1830068 - Optimize codegen for loading aliased lexicals

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, task, P3)

Description

[Jan de Mooij [:jandem]]

We often have a sequence of MLoadFixedSlot => MLexicalCheck => MUnbox when accessing lexicals in an outer scope. If we add support for MLexicalCheck to FoldLoadsWithUnbox, we can eliminate the lexical check and fold the load and unbox.

Comment 1

[Jan de Mooij [:jandem]]

Attachment: Bug 1830068 - Optimize MLexicalCheck in FoldLoadsWithUnbox. r?iain!

A common MIR pattern when loading lexicals from an outer scope is
MLoad{Fixed,Dynamic}Slot => MLexicalCheck => MUnbox(fallible).

This patch makes it possible to eliminate the lexical check in this case and to
fold the load and unbox into MLoad{Fixed,Dynamic}SlotAndUnbox, because this
instruction will bail out if we see the uninitialized-lexical magic value so a
separate check for it is redundant. This is similar to how we don't need a separate
hole check for MLoadElementAndUnbox.

We have test coverage for this case in multiple jit-tests.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/e8232b1436b9
Optimize MLexicalCheck in FoldLoadsWithUnbox. r=iain

Comment 3

[Cristian Tuns]

https://hg.mozilla.org/mozilla-central/rev/e8232b1436b9