1457365 - Do not add source note for useless expressions

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement, P2)

Description

[Tooru Fujisawa [:arai]]

https://searchfox.org/mozilla-central/rev/78dbe34925f04975f16cb9a5d4938be714d41897/js/src/frontend/BytecodeEmitter.cpp#8703-8704
> bool
> BytecodeEmitter::emitExpressionStatement(ParseNode* pn)
> {
> ...
> 
>     if (!updateSourceCoordNotes(pn->pn_pos.begin))
>         return false;
> ...
>     if (useful) {
>         JSOp op = wantval ? JSOP_SETRVAL : JSOP_POP;
>         ValueUsage valueUsage = wantval ? ValueUsage::WantValue : ValueUsage::IgnoreValue;
>         MOZ_ASSERT_IF(expr->isKind(ParseNodeKind::Assign), expr->isOp(JSOP_NOP));
>         if (!emitTree(expr, valueUsage))
>             return false;
>         if (!emit1(op))
>             return false;

Currently updateSourceCoordNotes is called regardless of useful-ness.
But we don't have to call it for useless expressions, since we don't emit bytecode for them.

Comment 1

[Tooru Fujisawa [:arai]]

here's try with the change.
https://treeherder.mozilla.org/#/jobs?repo=try&revision=bf823a24557d083ed20fb57a223cc2fb496da996&group_state=expanded

I'll post the patch once try finishes without unexpected failure.

Comment 2

[Tooru Fujisawa [:arai]]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=add06af6ef246587eb25efc7b96523e6f9f2c7db

Comment 3

[Tooru Fujisawa [:arai]]

Attachment: Do not add source note for useless expression.

This will reduce the source note size, and also help bug 1455548,
since now we have to handle source note only if we're emitting the code there.

Comment 4

[Tooru Fujisawa [:arai]]

https://hg.mozilla.org/integration/mozilla-inbound/rev/2c8741c94a0dd857b86f48aad3e8f82cb8c5e6d7
Bug 1457365 - Do not add source note for useless expression. r=jorendorff

Comment 5

[Andreea Pavel [:apavel]]

https://hg.mozilla.org/mozilla-central/rev/2c8741c94a0d