[GC] Precompute: Handle non-constant effects in recomputed GC values

src/passes/Precompute.cpp

@@ -210,7 +210,13 @@ class PrecomputingExpressionRunner
       if (hasEffects) {
         // Visit, so we recompute the effects. (This is rare, see comment
         // above.)
-        visitFunc();
+        auto flow = visitFunc();
+        // Also check the result of the effects - if it is non-constant, we
+        // cannot use it. (This can happen during propagation, when we see that
+        // other inputs exist to something we depend on.)
+        if (flow.breaking()) {
+          return flow;
+        }
       }
       // Refer to the same canonical GCData that we already created.
       return Literal(data, curr->type.getHeapType());

test/lit/passes/precompute-gc-loop.wast

@@ -0,0 +1,89 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+
+;; RUN: wasm-opt %s --remove-unused-names --precompute-propagate --fuzz-exec -all -S -o - \
+;; RUN:   | filecheck %s
+
+(module
+ ;; CHECK:      (type $struct (sub (struct (field i32))))
+ (type $struct (sub (struct (field i32))))
+
+ ;; CHECK:      (func $loop-different (type $1)
+ ;; CHECK-NEXT:  (local $struct (ref null $struct))
+ ;; CHECK-NEXT:  (local $int i32)
+ ;; CHECK-NEXT:  (local.set $int
+ ;; CHECK-NEXT:   (i32.const 0)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (loop $loop
+ ;; CHECK-NEXT:   (local.set $struct
+ ;; CHECK-NEXT:    (struct.new $struct
+ ;; CHECK-NEXT:     (local.get $int)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (local.set $int
+ ;; CHECK-NEXT:    (i32.const 1)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (br_if $loop
+ ;; CHECK-NEXT:    (i32.eqz
+ ;; CHECK-NEXT:     (block $block1 (result i32)
+ ;; CHECK-NEXT:      (block $block
+ ;; CHECK-NEXT:       (drop
+ ;; CHECK-NEXT:        (br_on_null $block
+ ;; CHECK-NEXT:         (local.get $struct)
+ ;; CHECK-NEXT:        )
+ ;; CHECK-NEXT:       )
+ ;; CHECK-NEXT:       (br $block1
+ ;; CHECK-NEXT:        (struct.get $struct 0
+ ;; CHECK-NEXT:         (local.get $struct)
+ ;; CHECK-NEXT:        )
+ ;; CHECK-NEXT:       )
+ ;; CHECK-NEXT:      )
+ ;; CHECK-NEXT:      (i32.const 0)
+ ;; CHECK-NEXT:     )
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $loop-different
+  (local $struct (ref null $struct))
+  (local $int i32)
+  ;; This int is set before the loop, and read at the loop top. The value there
+  ;; is *not* 0, because we set a new value in the loop, which takes effect in
+  ;; later iterations.
+  (local.set $int
+   (i32.const 0)
+  )
+  (loop $loop
+   (local.set $struct
+    (struct.new $struct
+     (local.get $int)
+    )
+   )
+   (local.set $int
+    (i32.const 1)
+   )
+
+   ;; The rest of the code is needed to prevent the testcase from getting
+   ;; optimized trivially away. This br should not be optimized away or
+   ;; simplfied - it executes in the first loop iteration but not the second.
+   (br_if $loop
+    (i32.eqz
+     (block $block1 (result i32)
+      (block $block
+       (drop
+        (br_on_null $block
+         (local.get $struct)
+        )
+       )
+       (br $block1
+        (struct.get $struct 0
+         (local.get $struct)
+        )
+       )
+      )
+      (i32.const 0)
+     )
+    )
+   )
+  )
+ )
+)