This affects the crypto-sha1 test in JetStream 2 (test originally from SunSpider). We spend time in str_charCodeAt because the string here is a rope initially:

  for(var i = 0; i < str.length * chrsz; i += chrsz) {
    bin[i>>5] |= (str.charCodeAt(i / chrsz) & mask) << (32 - chrsz - i%32);
  }

This is probably quite common.

The crypto-md5 test has the same issue.

charCodeAt() should normally flatten ropes - unless we're hitting bug 1382639 here?

(In reply to André Bargull [:anba] from comment #2)

charCodeAt() should normally flatten ropes

But if we fail to inline charCodeAt we attach a CallNative stub so we'll never try optimizing charCodeAt again..

We can't inline String.prototype.charAt and String.prototype.charCodeAt in a
couple of JetStream tests because the input is a nested rope.

Before this change, we can observe the following number of non-inlined calls to
these functions:

crypto
String.prototype.charCodeAt 360'000

pdfjs
String.prototype.charCodeAt 21'910'000

typescript
String.prototype.charCodeAt 17'840'000

base64-SP
String.prototype.charCodeAt 39'300'000

crypto-md5-SP
String.prototype.charCodeAt 41'750'000

crypto-sha1-SP
String.prototype.charCodeAt 30'180'000

acorn-wtb
String.prototype.charCodeAt 3'370'000

jshint-wtb
String.prototype.charAt 2'450'000
String.prototype.charCodeAt 200'000

uglify-js-wtb
String.prototype.charAt 450'000

After this change, only the WBT sub-benchmarks still have non-inlined calls:

acorn-wtb
String.prototype.charCodeAt 1'150'000

jshint-wtb
String.prototype.charAt 2'450'000
String.prototype.charCodeAt 200'000

uglify-js-wtb
String.prototype.charAt 450'000

Depends on D162725

https://hg.mozilla.org/mozilla-central/rev/6810bbf2119c