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
C++compiler concepts
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40