Bug 112836 – JSC should be a quadruple-tier VM

• Status: RESOLVED
• Resolution: FIXED
• Component: JavaScriptCore

[Filip Pizlo]

Our existing DFG optimizing compiler is pretty good, but not as good as it could be.  In fact, we've even gone so far as de-tuned it to do fewer optimizations.  This is because we care about compile times, and we want the DFG to be able to give profit to even very-short-running functions.  Right now it can produce significant speed-ups even for things that only run for ~2ms or less, and even in cases where the relevant functions are relatively complex.  But this approach also means that for longer-running code, we get less win.

This can be fixed by having an alternate, more powerful, but longer-compiling, tier of the DFG.  This would be the fourth tier of JSC.  Here's the wishlist for this tier:

1) Bring back the optimization fixpoint to allow CFG simplification to feed back into things like CSE.

2) Run the compiler on a separate thread.

3) Use a proper compiler backend that does global register allocation, GVN, PRE, LICM, etc.

[Filip Pizlo]

Branched for this work in http://trac.webkit.org/changeset/146384

[Sam Weinig]

Why not 5 onion.com/bWBRid ?

[Filip Pizlo]

(In reply to comment #2)
> Why not 5 onion.com/bWBRid ?

Assign that bug to Gavin.  He was saying something about doing AST interpretation for really simple code.

[Filip Pizlo]

(In reply to comment #1)
> Branched for this work in http://trac.webkit.org/changeset/146384

The branch is called "fourthTier".  I'm prefixing all bugs that are intended to land on this branch with "fourthTier: ".

[Filip Pizlo]

Aaaannnnnd we're done.