Index: flang/include/flang/Parser/parse-tree-visitor.h
===================================================================
--- flang/include/flang/Parser/parse-tree-visitor.h
+++ flang/include/flang/Parser/parse-tree-visitor.h
@@ -16,6 +16,7 @@
 #include <tuple>
 #include <utility>
 #include <variant>
+#include <vector>
 
 /// Parse tree visitor
 /// Call Walk(x, visitor) to visit x and, by default, each node under x.
@@ -483,20 +484,76 @@
     mutator.Post(x);
   }
 }
+
+// Expr traversal uses iteration rather than recursion to avoid
+// blowing out the stack on very deep expression parse trees.
+// It replaces implementations that looked like:
+//   template <typename V> void Walk(const Expr &x, V visitor) {
+//     if (visitor.Pre(x)) {      // Pre on the Expr
+//       Walk(x.source, visitor);
+//       // Pre on the operator, walk the operands, Post on operator
+//       Walk(x.u, visitor);
+//       visitor.Post(x);         // Post on the Expr
+//     }
+//   }
+template <typename A, typename V, typename UNARY, typename BINARY>
+static void IterativeWalk(A &start, V &visitor) {
+  struct ExprWorkList {
+    ExprWorkList(A &x) : expr(&x) {}
+    bool doPostExpr{false}, doPostOpr{false};
+    A *expr;
+  };
+  std::vector<ExprWorkList> stack;
+  stack.emplace_back(start);
+  do {
+    A &expr{*stack.back().expr};
+    if (stack.back().doPostOpr) {
+      stack.back().doPostOpr = false;
+      common::visit([&visitor](auto &y) { visitor.Post(y); }, expr.u);
+    } else if (stack.back().doPostExpr) {
+      visitor.Post(expr);
+      stack.pop_back();
+    } else if (!visitor.Pre(expr)) {
+      stack.pop_back();
+    } else {
+      stack.back().doPostExpr = true;
+      Walk(expr.source, visitor);
+      UNARY *unary{nullptr};
+      BINARY *binary{nullptr};
+      common::visit(
+          [&unary, &binary](auto &y) {
+            if constexpr (std::is_convertible_v<decltype(&y), UNARY *>) {
+              unary = &y;
+            } else if constexpr (std::is_convertible_v<decltype(&y),
+                                     BINARY *>) {
+              binary = &y;
+            }
+          },
+          expr.u);
+      if (!unary && !binary) {
+        Walk(expr.u, visitor);
+      } else if (common::visit(
+                     [&visitor](auto &y) { return visitor.Pre(y); }, expr.u)) {
+        stack.back().doPostOpr = true;
+        if (unary) {
+          stack.emplace_back(unary->v.value());
+        } else {
+          stack.emplace_back(std::get<1>(binary->t).value());
+          stack.emplace_back(std::get<0>(binary->t).value());
+        }
+      }
+    }
+  } while (!stack.empty());
+}
 template <typename V> void Walk(const Expr &x, V &visitor) {
-  if (visitor.Pre(x)) {
-    Walk(x.source, visitor);
-    Walk(x.u, visitor);
-    visitor.Post(x);
-  }
+  IterativeWalk<const Expr, V, const Expr::IntrinsicUnary,
+      const Expr::IntrinsicBinary>(x, visitor);
 }
 template <typename M> void Walk(Expr &x, M &mutator) {
-  if (mutator.Pre(x)) {
-    Walk(x.source, mutator);
-    Walk(x.u, mutator);
-    mutator.Post(x);
-  }
+  IterativeWalk<Expr, M, Expr::IntrinsicUnary, Expr::IntrinsicBinary>(
+      x, mutator);
 }
+
 template <typename V> void Walk(const Designator &x, V &visitor) {
   if (visitor.Pre(x)) {
     Walk(x.source, visitor);
Index: flang/include/flang/Semantics/expression.h
===================================================================
--- flang/include/flang/Semantics/expression.h
+++ flang/include/flang/Semantics/expression.h
@@ -378,6 +378,8 @@
   bool CheckIsValidForwardReference(const semantics::DerivedTypeSpec &);
   MaybeExpr AnalyzeComplex(MaybeExpr &&re, MaybeExpr &&im, const char *what);
 
+  MaybeExpr IterativelyAnalyzeSubexpressions(const parser::Expr &);
+
   semantics::SemanticsContext &context_;
   FoldingContext &foldingContext_{context_.foldingContext()};
   std::map<parser::CharBlock, int> impliedDos_; // values are INTEGER kinds
@@ -387,6 +389,7 @@
   bool inWhereBody_{false};
   bool inDataStmtConstant_{false};
   bool inStmtFunctionDefinition_{false};
+  bool iterativelyAnalyzingSubexpressions_{false};
   friend class ArgumentAnalyzer;
 };
 
Index: flang/lib/Semantics/expression.cpp
===================================================================
--- flang/lib/Semantics/expression.cpp
+++ flang/lib/Semantics/expression.cpp
@@ -29,6 +29,7 @@
 #include <functional>
 #include <optional>
 #include <set>
+#include <vector>
 
 // Typedef for optional generic expressions (ubiquitous in this file)
 using MaybeExpr =
@@ -3207,6 +3208,12 @@
     result = Analyze(x.u);
   }
   if (result) {
+    if constexpr (std::is_same_v<PARSED, parser::Expr>) {
+      if (!isNullPointerOk_ && IsNullPointer(*result)) {
+        Say(source,
+            "NULL() may not be used as an expression in this context"_err_en_US);
+      }
+    }
     SetExpr(x, Fold(std::move(*result)));
     return x.typedExpr->v;
   } else {
@@ -3222,15 +3229,76 @@
   }
 }
 
+// This is an optional preliminary pass over parser::Expr subtrees.
+// Given an expression tree, iteratively traverse it in a bottom-up order
+// to analyze all of its subexpressions.  A later normal top-down analysis
+// will then be able to use the results that will have been saved in the
+// parse tree without having to recurse deeply.  This technique keeps
+// absurdly deep expression parse trees from causing the analyzer to overflow
+// its stack.
+MaybeExpr ExpressionAnalyzer::IterativelyAnalyzeSubexpressions(
+    const parser::Expr &top) {
+  std::vector<const parser::Expr *> queue, finish;
+  queue.push_back(&top);
+  do {
+    const parser::Expr &expr{*queue.back()};
+    queue.pop_back();
+    if (!expr.typedExpr) {
+      const parser::Expr::IntrinsicUnary *unary{nullptr};
+      const parser::Expr::IntrinsicBinary *binary{nullptr};
+      common::visit(
+          [&unary, &binary](auto &y) {
+            if constexpr (std::is_convertible_v<decltype(&y),
+                              decltype(unary)>) {
+              // Don't evaluate a constant operand to Negate
+              if (!std::holds_alternative<parser::LiteralConstant>(
+                      y.v.value().u)) {
+                unary = &y;
+              }
+            } else if constexpr (std::is_convertible_v<decltype(&y),
+                                     decltype(binary)>) {
+              binary = &y;
+            }
+          },
+          expr.u);
+      if (unary) {
+        queue.push_back(&unary->v.value());
+      } else if (binary) {
+        queue.push_back(&std::get<0>(binary->t).value());
+        queue.push_back(&std::get<1>(binary->t).value());
+      }
+      finish.push_back(&expr);
+    }
+  } while (!queue.empty());
+  // Analyze the collected subexpressions in bottom-up order.
+  // On an error, bail out and leave partial results in place.
+  MaybeExpr result;
+  for (auto riter{finish.rbegin()}; riter != finish.rend(); ++riter) {
+    const parser::Expr &expr{**riter};
+    result = ExprOrVariable(expr, expr.source);
+    if (!result) {
+      return result;
+    }
+  }
+  return result; // last value was from analysis of "top"
+}
+
 MaybeExpr ExpressionAnalyzer::Analyze(const parser::Expr &expr) {
-  if (useSavedTypedExprs_ && expr.typedExpr) {
-    return expr.typedExpr->v;
+  bool wasIterativelyAnalyzing{iterativelyAnalyzingSubexpressions_};
+  MaybeExpr result;
+  if (useSavedTypedExprs_) {
+    if (expr.typedExpr) {
+      return expr.typedExpr->v;
+    }
+    if (!wasIterativelyAnalyzing) {
+      iterativelyAnalyzingSubexpressions_ = true;
+      result = IterativelyAnalyzeSubexpressions(expr);
+    }
   }
-  MaybeExpr result{ExprOrVariable(expr, expr.source)};
-  if (!isNullPointerOk_ && result && IsNullPointer(*result)) {
-    Say(expr.source,
-        "NULL() may not be used as an expression in this context"_err_en_US);
+  if (!result) {
+    result = ExprOrVariable(expr, expr.source);
   }
+  iterativelyAnalyzingSubexpressions_ = wasIterativelyAnalyzing;
   return result;
 }
 
@@ -3898,7 +3966,7 @@
     const parser::Expr &expr) {
   source_.ExtendToCover(expr.source);
   if (const Symbol *assumedTypeDummy{AssumedTypeDummy(expr)}) {
-    expr.typedExpr.Reset(new GenericExprWrapper{}, GenericExprWrapper::Deleter);
+    ResetExpr(expr);
     if (isProcedureCall_) {
       ActualArgument arg{ActualArgument::AssumedType{*assumedTypeDummy}};
       SetArgSourceLocation(arg, expr.source);
Index: flang/test/Evaluate/big-expr-tree.F90
===================================================================
--- /dev/null
+++ flang/test/Evaluate/big-expr-tree.F90
@@ -0,0 +1,9 @@
+! RUN: %python %S/test_folding.py %s %flang_fc1
+! Exercise parsing, expression analysis, and folding on a very tall expression tree
+! 32*32*32 = 32768 repetitions
+#define M0(x) x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x
+#define M1(x) x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x
+#define M2(x) x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x+x
+module m
+  logical, parameter :: test_1 = 32**3 .EQ. M2(M1(M0(1)))
+end module