1180211 - Don't emit bytecode for empty strings in emitTemplateString

Status: RESOLVED FIXED

(Core :: JavaScript Engine, defect)

Description

[Jan de Mooij [:jandem]]

Attachment: Patch

When filing bug 1180184, I noticed that for a template string like `${1}${2}` we currently emit:

00000:  string ""
00005:  int8 2
00007:  tostring
00008:  add
00009:  string ""
00014:  add
00015:  int8 3
00017:  tostring
00018:  add
00019:  string ""
00024:  add
00025:  return

With the attached patch we skip the empty strings and emit just 1 JSOP_ADD instead of 4:

00000:  int8 2
00002:  tostring
00003:  int8 3
00005:  tostring
00006:  add
00007:  return

I think doing simple optimizations like this in the emitter is still worthwhile.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Does this do the right thing with tagged templates?  That is, given:

  foo`${1}${2`

  function foo() {
  }

is foo passed the same arguments after this patch as before?

Comment 2

[Jan de Mooij [:jandem]]

(In reply to Boris Zbarsky [:bz] from comment #1)
> Does this do the right thing with tagged templates?

Yeah, bytecode for that is emitted somewhere else. It looks like this before and after the patch:

00005:  getgname "foo"
00010:  gimplicitthis "foo"
00015:  callsiteobj ["", "", ""]
00020:  one
00021:  int8 2
00023:  call 3

Comment 3

[Pulsebot]

https://hg.mozilla.org/integration/mozilla-inbound/rev/02187d02a60a

Comment 4

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/mozilla-central/rev/02187d02a60a