Bug 125480 – Impose and enforce some basic rules of sanity for where Phi functions are allowed to occur and where their (optional) corresponding MovHints can be

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

[Filip Pizlo]

Right now, if you wanted to insert some speculation right after where a value was produced, you'd get super confused if that value was produced by a Phi node.  You can't necessarily insert speculations after a Phi node because Phi nodes appear in this special sequence of Phis and MovHints that establish the OSR exit state for a block.  So, you'd probably want to search for the next place where it's safe to insert things.  We already do this "search for beginning of next bytecode instruction" search by looking at the next node that has a different CodeOrigin.  But this would be hard for a Phi because those Phis and MovHints have basically random CodeOrigins and they can all have different CodeOrigins.

This patch imposes some sanity for this situation:

- Phis must have unset CodeOrigins.

- In each basic block, all nodes that have unset CodeOrigins must come before all nodes that have set CodeOrigins.

This all ends up working out just great because prior to this patch we didn't have a use for unset CodeOrigins.  I think it's appropriate to make "unset CodeOrigin" mean that we're in the prologue of a basic block.

It's interesting what this means for block merging, which we don't yet do in SSA.  Consider merging the edge A->B.  One possibility is that the block merger is now required to clean up Phi/Upsilons, and reascribe the MovHints to have the CodeOrigin of the A's block terminal.  But an answer that might be better is that the originless nodes at the top of the B are just given the origin of the terminal and we keep the Phis.  That would require changing the above rules.  We'll see how it goes, and what we end up picking...

Overall, this special-things-at-the-top rule is analogous to what other SSA-based compilers do.  For example, LLVM has rules mandating that Phis appear at the top of a block.

[Filip Pizlo]

Created attachment 218826 [details]
the patch

[Geoffrey Garen]

Comment on attachment 218826 [details]
the patch

r=me

[Filip Pizlo]

Landed in http://trac.webkit.org/changeset/160348