infinite loops have side effects

Author: kripken

src/ast/effects.h

@@ -39,7 +39,7 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer> {
     if (breakNames.size() > 0) branches = true;
   }
 
-  bool branches = false; // branches out of this expression
+  bool branches = false; // branches out of this expression, returns, infinite loops, etc
   bool calls = false;
   std::set<Index> localsRead;
   std::set<Index> localsWritten;
@@ -138,6 +138,18 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer> {
   }
   void visitLoop(Loop* curr) {
     if (curr->name.is()) breakNames.erase(curr->name); // these were internal breaks
+    // if the loop is unreachable, then there is branching control flow:
+    //  (1) if the body is unreachable because of a (return), uncaught (br) etc., then we
+    //      already noted branching, so it is ok to mark it again  (if we have *caught*
+    //      (br)s, then they did not lead to the loop body being unreachable).
+    //      (same logic applies to blocks)
+    //  (2) if the loop is unreachable because it only has branches up to the loop
+    //      top, but no way to get out, then it is an infinite loop, and we consider
+    //      that a branching side effect (note how the same logic does not apply to
+    //      blocks).
+    if (curr->type == unreachable) {
+      branches = true;
+    }
   }
 
   void visitCall(Call *curr) { calls = true; }
@@ -182,6 +194,7 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer> {
         case TruncUFloat64ToInt32:
         case TruncUFloat64ToInt64: {
           implicitTrap = true;
+          break;
         }
         default: {}
       }
@@ -199,6 +212,7 @@ struct EffectAnalyzer : public PostWalker<EffectAnalyzer> {
         case RemSInt64:
         case RemUInt64: {
           implicitTrap = true;
+          break;
         }
         default: {}
       }

test/passes/remove-unused-brs_shrink-level=1.txt

@@ -123,4 +123,20 @@
    )
   )
  )
+ (func $join-and-it-becomes-unreachable (type $1)
+  (block $label$1
+   (block $block
+    (br_if $label$1
+     (i32.load8_u
+      (i32.const -93487262)
+     )
+    )
+    (br_if $label$1
+     (loop $label$5
+      (br $label$5)
+     )
+    )
+   )
+  )
+ )
 )

test/passes/remove-unused-brs_shrink-level=1.wast

@@ -77,5 +77,21 @@
       (br_if $out8 (call $b14)) ;; side effect
     )
   )
+  (func $join-and-it-becomes-unreachable
+   (block $label$1
+    (block
+     (br_if $label$1
+      (i32.load8_u
+       (i32.const -93487262)
+      )
+     )
+     (br_if $label$1
+      (loop $label$5 ;; this is unreachable (infinite loop, no exit)
+       (br $label$5)
+      )
+     )
+    )
+   )
+  )
 )
 

test/passes/remove-unused-names_vacuum.txt

@@ -4,11 +4,15 @@
  (memory $0 0)
  (func $return-i32-but-body-is-unreachable3 (type $0) (result i32)
   (local $label i32)
-  (unreachable)
+  (loop $while-in$1
+   (br $while-in$1)
+  )
  )
  (func $return-i32-but-body-is-unreachable4 (type $0) (result i32)
   (local $label i32)
-  (unreachable)
+  (loop $while-in$1
+   (br $while-in$1)
+  )
  )
  (func $to-drop-unreachable (type $1)
   (drop
@@ -17,4 +21,12 @@
    )
   )
  )
+ (func $return-i32-but-body-is-unreachable5 (type $0) (result i32)
+  (local $label i32)
+  (unreachable)
+ )
+ (func $return-i32-but-body-is-unreachable6 (type $0) (result i32)
+  (local $label i32)
+  (unreachable)
+ )
 )

test/passes/remove-unused-names_vacuum.wast

@@ -16,12 +16,25 @@
     )
     (i32.const 0)
   )
- (func $to-drop-unreachable
-  (drop
-   (block (result i32)
-    (unreachable)
+  (func $to-drop-unreachable
+   (drop
+    (block (result i32)
+     (unreachable)
+    )
    )
   )
- )
+  (func $return-i32-but-body-is-unreachable5 (result i32)
+    (local $label i32)
+    (block ;; without a name here, vaccum had a too-eager bug
+      (unreachable)
+    )
+  )
+  (func $return-i32-but-body-is-unreachable6 (result i32)
+    (local $label i32)
+    (block ;; without a name here, vaccum had a too-eager bug
+      (unreachable)
+    )
+    (i32.const 0)
+  )
 )