Change effect for pause (WebAssembly#8401)

The `pause` instruction doesn't have observable side effects, but for it
to fulfill its purpose of making spinlocks more efficient, we need to
avoid moving it out of loops. We previously did this by setting the
`isAtomic` effect, but that effect will soon go away in favor of a more
detailed analysis of how expressions may synchronize across threads.
Switch to using `branchesOut` as the effect to keep `pause` from being
moved instead and add a test demonstrating that it works as intended.

Author: tlively

src/ir/effects.h

@@ -644,9 +644,10 @@ class EffectAnalyzer {
       parent.isAtomic = true;
     }
     void visitPause(Pause* curr) {
-      // It's not much of a problem if pause gets reordered with anything, but
-      // we don't want it to be removed entirely.
-      parent.isAtomic = true;
+      // We don't want this to be moved out of loops, but it doesn't otherwises
+      // matter much how it gets reordered. Say we transfer control as a coarse
+      // approximation of this.
+      parent.branchesOut = true;
     }
     void visitSIMDExtract(SIMDExtract* curr) {}
     void visitSIMDReplace(SIMDReplace* curr) {}

test/lit/passes/licm.wast

@@ -1,6 +1,6 @@
 ;; NOTE: Assertions have been generated by update_lit_checks.py --all-items and should not be edited.
 
-;; RUN: foreach %s %t wasm-opt --licm -S -o - | filecheck %s
+;; RUN: foreach %s %t wasm-opt -all --licm -S -o - | filecheck %s
 
 (module
  (memory 10 20)
@@ -11,7 +11,7 @@
 
  ;; CHECK:      (memory $0 10 20)
 
- ;; CHECK:      (func $unreachable-get
+ ;; CHECK:      (func $unreachable-get (type $1)
  ;; CHECK-NEXT:  (local $x i32)
  ;; CHECK-NEXT:  (drop
  ;; CHECK-NEXT:   (local.get $x)
@@ -35,7 +35,7 @@
   )
  )
 
- ;; CHECK:      (func $unreachable-get-call (param $p i32)
+ ;; CHECK:      (func $unreachable-get-call (type $0) (param $p i32)
  ;; CHECK-NEXT:  (local $x i32)
  ;; CHECK-NEXT:  (loop $loop
  ;; CHECK-NEXT:   (unreachable)
@@ -56,7 +56,7 @@
   )
  )
 
- ;; CHECK:      (func $unreachable-get-store (param $p i32)
+ ;; CHECK:      (func $unreachable-get-store (type $0) (param $p i32)
  ;; CHECK-NEXT:  (local $x i32)
  ;; CHECK-NEXT:  (loop $loop
  ;; CHECK-NEXT:   (unreachable)
@@ -78,5 +78,30 @@
    )
   )
  )
-)
 
+ ;; CHECK:      (func $pause (type $0) (param $p i32)
+ ;; CHECK-NEXT:  (drop
+ ;; CHECK-NEXT:   (i32.const 0)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT:  (loop $loop
+ ;; CHECK-NEXT:   (nop)
+ ;; CHECK-NEXT:   (pause)
+ ;; CHECK-NEXT:   (drop
+ ;; CHECK-NEXT:    (i32.const 1)
+ ;; CHECK-NEXT:   )
+ ;; CHECK-NEXT:   (br $loop)
+ ;; CHECK-NEXT:  )
+ ;; CHECK-NEXT: )
+ (func $pause (param $p i32)
+  ;; We don't want pause to be moved out of loops. In principle we should be
+  ;; able to move side-effect-free expressions back before the pause, but we
+  ;; currently do not. Pause is rare and specialized enough that this shouldn't
+  ;; be a problem.
+  (loop $loop
+   (drop (i32.const 0))
+   (pause)
+   (drop (i32.const 1))
+   (br $loop)
+  )
+ )
+)