Account for global effects in LinearExecutionWalker

Author: stevenfontanella

src/ir/linear-execution.h

@@ -80,11 +80,10 @@ struct LinearExecutionWalker : public PostWalker<SubType, VisitorType> {
   static void scan(SubType* self, Expression** currp) {
     Expression* curr = *currp;
 
-    auto handleCall = [&](bool isReturn) {
+    auto handleCall = [&](bool mayThrow, bool isReturn) {
       if (!self->connectAdjacentBlocks) {
-        // Control is nonlinear if we return, or if EH is enabled or may be.
-        if (isReturn || !self->getModule() ||
-            self->getModule()->features.hasExceptionHandling()) {
+        // Control is nonlinear if we return or throw.
+        if (isReturn || !self->getModule() || mayThrow) {
           self->pushTask(SubType::doNoteNonLinear, currp);
         }
       }
@@ -153,11 +152,31 @@ struct LinearExecutionWalker : public PostWalker<SubType, VisitorType> {
         break;
       }
       case Expression::Id::CallId: {
-        handleCall(curr->cast<Call>()->isReturn);
+        auto* call = curr->cast<Call>();
+
+        bool mayThrow = !self->getModule() ||
+                        self->getModule()->features.hasExceptionHandling();
+        if (mayThrow && self->getModule()) {
+          auto* effects =
+            self->getModule()->getFunction(call->target)->effects.get();
+
+          if (effects && !effects->throws_) {
+            mayThrow = false;
+          }
+        }
+
+        handleCall(mayThrow, call->isReturn);
         return;
       }
       case Expression::Id::CallRefId: {
-        handleCall(curr->cast<CallRef>()->isReturn);
+        auto* callRef = curr->cast<CallRef>();
+
+        // TODO: Effect analysis for indirect calls isn't implemented yet.
+        // Assume any indirect call my throw for now.
+        bool mayThrow = !self->getModule() ||
+                        self->getModule()->features.hasExceptionHandling();
+
+        handleCall(mayThrow, callRef->isReturn);
         return;
       }
       case Expression::Id::TryId: {

test/lit/passes/simplify-locals-global-effects-eh.wast

@@ -0,0 +1,57 @@
+;; NOTE: Assertions have been generated by update_lit_checks.py and should not be edited.
+;; RUN: foreach %s %t wasm-opt --enable-exception-handling --generate-global-effects --simplify-locals -S -o - | filecheck %s
+
+(module
+  ;; CHECK:      (global $g (mut i32) (i32.const 0))
+
+  ;; CHECK:      (tag $t (type $0))
+  (tag $t)
+
+  (global $g (mut i32) (i32.const 0))
+
+  ;; CHECK:      (func $nop
+  ;; CHECK-NEXT: )
+  (func $nop
+  )
+
+  ;; CHECK:      (func $throws
+  ;; CHECK-NEXT:  (throw $t)
+  ;; CHECK-NEXT: )
+  (func $throws
+    (throw $t)
+  )
+
+  ;; CHECK:      (func $read-g (result i32)
+  ;; CHECK-NEXT:  (local $x i32)
+  ;; CHECK-NEXT:  (nop)
+  ;; CHECK-NEXT:  (call $nop)
+  ;; CHECK-NEXT:  (global.get $g)
+  ;; CHECK-NEXT: )
+  (func $read-g (result i32)
+    (local $x i32)
+    (local.set $x (global.get $g))
+
+    ;; With --global-effects, we can tell that this doesn't throw, so it
+    ;; doesn't act as a barrier to optimize. The local is optimized away.
+    (call $nop)
+    (local.get $x)
+  )
+
+  ;; CHECK:      (func $read-g-with-throw-in-between (result i32)
+  ;; CHECK-NEXT:  (local $x i32)
+  ;; CHECK-NEXT:  (local.set $x
+  ;; CHECK-NEXT:   (global.get $g)
+  ;; CHECK-NEXT:  )
+  ;; CHECK-NEXT:  (call $throws)
+  ;; CHECK-NEXT:  (local.get $x)
+  ;; CHECK-NEXT: )
+  (func $read-g-with-throw-in-between (result i32)
+    (local $x i32)
+    (local.set $x (global.get $g))
+
+    ;; A potential throw halts our optimizations.
+    (call $throws)
+
+    (local.get $x)
+  )
+)