1517235 - Add fast path for number inputs to parseFloat

Status: RESOLVED FIXED

(Core :: JavaScript: Standard Library, enhancement)

Description

[André Bargull [:anba]]

Similar to the existing fast path for numbers values in parseInt we should also fast path numbers in parseFloat.


The fps counter in the benchmark from bug 1515620 calls the following function to determine if a value is a number:
---
function isNumber(n) {
  return !isNaN(parseFloat(n)) && isFinite(n);
}
---

where |n| is always a number value. That means for us we're first stringifying the number in |num_parseFloat| and immediately after that we're calling |js_strtod| to convert the string back to a number.

Especially the string conversion is expensive, because we're allocation lots of strings which leads to more work for the GC. 

Without a number fast path in |num_parseFloat|, the |averageFpsFromTimes| function from the benchmark tool spends about 14.25% of the complete runtime. With the fast path it's only at 6.25% (*).


(*) The fps counter function is also creating lots of array objects in its |numbers| function, so we spend quite some time in |js::NativeObject::setOrExtendDenseElements| when called from |js::jit::ArrayPushDense|.

Comment 1

[André Bargull [:anba]]

Attachment: bug1517235.patch

We can directly return the input value if it's a number - except for negative zero, which must be returned as positive zero. (*)

While there I've also added a fast path for strings which contain an index-value.


(*) This case is already covered in <https://searchfox.org/mozilla-central/source/js/src/tests/test262/built-ins/parseFloat/S15.1.2.3_A1_T2.js>, so I didn't have to write a new test case. \o/

Comment 2

[Jason Orendorff [:jorendorff]]

Comment on attachment 9033955 [details] [diff] [review]
bug1517235.patch

Review of attachment 9033955 [details] [diff] [review]:
-----------------------------------------------------------------

Very nice!

Comment 3

[André Bargull [:anba]]

Try: https://treeherder.mozilla.org/#/jobs?repo=try&revision=9d24fe5068a1223e0fa3ef5b0322e2c1de631fe9

Comment 4

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/mozilla-inbound/rev/58633320257f
Add fast path for number inputs to parseFloat. r=jorendorff

Comment 5

[Andrei Ciure[:aciure]]

https://hg.mozilla.org/mozilla-central/rev/58633320257f