Clang error: region cannot be... nested inside 'B' region... (err_omp_prohibited_region)

Text	error: region cannot be   closely nested inside 'B' region   ; perhaps you forget to enclose 'omp D' directive into a parallel region? ; perhaps you forget to enclose 'omp D' directive into a for or a parallel for region with 'ordered' clause? ; perhaps you forget to enclose 'omp D' directive into a target region? ; perhaps you forget to enclose 'omp D' directive into a teams region? ; perhaps you forget to enclose 'omp D' directive into a for, simd, for simd, parallel for, or parallel for simd region? (since 11.0) error: region cannot be   closely nested inside 'B' region   ; perhaps you forget to enclose 'omp D' directive into a parallel region? ; perhaps you forget to enclose 'omp D' directive into a for or a parallel for region with 'ordered' clause? ; perhaps you forget to enclose 'omp D' directive into a target region? ; perhaps you forget to enclose 'omp D' directive into a teams region? (until 10.0)
Type	Error
Category	OpenMP Issue
Internal Id	err_omp_prohibited_region
Internal Message	region cannot be%select{| closely}0 nested inside '%1' region%select{|; perhaps you forget to enclose 'omp %3' directive into a parallel region?|; perhaps you forget to enclose 'omp %3' directive into a for or a parallel for region with 'ordered' clause?|; perhaps you forget to enclose 'omp %3' directive into a target region?|; perhaps you forget to enclose 'omp %3' directive into a teams region?|; perhaps you forget to enclose 'omp %3' directive into a for, simd, for simd, parallel for, or parallel for simd region?}2 (since 11.0) region cannot be%select{| closely}0 nested inside '%1' region%select{|; perhaps you forget to enclose 'omp %3' directive into a parallel region?|; perhaps you forget to enclose 'omp %3' directive into a for or a parallel for region with 'ordered' clause?|; perhaps you forget to enclose 'omp %3' directive into a target region?|; perhaps you forget to enclose 'omp %3' directive into a teams region?}2 (until 10.0)
Regular Expression	(?:error|fatal error)\: region cannot be(?:| closely) nested inside '(.*?)' region(?:|; perhaps you forget to enclose 'omp (.*?)' directive into a parallel region\?|; perhaps you forget to enclose 'omp (.*?)' directive into a for or a parallel for region with 'ordered' clause\?|; perhaps you forget to enclose 'omp (.*?)' directive into a target region\?|; perhaps you forget to enclose 'omp (.*?)' directive into a teams region\?|; perhaps you forget to enclose 'omp (.*?)' directive into a for, simd, for simd, parallel for, or parallel for simd region\?)
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 OpenMP region is incorrectly nested within another OpenMP region. OpenMP, being a parallel programming model, requires specific nesting rules to be followed for its directive-driven constructs. These rules ensure that the parallel or work-sharing regions are structured correctly to avoid ambiguities and conflicts in the execution flow of parallelized code. In essence, the error points out that an OpenMP region, specified by its directive, has been placed inside another OpenMP region in a way that is not permitted by the OpenMP specification. This could occur for several reasons, such as: An OpenMP directive that is intended to be used within a parallel region is found outside of any such region. A directive that requires to be enclosed in a specific type of OpenMP region (e.g., `for`, `parallel for`, `target`, `teams`, etc.) is placed incorrectly. Certain directives that have additional nesting requirements, possibly inclusive of specific clauses like `ordered`, are not correctly structured according to those requirements. The error message is tailored to provide a hint about the potential fix, suggesting the proper nesting or enclosing of the problematic directive within the appropriate OpenMP region. This is meant to guide the user in restructuring their OpenMP pragma directives to comply with the defined nesting rules, thus ensuring the correct execution flow of the parallel or work-sharing constructs. This error helps maintain the integrity of the parallel construct by enforcing the correct use of the OpenMP directives, as per the OpenMP standard guidelines. It underlines the importance of adhering to the structured block requirements and the hierarchical relationship between different OpenMP regions within a program.

AI Generated

Example

In the following example, the Clang compiler identifies an error resulting from improper nesting of OpenMP regions in a C++ program. The problem occurs when an omp ordered directive, intended to impose a specific execution order on loop iterations, is placed within a loop that has not been explicitly marked as ordered. The omp parallel directive initiates a parallel region, and the subsequent omp for directive distributes the loop iterations among threads within this parallel region. However, incorporating an omp ordered directive inside the loop mandates that the loop be associated with an 'ordered' clause, signifying that the loop iterations should execute according to their original sequence in the code. The error message provided by Clang suggests enclosing the omp ordered directive within a for or a parallel for region that includes an 'ordered' clause, rectifying the configuration to adhere to OpenMP standards and ensure the program's correctness when employing ordered parallel loops.

AI Generated

Flags	-fopenmp -xc	[Try out in Compiler Explorer]
Source	#include <omp.h> int main() { #pragma omp parallel #pragma omp for for(int i = 0; i < 10; i++) { #pragma omp ordered ; } return 0; }
Compiler Output	<source>:6:1: error: region cannot be closely nested inside 'for' region; perhaps you forget to enclose 'omp ordered' directive into a for or a parallel for region with 'ordered' clause?

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 5237)

static bool checkNestingOfRegions(Sema &SemaRef, const DSAStackTy *Stack, OpenMPDirectiveKind CurrentRegion, const DeclarationNameInfo &CurrentName, OpenMPDirectiveKind CancelRegion, OpenMPBindClauseKind BindKind, SourceLocation StartLoc) {
  if (Stack->getCurScope()) {
    // ...
    if (NestingProhibited) {
      if (OrphanSeen) {
      // ...
      } else {
        SemaRef.Diag(StartLoc, diag::err_omp_prohibited_region) << CloseNesting << getOpenMPDirectiveName(OffendingRegion) << Recommend << getOpenMPDirectiveName(CurrentRegion);

Triggered in Clang Tests

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

clang/test/OpenMP/target_teams_generic_loop_messages.cpp

• clang/test/OpenMP/target_teams_generic_loop_messages.cpp:21:5: error: region cannot be closely nested inside 'target teams loop' region

• clang/test/OpenMP/target_teams_generic_loop_messages.cpp:30:5: error: region cannot be closely nested inside 'target teams loop' region

• clang/test/OpenMP/target_teams_generic_loop_messages.cpp:41:5: error: region cannot be closely nested inside 'target teams loop' region; perhaps you forget to enclose 'omp ordered' directive into a for or a parallel for region with 'ordered' clause?

• clang/test/OpenMP/target_teams_generic_loop_messages.cpp:48:5: error: region cannot be closely nested inside 'target teams loop' region; perhaps you forget to enclose 'omp ordered' directive into a for or a parallel for region with 'ordered' clause?

• clang/test/OpenMP/target_teams_generic_loop_messages.cpp:55:5: error: region cannot be closely nested inside 'target teams loop' region; perhaps you forget to enclose 'omp ordered' directive into a for or a parallel for region with 'ordered' clause?