[flang-commits] [flang] [flang][cuda] Avoid crash when the force modifier is used (PR #160176)

[Valentin Clement バレンタイン クレメン via flang-commits]

https://github.com/clementval created https://github.com/llvm/llvm-project/pull/160176

When the force modifier is used on the collapse clause, statements can appear between the nested loops. Update the `getNextDoConstruct` lambda to take this into account.

Note that the force collapse modifier triggers a TODO later in lowering. 

>From c325c64b4f80febbdde4e6c6400fbe504d363f2f Mon Sep 17 00:00:00 2001
From: Valentin Clement <clementval at gmail.com>
Date: Mon, 22 Sep 2025 11:49:14 -0700
Subject: [PATCH] [flang][cuda] Avoid crash when the force modifier is used

---
 flang/lib/Semantics/resolve-directives.cpp    | 27 ++++++++++++++-----
 .../Semantics/OpenACC/acc-collapse-force.f90  | 19 +++++++++++++
 2 files changed, 40 insertions(+), 6 deletions(-)
 create mode 100644 flang/test/Semantics/OpenACC/acc-collapse-force.f90

diff --git a/flang/lib/Semantics/resolve-directives.cpp b/flang/lib/Semantics/resolve-directives.cpp
index caee3af8a7ca1..dd0a94f0b816f 100644
--- a/flang/lib/Semantics/resolve-directives.cpp
+++ b/flang/lib/Semantics/resolve-directives.cpp
@@ -315,6 +315,7 @@ class AccAttributeVisitor : DirectiveAttributeVisitor<llvm::acc::Directive> {
 
 private:
   std::int64_t GetAssociatedLoopLevelFromClauses(const parser::AccClauseList &);
+  bool HasForceCollapseModifier(const parser::AccClauseList &);
 
   Symbol::Flags dataSharingAttributeFlags{Symbol::Flag::AccShared,
       Symbol::Flag::AccPrivate, Symbol::Flag::AccFirstPrivate,
@@ -333,7 +334,7 @@ class AccAttributeVisitor : DirectiveAttributeVisitor<llvm::acc::Directive> {
       Symbol::Flag::AccDevicePtr, Symbol::Flag::AccDeviceResident,
       Symbol::Flag::AccLink, Symbol::Flag::AccPresent};
 
-  void CheckAssociatedLoop(const parser::DoConstruct &);
+  void CheckAssociatedLoop(const parser::DoConstruct &, bool forceCollapsed);
   void ResolveAccObjectList(const parser::AccObjectList &, Symbol::Flag);
   void ResolveAccObject(const parser::AccObject &, Symbol::Flag);
   Symbol *ResolveAcc(const parser::Name &, Symbol::Flag, Scope &);
@@ -1168,7 +1169,7 @@ bool AccAttributeVisitor::Pre(const parser::OpenACCLoopConstruct &x) {
   ClearDataSharingAttributeObjects();
   SetContextAssociatedLoopLevel(GetAssociatedLoopLevelFromClauses(clauseList));
   const auto &outer{std::get<std::optional<parser::DoConstruct>>(x.t)};
-  CheckAssociatedLoop(*outer);
+  CheckAssociatedLoop(*outer, HasForceCollapseModifier(clauseList));
   return true;
 }
 
@@ -1366,7 +1367,7 @@ bool AccAttributeVisitor::Pre(const parser::OpenACCCombinedConstruct &x) {
   const auto &clauseList{std::get<parser::AccClauseList>(beginBlockDir.t)};
   SetContextAssociatedLoopLevel(GetAssociatedLoopLevelFromClauses(clauseList));
   const auto &outer{std::get<std::optional<parser::DoConstruct>>(x.t)};
-  CheckAssociatedLoop(*outer);
+  CheckAssociatedLoop(*outer, HasForceCollapseModifier(clauseList));
   ClearDataSharingAttributeObjects();
   return true;
 }
@@ -1478,6 +1479,18 @@ bool AccAttributeVisitor::Pre(const parser::OpenACCCacheConstruct &x) {
   return true;
 }
 
+bool AccAttributeVisitor::HasForceCollapseModifier(
+    const parser::AccClauseList &x) {
+  for (const auto &clause : x.v) {
+    if (const auto *collapseClause{
+            std::get_if<parser::AccClause::Collapse>(&clause.u)}) {
+      const parser::AccCollapseArg &arg = collapseClause->v;
+      return std::get<bool>(arg.t);
+    }
+  }
+  return false;
+}
+
 std::int64_t AccAttributeVisitor::GetAssociatedLoopLevelFromClauses(
     const parser::AccClauseList &x) {
   std::int64_t collapseLevel{0};
@@ -1499,14 +1512,14 @@ std::int64_t AccAttributeVisitor::GetAssociatedLoopLevelFromClauses(
 }
 
 void AccAttributeVisitor::CheckAssociatedLoop(
-    const parser::DoConstruct &outerDoConstruct) {
+    const parser::DoConstruct &outerDoConstruct, bool forceCollapsed) {
   std::int64_t level{GetContext().associatedLoopLevel};
   if (level <= 0) { // collapse value was negative or 0
     return;
   }
 
   const auto getNextDoConstruct =
-      [this](const parser::Block &block,
+      [this, forceCollapsed](const parser::Block &block,
           std::int64_t &level) -> const parser::DoConstruct * {
     for (const auto &entry : block) {
       if (const auto *doConstruct = GetDoConstructIf(entry)) {
@@ -1524,7 +1537,9 @@ void AccAttributeVisitor::CheckAssociatedLoop(
             "LOOP directive not expected in COLLAPSE loop nest"_err_en_US);
         level = 0;
       } else {
-        break;
+        if (!forceCollapsed) {
+          break;
+        }
       }
     }
     return nullptr;
diff --git a/flang/test/Semantics/OpenACC/acc-collapse-force.f90 b/flang/test/Semantics/OpenACC/acc-collapse-force.f90
new file mode 100644
index 0000000000000..80b1060ebe6c7
--- /dev/null
+++ b/flang/test/Semantics/OpenACC/acc-collapse-force.f90
@@ -0,0 +1,19 @@
+! RUN: %python %S/../test_errors.py %s %flang -fopenacc -fsyntax-only
+
+! Check that loop with force collapse do not break in the semantic step.
+subroutine sub3()
+  integer :: i, j
+  integer, parameter :: n = 100, m = 200
+  real, dimension(n, m) :: a
+  real, dimension(n) :: bb
+  real :: r
+  a = 1
+  r = 0
+  !$acc parallel loop collapse(force:2) copy(a)
+  do i = 1, n
+    bb(i) = r
+    do j = 1, m
+      a(i,j) = r * a(i,j)
+    enddo
+  enddo
+end subroutine