RFR: 8218675: Reduce verification overhead in ClassFileParser

[Claes Redestad]

On 2019-02-11 09:44, David Holmes wrote:
> 
>> All of the modified UTF8-encoded bytes UTF8::next would skip over will
>> have values >= 128 (assuming legal UTF-8, which UTF8::next already 
>> assumes), so they can't match the special characters we're looking for 
>> anyway, so the switch will never match any of them. Thus the new method
>> is semantically equivalent, but measurably faster.
> 
> New code compares p1, p2, p3 ... to each of the special characters to 
> see that it doesn't match. Or are you saying the switch somehow 
> generates a set of range checks rather than comparisons??

No, I'm saying that any byte value that would have been skipped over by
UTF8::next will be skipped over by the switch, since they are all 128
and higher. It's impossible that, say, a ';' hides in one of the bytes
of a codepoint.

UTF8 was designed to allow scanning for ASCII characters byte-by-byte
like this, and modified UTF-8 is no different except for '\0' (which we
don't care about here).

/Claes