[GC] Add struct.new and start to test interesting execution (#3433)

With struct.new read/write support, we can start to do interesting
things! This adds a test of creating a struct and seeing that references
behave like references, that is, if we write to the value X refers to, and
if Y refers to the same thing, when reading from Y's value we see the
change as well.

The test is run through all of -O1, which uncovered a minor issue in
Precompute: We can't try to precompute a reference type, as we can't
replace a reference with a value.

Note btw that the test shows the optimizer properly running
CoalesceLocals on reference types, merging two locals.

1 files changed, 18 insertions, 4 deletions

diff --git a/src/wasm/wasm-s-parser.cpp b/src/wasm/wasm-s-parser.cpp
index 63b6e34bd..7efb2f2fb 100644
--- a/src/wasm/wasm-s-parser.cpp
+++ b/src/wasm/wasm-s-parser.cpp
@@ -2124,10 +2124,24 @@ Expression* SExpressionWasmBuilder::makeRttSub(Element& s) {
 }
 
 Expression* SExpressionWasmBuilder::makeStructNew(Element& s, bool default_) {
-  auto ret = allocator.alloc<StructNew>();
-  WASM_UNREACHABLE("TODO (gc): struct.new");
-  ret->finalize();
-  return ret;
+  auto heapType = parseHeapType(*s[1]);
+  auto* rtt = parseExpression(*s[2]);
+  if (rtt->type != Type::unreachable) {
+    if (!rtt->type.isRtt() || rtt->type.getHeapType() != heapType) {
+      throw ParseException("bad struct.new heap type", s.line, s.col);
+    }
+  }
+  auto numOperands = s.size() - 3;
+  if (default_ && numOperands > 0) {
+    throw ParseException(
+      "arguments provided for struct.new_with_default", s.line, s.col);
+  }
+  std::vector<Expression*> operands;
+  operands.resize(numOperands);
+  for (Index i = 0; i < numOperands; i++) {
+    operands[i] = parseExpression(*s[i + 3]);
+  }
+  return Builder(wasm).makeStructNew(rtt, operands);
 }
 
 Index SExpressionWasmBuilder::getStructIndex(const HeapType& type, Element& s) {