Update based on review comments, consisting of:

* Accept only if statements, rather than an assignment statement.
* Modify tests to match the if-statment required.
* Add test to check for the TODO in lowering.

Author: Leporacanthicus

flang/include/flang/Parser/dump-parse-tree.h

@@ -485,6 +485,7 @@ class ParseTreeDumper {
   NODE(OmpAtomicCapture, Stmt1)
   NODE(OmpAtomicCapture, Stmt2)
   NODE(parser, OmpAtomicCompare)
+  NODE(parser, OmpAtomicCompareIfStmt)
   NODE(parser, OmpAtomicRead)
   NODE(parser, OmpAtomicUpdate)
   NODE(parser, OmpAtomicWrite)

flang/include/flang/Parser/parse-tree.h

@@ -4126,12 +4126,17 @@ struct OmpAtomicCapture {
       t;
 };
 
-// ATOMCI COMPARE (OpenMP 5.1, Spec: 15.8.4)
+struct OmpAtomicCompareIfStmt {
+  UNION_CLASS_BOILERPLATE(OmpAtomicCompareIfStmt);
+  std::variant<common::Indirection<IfStmt>, common::Indirection<IfConstruct>> u;
+};
+
+// ATOMIC COMPARE (OpenMP 5.1, OPenMP 5.2 spec: 15.8.4)
 struct OmpAtomicCompare {
   TUPLE_CLASS_BOILERPLATE(OmpAtomicCompare);
   CharBlock source;
   std::tuple<OmpAtomicClauseList, Verbatim, OmpAtomicClauseList,
-      Statement<AssignmentStmt>, std::optional<OmpEndAtomic>>
+      OmpAtomicCompareIfStmt, std::optional<OmpEndAtomic>>
       t;
 };
 

flang/lib/Parser/openmp-parsers.cpp

@@ -10,6 +10,7 @@
 // See OpenMP-4.5-grammar.txt for documentation.
 
 #include "basic-parsers.h"
+#include "debug-parser.h"
 #include "expr-parsers.h"
 #include "misc-parsers.h"
 #include "stmt-parser.h"
@@ -912,10 +913,15 @@ TYPE_PARSER("ATOMIC" >>
         Parser<OmpAtomicClauseList>{} / endOmpLine, statement(assignmentStmt),
         statement(assignmentStmt), Parser<OmpEndAtomic>{} / endOmpLine)))
 
+TYPE_PARSER(construct<OmpAtomicCompareIfStmt>(indirect(Parser<IfStmt>{})) ||
+    construct<OmpAtomicCompareIfStmt>(indirect(Parser<IfConstruct>{})))
+
+// OMP ATOMIC [MEMORY-ORDER-CLAUSE-LIST] COMPARE [MEMORY-ORDER-CLAUSE-LIST]
 TYPE_PARSER("ATOMIC" >>
     sourced(construct<OmpAtomicCompare>(
         Parser<OmpAtomicClauseList>{} / maybe(","_tok), verbatim("COMPARE"_tok),
-        Parser<OmpAtomicClauseList>{} / endOmpLine, statement(assignmentStmt),
+        Parser<OmpAtomicClauseList>{} / endOmpLine,
+        Parser<OmpAtomicCompareIfStmt>{},
         maybe(Parser<OmpEndAtomic>{} / endOmpLine))))
 
 // OMP ATOMIC [MEMORY-ORDER-CLAUSE-LIST] UPDATE [MEMORY-ORDER-CLAUSE-LIST]

flang/lib/Parser/unparse.cpp

@@ -2528,7 +2528,7 @@ class UnparseVisitor {
     Walk(std::get<2>(x.t));
     Put("\n");
     EndOpenMP();
-    Walk(std::get<Statement<AssignmentStmt>>(x.t));
+    Walk(std::get<OmpAtomicCompareIfStmt>(x.t));
   }
   void Unparse(const OmpAtomicRead &x) {
     BeginOpenMP();

flang/lib/Semantics/check-omp-structure.cpp

@@ -2411,9 +2411,8 @@ void OmpStructureChecker::CheckAtomicUpdateStmt(
 void OmpStructureChecker::CheckAtomicCompareConstruct(
     const parser::OmpAtomicCompare &atomicCompareConstruct) {
 
-  CheckAtomicWriteStmt(std::get<parser::Statement<parser::AssignmentStmt>>(
-      atomicCompareConstruct.t)
-                           .statement);
+  // TODO: Check that the if-stmt is `if (var == expr) var = new`
+  //       [with or without then/end-do]
 
   unsigned version{context_.langOptions().OpenMPVersion};
   if (version < 51) {

flang/test/Lower/OpenMP/Todo/atomic-compare.f90

@@ -0,0 +1,11 @@
+! RUN: %not_todo_cmd %flang_fc1 -emit-fir -fopenmp -fopenmp-version=51 -o - %s 2>&1 | FileCheck %s
+
+! CHECK: not yet implemented: OpenMP atomic compare
+program p
+  integer :: x
+  logical :: r
+  !$omp atomic compare
+  if (x .eq. 0) then
+     x = 2
+  end if
+end program p

flang/test/Parser/OpenMP/atomic-unparse.f90

@@ -3,7 +3,7 @@
 program main
    implicit none
    integer :: i, j = 10
-   logical :: k
+   integer :: k
 !READ
 !$omp atomic read
    i = j
@@ -124,27 +124,46 @@ program main
 
 !COMPARE
 !$omp atomic compare
-   r = i .eq. j
+   if (k == i) k = j
 !$omp atomic seq_cst compare
-   r = i .eq. j
+   if (k == j) then
+      k = i
+   end if
 !$omp atomic compare seq_cst
-   r = i .eq. j
+   if (k .eq. j) then
+      k = i
+   end if
 !$omp atomic release compare
-   r = i .eq. j
+   if (i .eq. j) k = i
 !$omp atomic compare release
-   r = i .eq. j
+   if (i .eq. j) then
+      i = k
+   end if
 !$omp atomic acq_rel compare
-   r = i .eq. j
+   if (k .eq. j) then
+      j = i
+   end if
 !$omp atomic compare acq_rel
-   r = i .eq. j
+   if (i .eq. j) then
+      i = k
+   end if
 !$omp atomic acquire compare
-   r = i .eq. j
+   if (i .eq. j + 1) then
+      i = j
+   end if
+   
 !$omp atomic compare acquire
-   r = i .eq. j
+   if (i .eq. j) then
+      i = k
+   end if
 !$omp atomic relaxed compare
-   r = i .eq. j
+   if (i .eq. j) then
+      i = k
+   end if
 !$omp atomic compare relaxed
-   r = i .eq. j
+   if (i .eq. k) then
+      i = j
+   end if
 
 !ATOMIC
 !$omp atomic

flang/test/Semantics/OpenMP/atomic-compare.f90

@@ -6,71 +6,73 @@
   ! higher openmp version than the others, so need a separate file.
 
 
-  real a, b
-  logical r
+  real a, b, c
   a = 1.0
   b = 2.0
+  c = 3.0
   !$omp parallel num_threads(4)
   ! First a few things that should compile without error.
   !$omp atomic seq_cst, compare
-  r = b .ne. a
+  if (b .eq. a) then
+     b = c
+  end if
 
   !$omp atomic seq_cst compare
-  r = a .ge. b
+  if (a .eq. b) a = c
   !$omp end atomic
 
   !$omp atomic compare acquire hint(OMP_LOCK_HINT_CONTENDED)
-  r = a .lt. b
+  if (b .eq. a) b = c
 
   !$omp atomic release hint(OMP_LOCK_HINT_UNCONTENDED) compare
-  r = a .gt. b
+  if (b .eq. a) b = c
 
   !$omp atomic compare seq_cst
-  r = b .ne. a
+  if (b .eq. c) b = a
 
   !$omp atomic hint(1) acq_rel compare
-  r = b .eq. a
+  if (b .eq. a) b = c
   !$omp end atomic
 
   ! Check for error conidtions:
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one SEQ_CST clause can appear on the COMPARE directive
   !$omp atomic seq_cst seq_cst compare
-  r = a .le. b
+  if (b .eq. c) b = a
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one SEQ_CST clause can appear on the COMPARE directive
   !$omp atomic compare seq_cst seq_cst
-  r = b .gt. a
+  if (b .eq. c) b = a
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one SEQ_CST clause can appear on the COMPARE directive
   !$omp atomic seq_cst compare seq_cst
-  r = b .ge. b
+  if (b .eq. c) b = a
 
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one ACQUIRE clause can appear on the COMPARE directive
   !$omp atomic acquire acquire compare
-  r = a .le. b
+  if (b .eq. c) b = a
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one ACQUIRE clause can appear on the COMPARE directive
   !$omp atomic compare acquire acquire
-  r = b .gt. a
+  if (b .eq. c) b = a
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one ACQUIRE clause can appear on the COMPARE directive
   !$omp atomic acquire compare acquire
-  r = b .ge. b
+  if (b .eq. c) b = a
 
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one RELAXED clause can appear on the COMPARE directive
   !$omp atomic relaxed relaxed compare
-  r = a .le. b
+  if (b .eq. c) b = a
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one RELAXED clause can appear on the COMPARE directive
   !$omp atomic compare relaxed relaxed
-  r = b .gt. a
+  if (b .eq. c) b = a
   !ERROR: More than one memory order clause not allowed on OpenMP Atomic construct
   !ERROR: At most one RELAXED clause can appear on the COMPARE directive
   !$omp atomic relaxed compare relaxed
-  r = b .ge. b
+  if (b .eq. c) b = a
 
   !$omp end parallel
 end