Implement all Builder::replaceWithIdenticalType() cases as best we can (#3872)

The method had TODOs which it halted on. But we should not halt the
entire program, as this is a best-effort attempt to replace a node with
something simpler of the same type (we call it when we know the value
is not actually used).

2 files changed, 25 insertions, 6 deletions

diff --git a/src/wasm-builder.h b/src/wasm-builder.h
index bb739af89..fabaac22a 100644
--- a/src/wasm-builder.h
+++ b/src/wasm-builder.h
@@ -1040,9 +1040,8 @@ public:
     iff->condition = makeUnary(EqZInt32, iff->condition);
   }
 
-  // returns a replacement with the precise same type, and with
-  // minimal contents. as a replacement, this may reuse the
-  // input node
+  // Returns a replacement with the precise same type, and with minimal contents
+  // as best we can. As a replacement, this may reuse the input node.
   template<typename T> Expression* replaceWithIdenticalType(T* curr) {
     if (curr->type.isTuple()) {
       return makeConstantExpression(Literal::makeZeros(curr->type));
@@ -1050,13 +1049,12 @@ public:
     if (curr->type.isNullable()) {
       return ExpressionManipulator::refNull(curr, curr->type);
     }
-    if (curr->type.isFunction()) {
+    if (curr->type.isFunction() || !curr->type.isBasic()) {
       // We can't do any better, keep the original.
       return curr;
     }
     Literal value;
     // TODO: reuse node conditionally when possible for literals
-    TODO_SINGLE_COMPOUND(curr->type);
     switch (curr->type.getBasic()) {
       case Type::i32:
         value = Literal(int32_t(0));
@@ -1085,7 +1083,7 @@ public:
       case Type::i31ref:
         return makeI31New(makeConst(0));
       case Type::dataref:
-        WASM_UNREACHABLE("TODO: dataref");
+        return curr;
       case Type::none:
         return ExpressionManipulator::nop(curr);
       case Type::unreachable:
diff --git a/test/lit/passes/coalesce-locals-gc.wast b/test/lit/passes/coalesce-locals-gc.wast
new file mode 100644
index 000000000..5bad35001
--- /dev/null
+++ b/test/lit/passes/coalesce-locals-gc.wast
@@ -0,0 +1,21 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s --coalesce-locals -all -S -o - \
+;; RUN:   | filecheck %s
+
+(module
+ ;; CHECK:      (func $test-dead-get-non-nullable (param $0 dataref)
+ ;; CHECK-NEXT:  (unreachable)
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (local.get $0)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $test-dead-get-non-nullable (param $func (ref data))
+  (unreachable)
+  (drop
+   ;; A useless get (that does not read from any set, or from the inputs to the
+   ;; function). Normally we replace such gets with nops as best we can, but in
+   ;; this case the type is non-nullable, so we must leave it alone.
+   (local.get $func)
+  )
+ )
+)