From 2da18b9016454c86258f7f94cf8536d3309c1ffc Mon Sep 17 00:00:00 2001
From: Alon Zakai <azakai@google.com>
Date: Thu, 12 May 2022 11:15:04 -0700
Subject: Costs: Increase cost of casts (#4661)

Casts involve branches in the VM, so adding a cast in return for removing a branch
(like If=>Select) is not beneficial. We don't want to ever do any more casts than we
already are.
---
 test/lit/passes/remove-unused-brs-gc.wast | 109 ++++++++++++++++++++++++++++++
 1 file changed, 109 insertions(+)

(limited to 'test')

diff --git a/test/lit/passes/remove-unused-brs-gc.wast b/test/lit/passes/remove-unused-brs-gc.wast
index 458958d62..33aa08af5 100644
--- a/test/lit/passes/remove-unused-brs-gc.wast
+++ b/test/lit/passes/remove-unused-brs-gc.wast
@@ -215,4 +215,113 @@
    (unreachable)
   )
  )
+
+ ;; CHECK:      (func $casts-are-costly (param $x i32)
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (if (result i32)
+ ;; CHECK-NEXT:    (local.get $x)
+ ;; CHECK-NEXT:    (ref.test_static $struct
+ ;; CHECK-NEXT:     (ref.null any)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:    (i32.const 0)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (if (result anyref)
+ ;; CHECK-NEXT:    (local.get $x)
+ ;; CHECK-NEXT:    (ref.null any)
+ ;; CHECK-NEXT:    (ref.cast_static $struct
+ ;; CHECK-NEXT:     (ref.null any)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (if (result anyref)
+ ;; CHECK-NEXT:    (local.get $x)
+ ;; CHECK-NEXT:    (block $block (result anyref)
+ ;; CHECK-NEXT:     (block $something (result anyref)
+ ;; CHECK-NEXT:      (drop
+ ;; CHECK-NEXT:       (br_on_cast_static $something $struct
+ ;; CHECK-NEXT:        (ref.null $struct)
+ ;; CHECK-NEXT:       )
+ ;; CHECK-NEXT:      )
+ ;; CHECK-NEXT:      (ref.null any)
+ ;; CHECK-NEXT:     )
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:    (ref.null any)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (select (result anyref)
+ ;; CHECK-NEXT:    (block $block3 (result anyref)
+ ;; CHECK-NEXT:     (block $nothing
+ ;; CHECK-NEXT:      (drop
+ ;; CHECK-NEXT:       (br_on_null $nothing
+ ;; CHECK-NEXT:        (ref.null $struct)
+ ;; CHECK-NEXT:       )
+ ;; CHECK-NEXT:      )
+ ;; CHECK-NEXT:     )
+ ;; CHECK-NEXT:     (ref.null any)
+ ;; CHECK-NEXT:    )
+ ;; CHECK-NEXT:    (ref.null any)
+ ;; CHECK-NEXT:    (local.get $x)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $casts-are-costly (param $x i32)
+  ;; We never turn an if into a select if an arm has a cast of any kind, as
+  ;; those things involve branches internally, so we'd be adding more than we
+  ;; save.
+  (drop
+   (if (result i32)
+    (local.get $x)
+    (ref.test_static $struct
+     (ref.null any)
+    )
+    (i32.const 0)
+   )
+  )
+  (drop
+   (if (result anyref)
+    (local.get $x)
+    (ref.null any)
+    (ref.cast_static $struct
+     (ref.null any)
+    )
+   )
+  )
+  (drop
+   (if (result anyref)
+    (local.get $x)
+    (block (result anyref)
+     (block $something (result anyref)
+      (drop
+       (br_on_cast_static $something $struct
+        (ref.null $struct)
+       )
+      )
+      (ref.null any)
+     )
+    )
+    (ref.null any)
+   )
+  )
+  ;; However, null checks are fairly fast, and we will emit a select here.
+  (drop
+   (if (result anyref)
+    (local.get $x)
+    (block (result anyref)
+     (block $nothing
+      (drop
+       (br_on_null $nothing
+        (ref.null $struct)
+       )
+      )
+     )
+     (ref.null any)
+    )
+    (ref.null any)
+   )
+  )
+ )
 )
-- 
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