Optimistically assume that elements IC only transition once.

test/mjsunit/elements-transition-hoisting.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 40 matching lines @@
   function testDoubleConversion4(a) {
     var object = new Object();
     a[0] = 0;
     var count = 3;
     do {
       a[0] = object;
     } while (--count > 0);
   }
 
   testDoubleConversion4(new Array(5));
+  testDoubleConversion4(new Array(5)); // Call twice to make sure that second

[Jakob Kummerow, 2012/06/08 12:18:30]

nit: Two spaces between ; and //, and a full stop at the end.
Consider putting the comment on its own line to save a line break.

More of the same below.

+                                       // store is a transition and not
+                                       // optimistically MONOMORPHIC
   %OptimizeFunctionOnNextCall(testDoubleConversion4);
   testDoubleConversion4(new Array(5));
   testDoubleConversion4(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testDoubleConversion4));
 
   // Make sure that non-element related map checks that are not preceded by
   // transitions in a loop still get hoisted in a way that doesn't generate a
   // deopt in simple cases.
   function testExactMapHoisting(a) {
     var object = new Object();
     a.foo = 0;
     a[0] = 0;
     a[1] = 1;
     var count = 3;
     do {
       a.foo = object; // This map check should be hoistable
       a[1] = object;
       result = a.foo == object && a[1] == object;
     } while (--count > 0);
   }
 
   testExactMapHoisting(new Array(5));
+  testExactMapHoisting(new Array(5)); // Call twice to make sure that second
+                                      // store is a transition and not
+                                      // optimistically MONOMORPHIC
   %OptimizeFunctionOnNextCall(testExactMapHoisting);
   testExactMapHoisting(new Array(5));
   testExactMapHoisting(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testExactMapHoisting));
 
   // Make sure that non-element related map checks do NOT get hoisted if they
   // depend on an elements transition before them and it's not possible to hoist
   // that transition.
   function testExactMapHoisting2(a) {
     var object = new Object();
     a.foo = 0;
     a[0] = 0;
     a[1] = 1;
     var count = 3;
     do {
       if (a.bar === undefined) {
         a[1] = 2.5;
       }
       a.foo = object; // This map check should NOT be hoistable because it
                       // includes a check for the FAST_ELEMENTS map as well as
                       // the FAST_DOUBLE_ELEMENTS map, which depends on the
                       // double transition above in the if, which cannot be
                       // hoisted.
     } while (--count > 0);
   }
 
   testExactMapHoisting2(new Array(5));
+  testExactMapHoisting2(new Array(5)); // Call twice to make sure that second
+                                       // store is a transition and not
+                                       // optimistically MONOMORPHIC
   %OptimizeFunctionOnNextCall(testExactMapHoisting2);
   testExactMapHoisting2(new Array(5));
   testExactMapHoisting2(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testExactMapHoisting2));
 
   // Make sure that non-element related map checks do get hoisted if they use
   // the transitioned map for the check and all transitions that they depend
   // upon can hoisted, too.
   function testExactMapHoisting3(a) {
     var object = new Object();
     a.foo = 0;
     a[0] = 0;
     a[1] = 1;
     var count = 3;
     do {
       a[1] = 2.5;
       a.foo = object; // This map check should be hoistable because all elements
                       // transitions in the loop can also be hoisted.
     } while (--count > 0);
   }
 
   var add_transition = new Array(5);
   add_transition.foo = 0;
   add_transition[0] = new Object(); // For FAST_ELEMENT transition to be created
   testExactMapHoisting3(new Array(5));
+  testExactMapHoisting3(new Array(5)); // Call twice to make sure that second
+                                       // store is a transition and not
+                                       // optimistically MONOMORPHIC
   %OptimizeFunctionOnNextCall(testExactMapHoisting3);
   testExactMapHoisting3(new Array(5));
   testExactMapHoisting3(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testExactMapHoisting3));
 
   function testDominatingTransitionHoisting1(a) {
     var object = new Object();
     a[0] = 0;
     var count = 3;
     do {
       if (a.baz != true) {
         a[1] = 2.5;
       }
       a[0] = object;
     } while (--count > 3);
   }
 
   testDominatingTransitionHoisting1(new Array(5));
+  testDominatingTransitionHoisting1(new Array(5)); // Call twice to make sure
+                                                   // that second store is a
+                                                   // transition and not
+                                                   // optimistically MONOMORPHIC
   %OptimizeFunctionOnNextCall(testDominatingTransitionHoisting1);
   testDominatingTransitionHoisting1(new Array(5));
   testDominatingTransitionHoisting1(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testDominatingTransitionHoisting1));
 
   function testHoistingWithSideEffect(a) {
     var object = new Object();
     a[0] = 0;
     var count = 3;
     do {
       assertTrue(true);
       a[0] = object;
     } while (--count > 3);
   }
 
   testHoistingWithSideEffect(new Array(5));
+  testHoistingWithSideEffect(new Array(5)); // Call twice to make sure that
+                                            // second store is a transition and
+                                            // not optimistically MONOMORPHIC
   %OptimizeFunctionOnNextCall(testHoistingWithSideEffect);
   testHoistingWithSideEffect(new Array(5));
   testHoistingWithSideEffect(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testHoistingWithSideEffect));
 
   function testStraightLineDupeElinination(a,b,c,d,e,f) {
     var count = 3;
     do {
       assertTrue(true);
       a[0] = b;
@@ skipping 23 matching lines @@
   testStraightLineDupeElinination(new Array(5),.5,0,0,0,0);
   testStraightLineDupeElinination(new Array(5),0,.5,0,0,0);
   testStraightLineDupeElinination(new Array(5),0,0,.5,0,0);
   testStraightLineDupeElinination(new Array(5),0,0,0,.5,0);
   testStraightLineDupeElinination(new Array(5),0,0,0,0,.5);
   %OptimizeFunctionOnNextCall(testStraightLineDupeElinination);
   testStraightLineDupeElinination(new Array(5));
   testStraightLineDupeElinination(new Array(5));
   assertTrue(2 != %GetOptimizationStatus(testStraightLineDupeElinination));
 }