5 files changed, 82 insertions, 34 deletions
diff --git a/src/passes/Vacuum.cpp b/src/passes/Vacuum.cpp
index 89ef7359e..db42a994a 100644
--- a/src/passes/Vacuum.cpp
+++ b/src/passes/Vacuum.cpp
@@ -203,6 +203,25 @@ struct Vacuum : public WalkerPass<ExpressionStackWalker<Vacuum, Visitor<Vacuum>>
     // if the drop input has no side effects, it can be wiped out
     if (!EffectAnalyzer(curr->value).hasSideEffects()) {
       ExpressionManipulator::nop(curr);
+      return;
+    }
+    // sink a drop into an arm of an if-else if the other arm ends in an unreachable, as it if is a branch, this can make that branch optimizable and more vaccuming possible
+    auto* iff = curr->value->dynCast<If>();
+    if (iff && iff->ifFalse && isConcreteWasmType(iff->type)) {
+      // reuse the drop in both cases
+      if (iff->ifTrue->type == unreachable) {
+        assert(isConcreteWasmType(iff->ifFalse->type));
+        curr->value = iff->ifFalse;
+        iff->ifFalse = curr;
+        iff->type = none;
+        replaceCurrent(iff);
+      } else if (iff->ifFalse->type == unreachable) {
+        assert(isConcreteWasmType(iff->ifTrue->type));
+        curr->value = iff->ifTrue;
+        iff->ifTrue = curr;
+        iff->type = none;
+        replaceCurrent(iff);
+      }
     }
   }
 
diff --git a/test/passes/vacuum.txt b/test/passes/vacuum.txt
index c4004dd28..678c18cfa 100644
--- a/test/passes/vacuum.txt
+++ b/test/passes/vacuum.txt
@@ -5,6 +5,8 @@
   (type $2 (func (result f32)))
   (type $3 (func (result i32)))
   (type $4 (func (param i32 f64 i32 i32)))
+  (type $FUNCSIG$i (func (result i32)))
+  (import $int "env" "int" (result i32))
   (func $b (type $0)
     (nop)
   )
@@ -158,4 +160,22 @@
       (unreachable)
     )
   )
+  (func $if-drop (type $0)
+    (block $out
+      (if
+        (i32.const 0)
+        (drop
+          (call_import $int)
+        )
+        (br $out)
+      )
+      (if
+        (i32.const 1)
+        (br $out)
+        (drop
+          (call_import $int)
+        )
+      )
+    )
+  )
 )
diff --git a/test/passes/vacuum.wast b/test/passes/vacuum.wast
index 7b8d6a08a..3630512b4 100644
--- a/test/passes/vacuum.wast
+++ b/test/passes/vacuum.wast
@@ -5,6 +5,7 @@
   (type $2 (func (result f32)))
   (type $3 (func (result i32)))
   (type $4 (func (param i32 f64 i32 i32)))
+  (import $int "env" "int" (result i32))
   (func $b (type $0)
     (drop
       (i32.const 50)
@@ -297,4 +298,20 @@
       (unreachable)
     )
   )
+  (func $if-drop
+    (block $out
+      (drop
+        (if (i32.const 0)
+          (call_import $int)
+          (br $out)
+        )
+      )
+      (drop
+        (if (i32.const 1)
+          (br $out)
+          (call_import $int)
+        )
+      )
+    )
+  )
 )
diff --git a/test/unit.fromasm b/test/unit.fromasm
index f48dfcdb2..049b4dc8a 100644
--- a/test/unit.fromasm
+++ b/test/unit.fromasm
@@ -492,13 +492,12 @@
   )
   (func $smallIf
     (block $do-once$0
-      (drop
-        (if
-          (i32.const 2)
+      (if
+        (i32.const 2)
+        (drop
           (call $lb
             (i32.const 3)
           )
-          (br $do-once$0)
         )
       )
     )
@@ -544,21 +543,18 @@
   )
   (func $breakThroughMany (param $0 i32)
     (block $label$break$L1
-      (drop
-        (if
-          (get_local $0)
-          (loop $while-in$2
-            (br_if $label$break$L1
-              (i32.eqz
-                (get_local $0)
-              )
-            )
-            (call $zeroInit
-              (i32.const 0)
+      (if
+        (get_local $0)
+        (loop $while-in$2
+          (br_if $label$break$L1
+            (i32.eqz
+              (get_local $0)
             )
-            (br $while-in$2)
           )
-          (i32.const 1337)
+          (call $zeroInit
+            (i32.const 0)
+          )
+          (br $while-in$2)
         )
       )
     )
diff --git a/test/unit.fromasm.imprecise b/test/unit.fromasm.imprecise
index 06ab40915..d0d98452f 100644
--- a/test/unit.fromasm.imprecise
+++ b/test/unit.fromasm.imprecise
@@ -474,13 +474,12 @@
   )
   (func $smallIf
     (block $do-once$0
-      (drop
-        (if
-          (i32.const 2)
+      (if
+        (i32.const 2)
+        (drop
           (call $lb
             (i32.const 3)
           )
-          (br $do-once$0)
         )
       )
     )
@@ -526,21 +525,18 @@
   )
   (func $breakThroughMany (param $0 i32)
     (block $label$break$L1
-      (drop
-        (if
-          (get_local $0)
-          (loop $while-in$2
-            (br_if $label$break$L1
-              (i32.eqz
-                (get_local $0)
-              )
-            )
-            (call $zeroInit
-              (i32.const 0)
+      (if
+        (get_local $0)
+        (loop $while-in$2
+          (br_if $label$break$L1
+            (i32.eqz
+              (get_local $0)
             )
-            (br $while-in$2)
           )
-          (i32.const 1337)
+          (call $zeroInit
+            (i32.const 0)
+          )
+          (br $while-in$2)
         )
       )
     )