1788039 - Avoid unnecessary dup/pop sequences for post-increment/decrement operations

Status: RESOLVED FIXED

(Core :: JavaScript Engine, enhancement, P1)

Description

[André Bargull [:anba]]

We can use ValueUsage to determine when the result of the post-increment/decrement operation is unused, which allows to avoid having to emit extra dup/pop operations.

For example dis(function(){ var i = 0; i++ }) improves from:

00000:   1  Zero                        # 0
00001:   1  SetLocal 0                  # 0
00005:   1  Pop                         # 
00006:   1  GetLocal 0                  # i
00010:   1  ToNumeric                   # (tonumeric i)
00011:   1  Dup                         # (tonumeric i) (tonumeric i)
00012:   1  Inc                         # (tonumeric i) (inc (tonumeric i))
00013:   1  SetLocal 0                  # (tonumeric i) (inc (tonumeric i))
00017:   1  Pop                         # (tonumeric i)
00018:   1  Pop                         # 
00019:   1  RetRval                     #

to:

00000:   1  Zero                        # 0
00001:   1  SetLocal 0                  # 0
00005:   1  Pop                         # 
00006:   1  GetLocal 0                  # i
00010:   1  ToNumeric                   # (tonumeric i)
00011:   1  Inc                         # (inc (tonumeric i))
00012:   1  SetLocal 0                  # (inc (tonumeric i))
00016:   1  Pop                         # 
00017:   1  RetRval                     #

Comment 1

[André Bargull [:anba]]

Attachment: Bug 1788039: Avoid unnecessary stack operations for post-increment/decrement. r=arai!

Pass through ValueUsage to determine when the expression result is unused.
When the result isn't used, we can then avoid having to emit dup/pop byte codes.

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/ccec17d8541d
Avoid unnecessary stack operations for post-increment/decrement. r=arai

Comment 3

[Natalia Csoregi [:nataliaCs]]

https://hg.mozilla.org/mozilla-central/rev/ccec17d8541d