[MLIR][OpenMP] Support lowering of host_eval to LLVM IR

This patch updates the MLIR to LLVM IR lowering of `omp.target` to support
passing `num_teams`, `num_threads`, `thread_limit` and SPMD loop bounds through
the `host_eval` argument of `omp.target`.

This replaces the previous implementation where this information was directly
attached to the `omp.target` operation rather than captured to be used by the
corresponding nested operation.

llvm/include/llvm/Frontend/OpenMP/OMPIRBuilder.h

@@ -728,13 +728,12 @@ class OpenMPIRBuilder {
                       LoopBodyGenCallbackTy BodyGenCB, Value *TripCount,
                       const Twine &Name = "loop");
 
-  /// Generator for the control flow structure of an OpenMP canonical loop.
+  /// Calculate the trip count of a canonical loop.
   ///
-  /// Instead of a logical iteration space, this allows specifying user-defined
-  /// loop counter values using increment, upper- and lower bounds. To
-  /// disambiguate the terminology when counting downwards, instead of lower
-  /// bounds we use \p Start for the loop counter value in the first body
-  /// iteration.
+  /// This allows specifying user-defined loop counter values using increment,
+  /// upper- and lower bounds. To disambiguate the terminology when counting
+  /// downwards, instead of lower bounds we use \p Start for the loop counter
+  /// value in the first body iteration.
   ///
   /// Consider the following limitations:
   ///
@@ -758,7 +757,32 @@ class OpenMPIRBuilder {
   ///
   ///      for (int i = 0; i < 42; i -= 1u)
   ///
-  //
+  /// \param Loc       The insert and source location description.
+  /// \param Start     Value of the loop counter for the first iterations.
+  /// \param Stop      Loop counter values past this will stop the loop.
+  /// \param Step      Loop counter increment after each iteration; negative
+  ///                  means counting down.
+  /// \param IsSigned  Whether Start, Stop and Step are signed integers.
+  /// \param InclusiveStop Whether \p Stop itself is a valid value for the loop
+  ///                      counter.
+  /// \param Name      Base name used to derive instruction names.
+  ///
+  /// \returns The value holding the calculated trip count.
+  Value *calculateCanonicalLoopTripCount(const LocationDescription &Loc,
+                                         Value *Start, Value *Stop, Value *Step,
+                                         bool IsSigned, bool InclusiveStop,
+                                         const Twine &Name = "loop");
+
+  /// Generator for the control flow structure of an OpenMP canonical loop.
+  ///
+  /// Instead of a logical iteration space, this allows specifying user-defined
+  /// loop counter values using increment, upper- and lower bounds. To
+  /// disambiguate the terminology when counting downwards, instead of lower
+  /// bounds we use \p Start for the loop counter value in the first body
+  ///
+  /// It calls \see calculateCanonicalLoopTripCount for trip count calculations,
+  /// so limitations of that method apply here as well.
+  ///
   /// \param Loc       The insert and source location description.
   /// \param BodyGenCB Callback that will generate the loop body code.
   /// \param Start     Value of the loop counter for the first iterations.

llvm/lib/Frontend/OpenMP/OMPIRBuilder.cpp

@@ -4032,10 +4032,10 @@ OpenMPIRBuilder::createCanonicalLoop(const LocationDescription &Loc,
   return CL;
 }
 
-Expected<CanonicalLoopInfo *> OpenMPIRBuilder::createCanonicalLoop(
-    const LocationDescription &Loc, LoopBodyGenCallbackTy BodyGenCB,
-    Value *Start, Value *Stop, Value *Step, bool IsSigned, bool InclusiveStop,
-    InsertPointTy ComputeIP, const Twine &Name) {
+Value *OpenMPIRBuilder::calculateCanonicalLoopTripCount(
+    const LocationDescription &Loc, Value *Start, Value *Stop, Value *Step,
+    bool IsSigned, bool InclusiveStop, const Twine &Name) {
+  updateToLocation(Loc);
 
   // Consider the following difficulties (assuming 8-bit signed integers):
   //  * Adding \p Step to the loop counter which passes \p Stop may overflow:
@@ -4048,10 +4048,6 @@ Expected<CanonicalLoopInfo *> OpenMPIRBuilder::createCanonicalLoop(
   assert(IndVarTy == Stop->getType() && "Stop type mismatch");
   assert(IndVarTy == Step->getType() && "Step type mismatch");
 
-  LocationDescription ComputeLoc =
-      ComputeIP.isSet() ? LocationDescription(ComputeIP, Loc.DL) : Loc;
-  updateToLocation(ComputeLoc);
-
   ConstantInt *Zero = ConstantInt::get(IndVarTy, 0);
   ConstantInt *One = ConstantInt::get(IndVarTy, 1);
 
@@ -4090,8 +4086,20 @@ Expected<CanonicalLoopInfo *> OpenMPIRBuilder::createCanonicalLoop(
     Value *OneCmp = Builder.CreateICmp(CmpInst::ICMP_ULE, Span, Incr);
     CountIfLooping = Builder.CreateSelect(OneCmp, One, CountIfTwo);
   }
-  Value *TripCount = Builder.CreateSelect(ZeroCmp, Zero, CountIfLooping,
-                                          "omp_" + Name + ".tripcount");
+
+  return Builder.CreateSelect(ZeroCmp, Zero, CountIfLooping,
+                              "omp_" + Name + ".tripcount");
+}
+
+Expected<CanonicalLoopInfo *> OpenMPIRBuilder::createCanonicalLoop(
+    const LocationDescription &Loc, LoopBodyGenCallbackTy BodyGenCB,
+    Value *Start, Value *Stop, Value *Step, bool IsSigned, bool InclusiveStop,
+    InsertPointTy ComputeIP, const Twine &Name) {
+  LocationDescription ComputeLoc =
+      ComputeIP.isSet() ? LocationDescription(ComputeIP, Loc.DL) : Loc;
+
+  Value *TripCount = calculateCanonicalLoopTripCount(
+      ComputeLoc, Start, Stop, Step, IsSigned, InclusiveStop, Name);
 
   auto BodyGen = [=](InsertPointTy CodeGenIP, Value *IV) {
     Builder.restoreIP(CodeGenIP);

mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp

@@ -1772,55 +1772,34 @@ LogicalResult TargetOp::verify() {
 Operation *TargetOp::getInnermostCapturedOmpOp() {
   Dialect *ompDialect = (*this)->getDialect();
   Operation *capturedOp = nullptr;
-  Region *capturedParentRegion = nullptr;
 
-  walk<WalkOrder::PostOrder>([&](Operation *op) {
+  // Process in pre-order to check operations from outermost to innermost,
+  // ensuring we only enter the region of an operation if it meets the criteria
+  // for being captured. We stop the exploration of nested operations as soon as
+  // we process a region with no operation to be captured.
+  walk<WalkOrder::PreOrder>([&](Operation *op) {
     if (op == *this)
-      return;
-
-    // Reset captured op if crossing through an omp.loop_nest, so that the top
-    // level one will be the one captured.
-    if (llvm::isa<LoopNestOp>(op)) {
-      capturedOp = nullptr;
-      capturedParentRegion = nullptr;
-    }
+      return WalkResult::advance();
 
+    // Ignore operations of other dialects or omp operations with no regions,
+    // because these will only be checked if they are siblings of an omp
+    // operation that can potentially be captured.
     bool isOmpDialect = op->getDialect() == ompDialect;
     bool hasRegions = op->getNumRegions() > 0;
-
-    if (capturedOp) {
-      bool isImmediateParent = false;
-      for (Region &region : op->getRegions()) {
-        if (&region == capturedParentRegion) {
-          isImmediateParent = true;
-          capturedParentRegion = op->getParentRegion();
-          break;
-        }
-      }
-
-      // Make sure the captured op is part of a (possibly multi-level) nest of
-      // OpenMP-only operations containing no unsupported siblings at any level.
-      if ((hasRegions && isOmpDialect != isImmediateParent) ||
-          (!isImmediateParent && !siblingAllowedInCapture(op))) {
-        capturedOp = nullptr;
-        capturedParentRegion = nullptr;
-      }
-    } else {
-      //  The first OpenMP dialect op containing a region found while visiting
-      //  in post-order should be the innermost captured OpenMP operation.
-      if (isOmpDialect && hasRegions) {
-        capturedOp = op;
-        capturedParentRegion = op->getParentRegion();
-
-        // Don't capture this op if it has a not-allowed sibling.
-        for (Operation &sibling : op->getParentRegion()->getOps()) {
-          if (&sibling != op && !siblingAllowedInCapture(&sibling)) {
-            capturedOp = nullptr;
-            capturedParentRegion = nullptr;
-          }
-        }
-      }
-    }
+    if (!isOmpDialect || !hasRegions)
+      return WalkResult::skip();
+
+    // Don't capture this op if it has a not-allowed sibling, and stop recursing
+    // into nested operations.
+    for (Operation &sibling : op->getParentRegion()->getOps())
+      if (&sibling != op && !siblingAllowedInCapture(&sibling))
+        return WalkResult::interrupt();
+
+    // Don't continue capturing nested operations if we reach an omp.loop_nest.
+    // Otherwise, process the contents of this operation.
+    capturedOp = op;
+    return llvm::isa<LoopNestOp>(op) ? WalkResult::interrupt()
+                                     : WalkResult::advance();
   });
 
   return capturedOp;