[EH][GC] Fix nested pop after removing ref.cast (#4407)

`ref.cast` can be statically removed when the ref's type is a subtype of
the intended RTT type and either of `--ignore-implicit-traps` or
`--traps-never-happen` is given: https://github.com/WebAssembly/binaryen/blob/083ab9842ec3d4ca278c95e1a33112ae7cd4d9e5/src/passes/OptimizeInstructions.cpp#L1603-L1624

Some more context: https://github.com/WebAssembly/binaryen/pull/4097#discussion_r694456784

But this can create a block in which a `pop` is nested, which makes the
`catch` invalid. The test in this PR is the same as the example given by
@kripken in #4237. This calls the fixup function
`EHUtils::handleBlockNestedPops` at the end of the pass to fix this.

Also, because this pass creates a lot of blocks in other patterns, I
think it is possible there can be other patterns to cause this kind of
`pop` nesting.

2 files changed, 62 insertions, 0 deletions

diff --git a/src/passes/OptimizeInstructions.cpp b/src/passes/OptimizeInstructions.cpp
index 80ca7162c..347ebbbe4 100644
--- a/src/passes/OptimizeInstructions.cpp
+++ b/src/passes/OptimizeInstructions.cpp
@@ -26,6 +26,7 @@
 #include <ir/bits.h>
 #include <ir/cost.h>
 #include <ir/effects.h>
+#include <ir/eh-utils.h>
 #include <ir/find_all.h>
 #include <ir/gc-type-utils.h>
 #include <ir/iteration.h>
@@ -229,6 +230,9 @@ struct OptimizeInstructions
     // Some patterns create locals (like when we use getResultOfFirst), which we
     // may need to fix up.
     TypeUpdating::handleNonDefaultableLocals(func, *getModule());
+    // Some patterns create blocks that can interfere 'catch' and 'pop', nesting
+    // the 'pop' into a block making it invalid.
+    EHUtils::handleBlockNestedPops(func, *getModule());
   }
 
   // Set to true when one of the visitors makes a change (either replacing the
diff --git a/test/lit/passes/optimize-instructions-iit-eh.wast b/test/lit/passes/optimize-instructions-iit-eh.wast
new file mode 100644
index 000000000..b297cb7df
--- /dev/null
+++ b/test/lit/passes/optimize-instructions-iit-eh.wast
@@ -0,0 +1,58 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: wasm-opt %s --ignore-implicit-traps --optimize-instructions -all -S -o - \
+;; RUN:   | filecheck %s
+
+(module
+  ;; CHECK:      (type $struct.A (struct (field i32)))
+  (type $struct.A (struct i32))
+  ;; CHECK:      (global $g-struct.A (rtt $struct.A) (rtt.canon $struct.A))
+
+  ;; CHECK:      (tag $e (param (ref null $struct.A)))
+  (tag $e (param (ref null $struct.A)))
+  (global $g-struct.A (rtt $struct.A) (rtt.canon $struct.A))
+
+  ;; CHECK:      (func $ref-cast-statically-removed
+  ;; CHECK-NEXT:  (local $0 (ref null $struct.A))
+  ;; CHECK-NEXT:  (local $1 (ref null $struct.A))
+  ;; CHECK-NEXT:  (try $try
+  ;; CHECK-NEXT:   (do
+  ;; CHECK-NEXT:    (nop)
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:   (catch $e
+  ;; CHECK-NEXT:    (local.set $1
+  ;; CHECK-NEXT:     (pop (ref null $struct.A))
+  ;; CHECK-NEXT:    )
+  ;; CHECK-NEXT:    (throw $e
+  ;; CHECK-NEXT:     (block (result (ref null $struct.A))
+  ;; CHECK-NEXT:      (local.set $0
+  ;; CHECK-NEXT:       (local.get $1)
+  ;; CHECK-NEXT:      )
+  ;; CHECK-NEXT:      (drop
+  ;; CHECK-NEXT:       (global.get $g-struct.A)
+  ;; CHECK-NEXT:      )
+  ;; CHECK-NEXT:      (local.get $0)
+  ;; CHECK-NEXT:     )
+  ;; CHECK-NEXT:    )
+  ;; CHECK-NEXT:   )
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT: )
+  (func $ref-cast-statically-removed
+    (try
+      (do)
+      (catch $e
+        (throw $e
+          ;; Because --ignore-implicit-traps is given, this ref.cast is assumed
+          ;; not to throw so this ref.cast can be statically removed. But that
+          ;; creates a block around this, making 'pop' nested into it, which is
+          ;; invalid. We fix this up at the end up OptimizeInstruction,
+          ;; assigning the 'pop' to a local at the start of this 'catch' body
+          ;; and later using 'local.get' to get it.
+          (ref.cast
+            (pop (ref null $struct.A))
+            (global.get $g-struct.A)
+          )
+        )
+      )
+    )
+  )
+)