[flang][OpenMP] Support bind clause code-gen for standalone loops (#122674)

Extends rewriting of `loop` directives by supporting `bind` clause for
standalone directives. This follows both the spec and the current state
of clang as follows:
* No `bind` or `bind(thread)`: the `loop` is rewritten to `simd`.
* `bind(parallel)`: the `loop` is rewritten to `do`.
* `bind(teams)`: the `loop` is rewritten to `distribute`.

This is a follow-up PR for
#122632, only the latest commit
in this PR is relevant to the PR.

Author: ergawy

flang/lib/Optimizer/OpenMP/GenericLoopConversion.cpp

@@ -53,7 +53,7 @@ class GenericLoopConversionPattern
 
     switch (combinedInfo) {
     case GenericLoopCombinedInfo::Standalone:
-      rewriteToSimdLoop(loopOp, rewriter);
+      rewriteStandaloneLoop(loopOp, rewriter);
       break;
     case GenericLoopCombinedInfo::TargetParallelLoop:
       llvm_unreachable("not yet implemented: `parallel loop` direcitve");
@@ -87,7 +87,10 @@ class GenericLoopConversionPattern
              << loopOp->getName() << " operation";
     };
 
-    if (loopOp.getBindKind())
+    // For standalone directives, `bind` is already supported. Other combined
+    // forms will be supported in a follow-up PR.
+    if (combinedInfo != GenericLoopCombinedInfo::Standalone &&
+        loopOp.getBindKind())
       return todo("bind");
 
     if (loopOp.getOrder())
@@ -119,7 +122,27 @@ class GenericLoopConversionPattern
     return result;
   }
 
-  /// Rewrites standalone `loop` directives to equivalent `simd` constructs.
+  void rewriteStandaloneLoop(mlir::omp::LoopOp loopOp,
+                             mlir::ConversionPatternRewriter &rewriter) const {
+    using namespace mlir::omp;
+    std::optional<ClauseBindKind> bindKind = loopOp.getBindKind();
+
+    if (!bindKind.has_value())
+      return rewriteToSimdLoop(loopOp, rewriter);
+
+    switch (*loopOp.getBindKind()) {
+    case ClauseBindKind::Parallel:
+      return rewriteToWsloop(loopOp, rewriter);
+    case ClauseBindKind::Teams:
+      return rewriteToDistrbute(loopOp, rewriter);
+    case ClauseBindKind::Thread:
+      return rewriteToSimdLoop(loopOp, rewriter);
+    }
+  }
+
+  /// Rewrites standalone `loop` (without `bind` clause or with
+  /// `bind(parallel)`) directives to equivalent `simd` constructs.
+  ///
   /// The reasoning behind this decision is that according to the spec (version
   /// 5.2, section 11.7.1):
   ///
@@ -147,30 +170,57 @@ class GenericLoopConversionPattern
   /// the directive.
   void rewriteToSimdLoop(mlir::omp::LoopOp loopOp,
                          mlir::ConversionPatternRewriter &rewriter) const {
-    loopOp.emitWarning("Detected standalone OpenMP `loop` directive, the "
-                       "associated loop will be rewritten to `simd`.");
-    mlir::omp::SimdOperands simdClauseOps;
-    simdClauseOps.privateVars = loopOp.getPrivateVars();
+    loopOp.emitWarning(
+        "Detected standalone OpenMP `loop` directive with thread binding, "
+        "the associated loop will be rewritten to `simd`.");
+    rewriteToSingleWrapperOp<mlir::omp::SimdOp, mlir::omp::SimdOperands>(
+        loopOp, rewriter);
+  }
+
+  void rewriteToDistrbute(mlir::omp::LoopOp loopOp,
+                          mlir::ConversionPatternRewriter &rewriter) const {
+    rewriteToSingleWrapperOp<mlir::omp::DistributeOp,
+                             mlir::omp::DistributeOperands>(loopOp, rewriter);
+  }
+
+  void rewriteToWsloop(mlir::omp::LoopOp loopOp,
+                       mlir::ConversionPatternRewriter &rewriter) const {
+    rewriteToSingleWrapperOp<mlir::omp::WsloopOp, mlir::omp::WsloopOperands>(
+        loopOp, rewriter);
+  }
+
+  // TODO Suggestion by Sergio: tag auto-generated operations for constructs
+  // that weren't part of the original program, that would be useful
+  // information for debugging purposes later on. This new attribute could be
+  // used for `omp.loop`, but also for `do concurrent` transformations,
+  // `workshare`, `workdistribute`, etc. The tag could be used for all kinds of
+  // auto-generated operations using a dialect attribute (named something like
+  // `omp.origin` or `omp.derived`) and perhaps hold the name of the operation
+  // it was derived from, the reason it was transformed or something like that
+  // we could use when emitting any messages related to it later on.
+  template <typename OpTy, typename OpOperandsTy>
+  void
+  rewriteToSingleWrapperOp(mlir::omp::LoopOp loopOp,
+                           mlir::ConversionPatternRewriter &rewriter) const {
+    OpOperandsTy clauseOps;
+    clauseOps.privateVars = loopOp.getPrivateVars();
 
     auto privateSyms = loopOp.getPrivateSyms();
     if (privateSyms)
-      simdClauseOps.privateSyms.assign(privateSyms->begin(),
-                                       privateSyms->end());
+      clauseOps.privateSyms.assign(privateSyms->begin(), privateSyms->end());
 
-    Fortran::common::openmp::EntryBlockArgs simdArgs;
-    simdArgs.priv.vars = simdClauseOps.privateVars;
+    Fortran::common::openmp::EntryBlockArgs args;
+    args.priv.vars = clauseOps.privateVars;
 
-    auto simdOp =
-        rewriter.create<mlir::omp::SimdOp>(loopOp.getLoc(), simdClauseOps);
-    mlir::Block *simdBlock =
-        genEntryBlock(rewriter, simdArgs, simdOp.getRegion());
+    auto wrapperOp = rewriter.create<OpTy>(loopOp.getLoc(), clauseOps);
+    mlir::Block *opBlock = genEntryBlock(rewriter, args, wrapperOp.getRegion());
 
     mlir::IRMapping mapper;
     mlir::Block &loopBlock = *loopOp.getRegion().begin();
 
-    for (auto [loopOpArg, simdopArg] :
-         llvm::zip_equal(loopBlock.getArguments(), simdBlock->getArguments()))
-      mapper.map(loopOpArg, simdopArg);
+    for (auto [loopOpArg, opArg] :
+         llvm::zip_equal(loopBlock.getArguments(), opBlock->getArguments()))
+      mapper.map(loopOpArg, opArg);
 
     rewriter.clone(*loopOp.begin(), mapper);
   }

flang/test/Lower/OpenMP/loop-directive.f90

@@ -92,7 +92,7 @@ subroutine test_reduction()
 ! CHECK-LABEL: func.func @_QPtest_bind
 subroutine test_bind()
   integer :: i, dummy = 1
-  ! CHECK: omp.loop bind(thread) private(@{{.*}} %{{.*}}#0 -> %{{.*}} : {{.*}}) {
+  ! CHECK: omp.simd private(@{{.*}} %{{.*}}#0 -> %{{.*}} : {{.*}}) {
   ! CHECK: }
   !$omp loop bind(thread)
   do i=1,10
@@ -139,3 +139,43 @@ subroutine test_nested_directives
   end do
   !$omp end target teams
 end subroutine
+
+! CHECK-LABEL: func.func @_QPtest_standalone_bind_teams
+subroutine test_standalone_bind_teams
+  implicit none
+  integer, parameter :: N = 100000
+  integer a(N), b(N), c(N)
+  integer j,i, num, flag;
+  num = N
+
+  ! CHECK:     omp.distribute
+  ! CHECK-SAME:  private(@{{.*}}Ea_private_ref_100000xi32 {{[^,]*}},
+  ! CHECK-SAME:          @{{.*}}Ei_private_ref_i32 {{.*}} : {{.*}}) {
+  ! CHECK:       omp.loop_nest {{.*}} {
+  ! CHECK:       }
+  ! CHECK:     }
+  !$omp loop bind(teams) private(a)
+  do i=1,N
+    c(i) = a(i) * b(i)
+  end do
+end subroutine
+
+! CHECK-LABEL: func.func @_QPtest_standalone_bind_parallel
+subroutine test_standalone_bind_parallel
+  implicit none
+  integer, parameter :: N = 100000
+  integer a(N), b(N), c(N)
+  integer j,i, num, flag;
+  num = N
+
+  ! CHECK:     omp.wsloop
+  ! CHECK-SAME:  private(@{{.*}}Ea_private_ref_100000xi32 {{[^,]*}},
+  ! CHECK-SAME:          @{{.*}}Ei_private_ref_i32 {{.*}} : {{.*}}) {
+  ! CHECK:       omp.loop_nest {{.*}} {
+  ! CHECK:       }
+  ! CHECK:     }
+  !$omp loop bind(parallel) private(a)
+  do i=1,N
+    c(i) = a(i) * b(i)
+  end do
+end subroutine