Fix printing of unreachable br_on_cast{_fail} (#7102)

br_on_cast and br_on_cast_fail have two type annotations: one for their
input type and one for their cast type. In cases where their operands
were unreachable, we were previously printing "unreachable" for the
input type annotation. This is not valid wat because "unreachable" is
not a reference type.

To fix the problem, print the bottom type of the cast type's hierarchy
as the input type for br_on_cast and br_on_cast_fail when the operand is
unreachable. This ensures that the instructions have the most precise
possible output type according to Wasm typing rules, so it maximizes the
number of contexts in which the printed instructions are valid.

1 files changed, 15 insertions, 2 deletions

diff --git a/src/passes/Print.cpp b/src/passes/Print.cpp
index 5a5d01299..5c8463f78 100644
--- a/src/passes/Print.cpp
+++ b/src/passes/Print.cpp
@@ -2219,7 +2219,15 @@ struct PrintExpressionContents
         printMedium(o, "br_on_cast ");
         curr->name.print(o);
         o << ' ';
-        printType(curr->ref->type);
+        if (curr->ref->type == Type::unreachable) {
+          // Need to print some reference type in the correct hierarchy rather
+          // than unreachable, and the bottom type is valid in the most
+          // contexts.
+          printType(
+            Type(curr->castType.getHeapType().getBottom(), NonNullable));
+        } else {
+          printType(curr->ref->type);
+        }
         o << ' ';
         printType(curr->castType);
         return;
@@ -2227,7 +2235,12 @@ struct PrintExpressionContents
         printMedium(o, "br_on_cast_fail ");
         curr->name.print(o);
         o << ' ';
-        printType(curr->ref->type);
+        if (curr->ref->type == Type::unreachable) {
+          printType(
+            Type(curr->castType.getHeapType().getBottom(), NonNullable));
+        } else {
+          printType(curr->ref->type);
+        }
         o << ' ';
         printType(curr->castType);
         return;