Clang error: OpenMP constructs may not be nested inside a simd region... (err_omp_prohibited_region_simd)

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error: OpenMP constructs may not be nested inside a simd region
 
except for ordered simd, simd, scan, or atomic directive

(since 11.0)

error: OpenMP constructs may not be nested inside a simd region
 
except for ordered simd, simd or atomic directive

(10.0)
error: OpenMP constructs may not be nested inside a simd region (until 9.0)

Type Error
Category OpenMP Issue
Internal Id err_omp_prohibited_region_simd
Internal Message OpenMP constructs may not be nested inside a simd region%select{| except for ordered simd, simd, scan, or atomic directive}0 (since 11.0)
OpenMP constructs may not be nested inside a simd region%select{| except for ordered simd, simd or atomic directive}0 (10.0)
OpenMP constructs may not be nested inside a simd region (until 9.0)
Regular Expression (?:error|fatal error)\: OpenMP constructs may not be nested inside a simd region(?:| except for ordered simd, simd, scan, or atomic directive)
First Commit 2014-06-24 549210e783c7 [OPENMP] Added initial checking of nesting of OpenMP regions.

Description

The error is issued by the Clang compiler when an attempt is made to nest OpenMP directives within a SIMD region in a manner that is not allowed by the OpenMP specification. Clang supports OpenMP, a parallel programming model that accelerates the execution of code across multiple processor cores. A SIMD (Single Instruction, Multiple Data) region is a specific type of OpenMP construct designed to vectorize code, executing the same operation on multiple data points simultaneously to improve performance.

When programming with OpenMP, certain constructs and regions can be nested within one another, allowing for complex parallelism strategies. However, the ability to nest these constructs is regulated by the OpenMP specification to ensure that the resulting parallel code can be executed correctly and efficiently. This particular error is triggered when an OpenMP directive that is not permitted to be nested inside a SIMD region is encountered. The OpenMP specification allows for some directives, like ordered simd, simd, scan, and atomic, to be nested within a SIMD region under specific conditions.

The versions of the error message provide insight into how the rules for nesting these constructs have evolved over time with different versions of the OpenMP standard and the Clang compiler. Starting from version 11.0 of Clang, certain directives are explicitly allowed within SIMD regions, reflecting changes or clarifications in the OpenMP specification. In contrast, before version 9.0, the error message was less specific, indicating a time when the restrictions on nesting within SIMD regions were either more stringent or less clearly defined.

This error is critical for developers working with OpenMP to understand, as nesting constructs improperly can lead to incorrect parallelism logic and potentially hinder performance optimizations that SIMD regions aim to provide. By adhering to the OpenMP nesting rules signaled by such errors, developers can write more efficient and correct parallel code.

 
AI Generated

Example

In the following example, an OpenMP parallel region is defined, encapsulating a series of nested OpenMP directives aimed at parallel execution. Initially, a task is created within a single worker context by defining a dependency on the worker's thread number. This setup is intended to demonstrate parallel execution while managing dependencies between tasks for correctness and efficiency. Subsequently, an attempt is made to vectorize a loop using #pragma omp for simd, signifying the intent to execute iterations of the loop concurrently using SIMD capabilities. Inside this SIMD-enabled loop, an additional parallel construct is introduced with #pragma omp taskloop simd to further parallelize the workload across different iterations of the nested loop, aiming to leverage both task parallelism and vectorization for performance gains.

The example is designed to showcase the interplay between different types of parallelism in OpenMP: task parallelism, vectorization, and nested parallel constructs. However, this configuration triggers the error message due to violating the OpenMP specifications regarding nesting certain constructs inside a SIMD region. The compiler error output elucidates the specific constraint being violated, emphasizing the conditions under which OpenMP constructs can be nested within SIMD regions according to the Clang compiler's interpretation of the OpenMP standards. Through this, the example illustrates a critical compilation constraint for developers utilizing OpenMP for parallel programming, especially in scenarios where maximized performance is sought through the combined use of SIMD vectorization and task parallelism.

 
AI Generated


Flags -xc++ -fopenmp

[Try out in Compiler Explorer]

Source 
#include <omp.h>
int main() {
  #pragma omp parallel
  {
    #pragma omp single
    {
      #pragma omp task depend(inout: omp_get_thread_num())
    }
  }
  #pragma omp for simd
  for(int i = 0; i < 10; ++i) {
    #pragma omp taskloop simd
    for(int j = 0; j < 10; ++j) {
    }
  }
  return 0;
}
Compiler Output 
<source>:7:38: error: expected addressable lvalue expression, array element, array section or array shaping expression of non 'omp_depend_t' type
<source>:8:5: error: expected statement
<source>:12:5: error: OpenMP constructs may not be nested inside a simd region except for ordered simd, simd, scan, or atomic directive


Clang Internals (17.0.6)

Git Commit Message

[OPENMP] Added initial checking of nesting of OpenMP regions.

llvm-svn: 211566

Used in Clang Sources

This section lists all occurrences of the diagnostic within the Clang's codebase. For each occurrence, an auto-extracted snipped from the source code is listed including key elements like control structures, functions, or classes. It should illustrate the conditions under which the diagnostic is activated.

clang/lib/Sema/SemaOpenMP.cpp (line 5009)

static bool checkNestingOfRegions(Sema &SemaRef, const DSAStackTy *Stack, OpenMPDirectiveKind CurrentRegion, const DeclarationNameInfo &CurrentName, OpenMPDirectiveKind CancelRegion, OpenMPBindClauseKind BindKind, SourceLocation StartLoc) {
  if (Stack->getCurScope()) {
    // ...
    if (isOpenMPSimdDirective(ParentRegion) && ((SemaRef.LangOpts.OpenMP <= 45 && CurrentRegion != OMPD_ordered) || (SemaRef.LangOpts.OpenMP >= 50 && CurrentRegion != OMPD_ordered && CurrentRegion != OMPD_simd && CurrentRegion != OMPD_atomic && CurrentRegion != OMPD_scan))) {
      // ...
      SemaRef.Diag(StartLoc, (CurrentRegion != OMPD_simd) ? diag::err_omp_prohibited_region_simd : diag::warn_omp_nesting_simd) << (SemaRef.LangOpts.OpenMP >= 50 ? 1 : 0);

clang/lib/Sema/SemaOpenMP.cpp (line 11361)

StmtResult Sema::ActOnOpenMPOrderedDirective(ArrayRef<OMPClause *> Clauses, Stmt *AStmt, SourceLocation StartLoc, SourceLocation EndLoc) {
  // ...
  if (!ErrorFound && !SC && isOpenMPSimdDirective(DSAStack->getParentDirective())) {
    // ...
    Diag(StartLoc, diag::err_omp_prohibited_region_simd) << (LangOpts.OpenMP >= 50 ? 1 : 0);

Triggered in Clang Tests

This section lists all internal Clang test cases that trigger the diagnostic.

clang/test/OpenMP/target_teams_distribute_parallel_for_simd_misc_messages.c

  • clang/test/OpenMP/target_teams_distribute_parallel_for_simd_misc_messages.c:172:1: error: OpenMP constructs may not be nested inside a simd region except for ordered simd, simd, scan, or atomic directive


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