[mlir][AsmPrinter] Remove recursion while SSA naming

Address the TODO of removing recursion while SSA naming.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D102226

mlir/lib/IR/AsmPrinter.cpp

@@ -36,6 +36,9 @@
 #include "llvm/Support/Endian.h"
 #include "llvm/Support/Regex.h"
 #include "llvm/Support/SaveAndRestore.h"
+
+#include <tuple>
+
 using namespace mlir;
 using namespace mlir::detail;
 
@@ -835,11 +838,58 @@ private:
 SSANameState::SSANameState(
     Operation *op,
     DialectInterfaceCollection<OpAsmDialectInterface> &interfaces) {
-  llvm::ScopedHashTable<StringRef, char>::ScopeTy usedNamesScope(usedNames);
+  llvm::SaveAndRestore<unsigned> valueIDSaver(nextValueID);
+  llvm::SaveAndRestore<unsigned> argumentIDSaver(nextArgumentID);
+  llvm::SaveAndRestore<unsigned> conflictIDSaver(nextConflictID);
+
+  // The context includes nextValueID, nextArgumentID, nextConflictID and scoped
+  // HashTable.
+  using hashTableScopeTy = llvm::ScopedHashTable<StringRef, char>::ScopeTy;
+  // A namingContext carries the information inherits from parent region.
+  using namingContext =
+      std::tuple<Region *, unsigned, unsigned, unsigned, hashTableScopeTy *>;
+  // Allocator for hashTableScopeTy
+  llvm::BumpPtrAllocator allocator;
+
+  SmallVector<namingContext, 8> nameContext;
+  for (Region &region : op->getRegions())
+    nameContext.push_back(std::make_tuple(&region, nextValueID, nextArgumentID,
+                                          nextConflictID, nullptr));
+
   numberValuesInOp(*op, interfaces);
 
-  for (auto &region : op->getRegions())
+  while (!nameContext.empty()) {
-    numberValuesInRegion(region, interfaces);
+    Region *region;
+    hashTableScopeTy *parentScope;
+    std::tie(region, nextValueID, nextArgumentID, nextConflictID, parentScope) =
+        nameContext.pop_back_val();
+
+    // When we switch from one subtree to another, pop the scopes(needless)
+    // until the parent scope.
+    while (usedNames.getCurScope() != parentScope) {
+      usedNames.getCurScope()->~hashTableScopeTy();
+      assert((usedNames.getCurScope() != nullptr || parentScope == nullptr) &&
+             "top level parentScope must be a nullptr");
+    }
+
+    // Add a scope for the current region.
+    auto *curNamesScope = allocator.Allocate<hashTableScopeTy>();
+    new (curNamesScope) hashTableScopeTy(usedNames);
+
+    numberValuesInRegion(*region, interfaces);
+
+    for (Block &block : *region) {
+      for (Operation &op : block)
+        for (Region &region : op.getRegions())
+          nameContext.push_back(std::make_tuple(&region, nextValueID,
+                                                nextArgumentID, nextConflictID,
+                                                curNamesScope));
+    }
+  }
+
+  // Manually remove all the scopes.
+  while (usedNames.getCurScope() != nullptr)
+    usedNames.getCurScope()->~hashTableScopeTy();
 }
 
 void SSANameState::printValueID(Value value, bool printResultNo,
@@ -918,15 +968,6 @@ void SSANameState::shadowRegionArgs(Region &region, ValueRange namesToUse) {
 void SSANameState::numberValuesInRegion(
     Region &region,
     DialectInterfaceCollection<OpAsmDialectInterface> &interfaces) {
-  // Save the current value ids to allow for numbering values in sibling regions
-  // the same.
-  llvm::SaveAndRestore<unsigned> valueIDSaver(nextValueID);
-  llvm::SaveAndRestore<unsigned> argumentIDSaver(nextArgumentID);
-  llvm::SaveAndRestore<unsigned> conflictIDSaver(nextConflictID);
-
-  // Push a new used names scope.
-  llvm::ScopedHashTable<StringRef, char>::ScopeTy usedNamesScope(usedNames);
-
   // Number the values within this region in a breadth-first order.
   unsigned nextBlockID = 0;
   for (auto &block : region) {
@@ -935,14 +976,6 @@ void SSANameState::numberValuesInRegion(
     blockIDs[&block] = nextBlockID++;
     numberValuesInBlock(block, interfaces);
   }
-
-  // After that we traverse the nested regions.
-  // TODO: Rework this loop to not use recursion.
-  for (auto &block : region) {
-    for (auto &op : block)
-      for (auto &nestedRegion : op.getRegions())
-        numberValuesInRegion(nestedRegion, interfaces);
-  }
 }
 
 void SSANameState::numberValuesInBlock(