fix bugs in if_else code generation

The size of the value stack must be the same after each branch of an
if_else op. If the true branch has a value and the false branch doesn't,
then we'll have kept the value on the stack on the true branch, but
won't have a value on the false branch. To fix this, we write a NOP at
the end of the true branch, and if the value needs to be discarded, we
update it to a DISCARD.

1 files changed, 25 insertions, 0 deletions

diff --git a/test/interp/nested-if.txt b/test/interp/nested-if.txt
new file mode 100644
index 00000000..f0baac5b
--- /dev/null
+++ b/test/interp/nested-if.txt
@@ -0,0 +1,25 @@
+;;; TOOL: run-interp
+
+;; This tests two paths branching to the same location, with different value
+;; stack heights.
+
+;; The (i32.const 2) value needs to be discarded because the false branch of the
+;; outer if does not return a value. However, in a bottom-up parser, this is not
+;; known until the inner if expression is reduced. So the previously generated
+;; code for the true branch of the outer if must be fixed-up later, when the
+;; outer if expression is reduced.
+
+(module
+  (export "f" $f)
+  (func $f (result i32)
+    (block $exit
+      (if                     ;; outer if
+        (i32.const 1)
+        (i32.const 2)
+        (if                   ;; inner if
+          (i32.const 3)
+          (br $exit))))
+    (i32.const 4)))
+(;; STDOUT ;;;
+f() => i32:4
+;;; STDOUT ;;)