Simplify BinaryenIRWriter (#3110)

BinaryenIRWriter was previously inconsistent about whether or not it
emitted an instruction if that instruction was not reachable.
Instructions that produced values were not emitted if they were
unreachable, but instructions that did not produce values were always
emitted. Additionally, blocks continued to emit their children even
after emitting an unreachable child.

Since it was not possible to tell whether an unreachable instruction's
parent would be emitted, BinaryenIRWriter had to be very defensive and
emit many extra `unreachable` instructions around unreachable code to
avoid type errors.

This PR unifies the logic for emitting all non-control flow
instructions and changes the behavior of BinaryenIRWriter so that it
never emits instructions that cannot be reached due to having
unreachable children. This means that extra `unreachable` instructions
now only need to be emitted after unreachable control flow
constructs. BinaryenIRWriter now also stops emitting instructions
inside blocks after the first unreachable instruction as an extra
optimization.

This change will also simplify Poppy IR stackification (see #3059) by
guaranteeing that instructions with unreachable children will not be
emitted into the stackifier. This makes satisfying the Poppy IR rule
against unreachable Pops trivial, whereas previously satisfying this
rule would have required about about 700 additional lines of code to
recompute the types of all unreachable children for any instruction.

src/ir/iteration.h

@@ -17,6 +17,7 @@
 #ifndef wasm_ir_iteration_h
 #define wasm_ir_iteration_h
 
+#include "ir/properties.h"
 #include "wasm-traversal.h"
 #include "wasm.h"
 
@@ -29,18 +30,24 @@ namespace wasm {
 //  * This skips missing children, e.g. if an if has no else, it is represented
 //    as having 2 children (and not 3 with the last a nullptr).
 //
-// In general, it is preferable not to use this class and to directly access
-// the children (using e.g. iff->ifTrue etc.), as that is faster. However, in
-// cases where speed does not matter, this can be convenient.
+// In general, it is preferable not to use this class and to directly access the
+// children (using e.g. iff->ifTrue etc.), as that is faster. However, in cases
+// where speed does not matter, this can be convenient. TODO: reimplement these
+// to avoid materializing all the chilren at once.
 //
-
-class ChildIterator {
+//   ChildIterator - Iterates over all children
+//
+//   ValueChildIterator - Iterates over all children that produce values used by
+//                        this instruction. For example, includes If::condition
+//                        but not If::ifTrue.
+//
+template<template<class, class> class Scanner> class AbstractChildIterator {
+  using Self = AbstractChildIterator<Scanner>;
   struct Iterator {
-    const ChildIterator& parent;
+    const Self& parent;
     Index index;
 
-    Iterator(const ChildIterator& parent, Index index)
-      : parent(parent), index(index) {}
+    Iterator(const Self& parent, Index index) : parent(parent), index(index) {}
 
     bool operator!=(const Iterator& other) const {
       return index != other.index || &parent != &(other.parent);
@@ -52,21 +59,21 @@ class ChildIterator {
   };
 
 public:
-  std::vector<Expression*> children;
+  SmallVector<Expression*, 4> children;
 
-  ChildIterator(Expression* parent) {
+  AbstractChildIterator(Expression* parent) {
     struct Traverser : public PostWalker<Traverser> {
       Expression* parent;
-      std::vector<Expression*>* children;
+      SmallVector<Expression*, 4>* children;
 
       // We need to scan subchildren exactly once - just the parent.
       bool scanned = false;
 
       static void scan(Traverser* self, Expression** currp) {
         if (!self->scanned) {
           self->scanned = true;
-          PostWalker<Traverser, UnifiedExpressionVisitor<Traverser>>::scan(
-            self, currp);
+          Scanner<Traverser, UnifiedExpressionVisitor<Traverser>>::scan(self,
+                                                                        currp);
         } else {
           // This is one of the children. Do not scan further, just note it.
           self->children->push_back(*currp);
@@ -82,6 +89,25 @@ class ChildIterator {
   Iterator end() const { return Iterator(*this, children.size()); }
 };
 
+template<class SubType, class Visitor>
+struct ValueChildScanner : PostWalker<SubType, Visitor> {
+  static void scan(SubType* self, Expression** currp) {
+    auto* curr = *currp;
+    if (Properties::isControlFlowStructure(curr)) {
+      // If conditions are the only value children of control flow structures
+      if (auto* iff = curr->dynCast<If>()) {
+        self->pushTask(SubType::scan, &iff->condition);
+      }
+    } else {
+      // All children on non-control flow expressions are value children
+      PostWalker<SubType, Visitor>::scan(self, currp);
+    }
+  }
+};
+
+using ChildIterator = AbstractChildIterator<PostWalker>;
+using ValueChildIterator = AbstractChildIterator<ValueChildScanner>;
+
 // Returns true if the current expression contains a certain kind of expression,
 // within the given depth of BFS. If depth is -1, this searches all children.
 template<typename T> bool containsChild(Expression* parent, int depth = -1) {

src/ir/properties.h

@@ -19,7 +19,6 @@
 
 #include "ir/bits.h"
 #include "ir/effects.h"
-#include "ir/iteration.h"
 #include "wasm.h"
 
 namespace wasm {

src/ir/stack-utils.cpp

@@ -15,6 +15,7 @@
  */
 
 #include "stack-utils.h"
+#include "ir/iteration.h"
 #include "ir/properties.h"
 
 namespace wasm {
@@ -56,20 +57,13 @@ bool mayBeUnreachable(Expression* expr) {
 } // namespace StackUtils
 
 StackSignature::StackSignature(Expression* expr) {
-  params = Type::none;
-  if (Properties::isControlFlowStructure(expr)) {
-    if (expr->is<If>()) {
-      params = Type::i32;
-    }
-  } else {
-    std::vector<Type> inputs;
-    for (auto* child : ChildIterator(expr)) {
-      assert(child->type.isConcrete());
-      // Children might be tuple pops, so expand their types
-      inputs.insert(inputs.end(), child->type.begin(), child->type.end());
-    }
-    params = Type(inputs);
+  std::vector<Type> inputs;
+  for (auto* child : ValueChildIterator(expr)) {
+    assert(child->type.isConcrete());
+    // Children might be tuple pops, so expand their types
+    inputs.insert(inputs.end(), child->type.begin(), child->type.end());
   }
+  params = Type(inputs);
   if (expr->type == Type::unreachable) {
     unreachable = true;
     results = Type::none;