[Wasm GC] Avoid dangerous StackIR opts on GC (#4007)

removeUnneededBlocks() can force us to use a local when we
load the emitted wasm, which can't work for something nondefaultable
like an RTT.

For now, just don't run that optimization if GC is enabled. Eventually,
perhaps we'll want to enable this optimization in a different way.

4 files changed, 77 insertions, 4 deletions

diff --git a/src/passes/StackIR.cpp b/src/passes/StackIR.cpp
index fbb990fe0..a3d9442bf 100644
--- a/src/passes/StackIR.cpp
+++ b/src/passes/StackIR.cpp
@@ -51,10 +51,14 @@ class StackIROptimizer {
   Function* func;
   PassOptions& passOptions;
   StackIR& insts;
+  FeatureSet features;
 
 public:
-  StackIROptimizer(Function* func, PassOptions& passOptions)
-    : func(func), passOptions(passOptions), insts(*func->stackIR.get()) {
+  StackIROptimizer(Function* func,
+                   PassOptions& passOptions,
+                   FeatureSet features)
+    : func(func), passOptions(passOptions), insts(*func->stackIR.get()),
+      features(features) {
     assert(func->stackIR);
   }
 
@@ -65,7 +69,39 @@ public:
     if (passOptions.optimizeLevel >= 3 || passOptions.shrinkLevel >= 1) {
       local2Stack();
     }
-    removeUnneededBlocks();
+    // Removing unneeded blocks is dangerous with GC, as if we do this:
+    //
+    //   (call
+    //     (rtt)
+    //     (block
+    //       (nop)
+    //       (i32)
+    //     )
+    //   )
+    // === remove inner block ==>
+    //   (call
+    //     (rtt)
+    //     (nop)
+    //     (i32)
+    //   )
+    //
+    // Then we end up with a nop that forces us to emit this during load:
+    //
+    //   (call
+    //     (block
+    //       (local.set
+    //         (rtt)
+    //       )
+    //       (nop)
+    //       (local.get)
+    //     )
+    //     (i32)
+    //   )
+    //
+    // However, that is not valid as an rtt cannot be set to a local.
+    if (!features.hasGC()) {
+      removeUnneededBlocks();
+    }
     dce();
   }
 
@@ -340,7 +376,7 @@ struct OptimizeStackIR : public WalkerPass<PostWalker<OptimizeStackIR>> {
     if (!func->stackIR) {
       return;
     }
-    StackIROptimizer(func, getPassOptions()).run();
+    StackIROptimizer(func, getPassOptions(), getModule()->features).run();
   }
 };
 
diff --git a/test/lit/passes/roundtrip-gc.wast b/test/lit/passes/roundtrip-gc.wast
new file mode 100644
index 000000000..b26f32f8c
--- /dev/null
+++ b/test/lit/passes/roundtrip-gc.wast
@@ -0,0 +1,37 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s -all --generate-stack-ir --optimize-stack-ir --roundtrip -S -o - | filecheck %s
+
+(module
+ (type ${i32} (struct (field i32)))
+ ;; CHECK:      (export "export" (func $test))
+ (export "export" (func $test))
+ ;; CHECK:      (func $test
+ ;; CHECK-NEXT:  (call $help
+ ;; CHECK-NEXT:   (rtt.canon $\7bi32\7d)
+ ;; CHECK-NEXT:   (block $label$1 (result i32)
+ ;; CHECK-NEXT:    (nop)
+ ;; CHECK-NEXT:    (i32.const 1)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $test
+  (call $help
+   (rtt.canon ${i32})
+   ;; Stack IR optimizations can remove this block, leaving a nop in an odd
+   ;; "stacky" location. On load, we would normally use a local to work around
+   ;; that, creating a block to contain the rtt before us and the nop, and then
+   ;; returning the local. But we can't use a local for an rtt, so we should not
+   ;; optimize this sort of thing in stack IR.
+   (block (result i32)
+    (nop)
+    (i32.const 1)
+   )
+  )
+ )
+ ;; CHECK:      (func $help (param $3 (rtt $\7bi32\7d)) (param $4 i32)
+ ;; CHECK-NEXT:  (nop)
+ ;; CHECK-NEXT: )
+ (func $help (param $3 (rtt ${i32})) (param $4 i32)
+  (nop)
+ )
+)
diff --git a/test/passes/Os_print-stack-ir_all-features.txt b/test/passes/Os_print-stack-ir_all-features_disable-gc.txt
index 7184df236..7184df236 100644
--- a/test/passes/Os_print-stack-ir_all-features.txt
+++ b/test/passes/Os_print-stack-ir_all-features_disable-gc.txt
diff --git a/test/passes/Os_print-stack-ir_all-features.wast b/test/passes/Os_print-stack-ir_all-features_disable-gc.wast
index 5c03b5e23..5c03b5e23 100644
--- a/test/passes/Os_print-stack-ir_all-features.wast
+++ b/test/passes/Os_print-stack-ir_all-features_disable-gc.wast