669796 - IonMonkey: perform unreachable code elimination during GVN

Status: RESOLVED DUPLICATE of bug 1029830

(Core :: JavaScript Engine, defect)

Description

[Ryan Pearl [:rpearl]]

Follow up to Bug 659729.

I think it's probably natural to perform unreachable code elimination during the GVN pass--I'll see if I can make this work.

Comment 1

[David Mandelin [:dmandelin]]

Could you summarize your design/algorithm idea here?

Comment 2

[Ryan Pearl [:rpearl]]

Sorry, this dropped out of sight. The algorithm would be something like:

Assume that the start block is reachable. Assume that all other blocks and edges are unreachable.

Examine control flow instructions during GVN: If an outgoing edge can be followed, mark it reachable. Once an edge becomes reachable, the target blocks of that edge become reachable. 
 
Inputs of phi nodes from unreachable edges are ignored.

Comment 3

[Paul Wright]

Is this still needed?

Comment 4

[Niko Matsakis [:nmatsakis]]

Bug 620767 provides a similar optimization, though it is not integrated with GVN.

Comment 5

[Niko Matsakis [:nmatsakis]]

My mistake, bug 620676

Comment 6

[Niko Matsakis [:nmatsakis]]

Sorry for the spam! I cannot type, apparently. Bug 820676!

Comment 8

[Niko Matsakis [:nmatsakis]]

One advantage to doing the integration proposed in this bug would be better handling of code like the following:

    if (foo() && false) { ... }

In this case we currently fail to optimize away the body.  This is because the |foo() && false| is first stored into a variable, and at the time the pass runs the constants have not been fully propagated.  This situation arises in self-hosted code for ParallelArray, where inlined functions are often evaluated to true or false to select different control flow in sequential or parallel mode.

Comment 9

[Dan Gohman [:sunfish]]

The new GVN implementation (bug 1004363) is capable of this. Bug 1029830 tracks enabling it.