[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, 7 insertions, 1 deletions

diff --git a/src/wasm/wasm-stack.cpp b/src/wasm/wasm-stack.cpp
index cca351b6f..2e4e9d332 100644
--- a/src/wasm/wasm-stack.cpp
+++ b/src/wasm/wasm-stack.cpp
@@ -1908,7 +1908,13 @@ void BinaryInstWriter::visitRttSub(RttSub* curr) {
 }
 
 void BinaryInstWriter::visitStructNew(StructNew* curr) {
-  WASM_UNREACHABLE("TODO (gc): struct.new");
+  o << int8_t(BinaryConsts::GCPrefix);
+  if (curr->isWithDefault()) {
+    o << U32LEB(BinaryConsts::StructNewDefaultWithRtt);
+  } else {
+    o << U32LEB(BinaryConsts::StructNewWithRtt);
+  }
+  parent.writeHeapType(curr->rtt->type.getHeapType());
 }
 
 void BinaryInstWriter::visitStructGet(StructGet* curr) {