[flang-commits] [flang] Skip compiler directives between OMP PARALLEL DO and the loop (PR #81021)

[Mats Petersson via flang-commits]

https://github.com/Leporacanthicus created https://github.com/llvm/llvm-project/pull/81021

This fixes a compilation error when code like this is presented to the compiler:

  !$OMP PARALLEL DO
  !DIR$ VECTOR ALIGNED
  DO 20 i=1,N
     a = a + 0.5
20   CONTINUE

The directive itself is later ignored (with a warning that this is happening), but because the compiler already errored out before that point, it completely fails to compile this code. Other compilers accept the code without complaints.

>From 851b4173418073afa23cdf5cecdcb4a52b1b5251 Mon Sep 17 00:00:00 2001
From: Mats Petersson <mats.petersson at arm.com>
Date: Wed, 7 Feb 2024 17:29:36 +0000
Subject: [PATCH] Skip compiler directives between OMP PARALLEL DO and the loop

This fixes a compilation error when code like this is presented to the
compiler:

  !$OMP PARALLEL DO
  !DIR$ VECTOR ALIGNED
  DO 20 i=1,N
     a = a + 0.5
20   CONTINUE

The directive itself is later ignored (with a warning that this is
happening), but because the compiler already errored out before that
point, it completely fails to compile this code. Other compilers
accept the code without complaints.
---
 flang/lib/Semantics/canonicalize-omp.cpp         | 16 +++++++++++-----
 flang/test/Semantics/OpenMP/loop-association.f90 |  7 +++++++
 2 files changed, 18 insertions(+), 5 deletions(-)

diff --git a/flang/lib/Semantics/canonicalize-omp.cpp b/flang/lib/Semantics/canonicalize-omp.cpp
index 013fb408214ee..01adcf5372842 100644
--- a/flang/lib/Semantics/canonicalize-omp.cpp
+++ b/flang/lib/Semantics/canonicalize-omp.cpp
@@ -90,7 +90,11 @@ class CanonicalizationOfOmp {
     auto &dir{std::get<parser::OmpLoopDirective>(beginDir.t)};
 
     nextIt = it;
-    if (++nextIt != block.end()) {
+    while (++nextIt != block.end()) {
+      // Ignore compiler directives.
+      if (auto *directive{GetConstructIf<parser::CompilerDirective>(*nextIt)})
+        continue;
+
       if (auto *doCons{GetConstructIf<parser::DoConstruct>(*nextIt)}) {
         if (doCons->GetLoopControl()) {
           // move DoConstruct
@@ -111,12 +115,14 @@ class CanonicalizationOfOmp {
               "DO loop after the %s directive must have loop control"_err_en_US,
               parser::ToUpperCaseLetters(dir.source.ToString()));
         }
-        return; // found do-loop
+      } else {
+        messages_.Say(dir.source,
+            "A DO loop must follow the %s directive"_err_en_US,
+            parser::ToUpperCaseLetters(dir.source.ToString()));
       }
+      // If we get here, we either found a loop, or issued an error message.
+      return;
     }
-    messages_.Say(dir.source,
-        "A DO loop must follow the %s directive"_err_en_US,
-        parser::ToUpperCaseLetters(dir.source.ToString()));
   }
 
   void RewriteOmpAllocations(parser::ExecutionPart &body) {
diff --git a/flang/test/Semantics/OpenMP/loop-association.f90 b/flang/test/Semantics/OpenMP/loop-association.f90
index 8a28fd8878f49..29fca3b4be590 100644
--- a/flang/test/Semantics/OpenMP/loop-association.f90
+++ b/flang/test/Semantics/OpenMP/loop-association.f90
@@ -30,6 +30,13 @@
      c = c - 1
   END DO outer
 
+  ! Accept directives between parallel do and actual loop.
+  !$OMP PARALLEL DO
+  !DIR$ VECTOR ALIGNED
+  DO 20 i=1,N
+     a = a + 0.5
+20   CONTINUE
+
   c = 16
   !ERROR: DO loop after the PARALLEL DO directive must have loop control
   !$omp parallel do