754713 - IonMonkey: Assertion failure: [infer failure] Missing type pushed 0: void, at js/src/jsinfer.cpp:353

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Christian Holler (:decoder)]

The following testcase asserts on ionmonkey revision e8de64e7e9fe (run with --ion -n):


var t = 100;
function foo(ox) {
  var x = ox;
  var n = 0;
  for (var i = 0; i < 90; i++) {
    n += x.f.g;
    if (i >= 80)
      t;
  }
}
function f() {}
function g() {}
f.prototype = {f:g};
foo(new f());

Comment 1

[Jan de Mooij [:jandem]]

The problem is that IonBuilder::jsop_getprop pushes a constant value for "x.f" and therefore "x" has no SSA uses. IsPhiObservable does not like this:

    // Note that this skips reading resume points, which we don't count as
    // actual uses. This is safe as long as the SSA still mimics the actual
    // bytecode, i.e. no elimination has occurred. If the only uses are resume
    // points, then the SSA name is never consumed by the program.

So it's basically the same issue as bug 724517. Requiring SSA uses to mimic bytecode uses seems too error-prone. I will change IsPhiObservable to not skip resume points and see if it hurts benchmark performance.

Comment 3

[Jan de Mooij [:jandem]]

Bug 741203 has another great testcase, I reduced it to:


function f(x) {
    var y = (x < 0) ? 1 : 2;
    Math.floor(0); // bailout
}
Math.floor(0);
f(1);


Here we eliminate the phi for "y", then bailout and resume at SETLOCAL y. The type check fails because we set y to |undefined| instead of 2.

Comment 4

[Jan de Mooij [:jandem]]

Attachment: Patch

This is one of the simpler solutions we discussed yesterday. My initial patch fixed all places manually and modified traverseBytecode to ensure that if there's a bytecode use there must be at least one SSA use. The final patch is much simpler though: if a bytecode instruction uses a phi, it marks that phi as having bytecode uses and we always consider such phis observable. It fixes all testcases I found and does not noticeably regress performance.

Comment 5

[Jan de Mooij [:jandem]]

Attachment: Patch

Comment 6

[David Anderson [:dvander] - inactive, e-mail if emergency]

Comment on attachment 624454 [details] [diff] [review]
Patch

Review of attachment 624454 [details] [diff] [review]:
-----------------------------------------------------------------

Nice - glad to see there was both a simple and generic fix.

::: js/src/ion/IonBuilder.cpp
@@ +622,5 @@
>  
> +void
> +IonBuilder::markPhiBytecodeUses(JSOp op)
> +{
> +    unsigned nuses = analyze::GetUseCount(script, pc - script->code);

What's the difference between this and js_CodeSpec[op].nuses?

Comment 7

[Jan de Mooij [:jandem]]

(In reply to David Anderson [:dvander] from comment #6)
> 
> What's the difference between this and js_CodeSpec[op].nuses?

js_CodeSpec[op].nuses is -1 if the number of uses is dynamic (JSOP_CALL, JSOP_POPN etc). GetUseCount returns the right value in these cases.

http://hg.mozilla.org/projects/ionmonkey/rev/46eda2edc3b7

Comment 8

[Christian Holler (:decoder)]

A testcase for this bug was automatically identified at js/src/jit-test/tests/ion/bug754713-2.js.