> On Feb 19, 2016, at 4:05 AM, Peter Levart <peter.lev...@gmail.com> wrote:
>
> Hi,
>
> Just a stupid question.
>
> Your comment in String:
>
> 140 * @implNote Note this field is not {@link Stable}, because we want
> 141 * LATIN1 (0) coder to fold too. {@link Stable} would not allow that,
> 142 * as it reserves the default value as "not initialized" value.
> 143 * Constant-folding this field is handled internally in VM.
> 144 */
> 145 private final byte coder;
Why don’t you change the values to:
static final byte LATIN1 = 1;
static final byte UTF16 = 2;
?
>
>
> Couldn't @Stable final instance fields constant-fold the default value too in
> general? I mean can't it be assumed that the final field has been set before
> JIT kicks in?
>
> Regards, Peter
>
> On 02/19/2016 12:55 PM, Aleksey Shipilev wrote:
>> Hi,
>>
>> Please review a simple performance improvement in Strings:
>> https://bugs.openjdk.java.net/browse/JDK-8150180
>>
>> In short, we want VM to trust constant String contents, so that
>> "Foo".charAt(0) is folded. As far as current Hotspot goes, this is only
>> achievable with @Stable -- we need to trust the array contents:
>> http://cr.openjdk.java.net/~shade/8150180/webrev.jdk.01/
>>
>> This, however, steps into the compiler bug caused by StringUTF16.getChar
>> intrinsic producing a mismatched load, that is then folded incorrectly.
>> So we instead rely on Java level folding in cases like these:
>> http://cr.openjdk.java.net/~shade/8150180/webrev.hs.01/
>>
>> ...and it works:
>> http://cr.openjdk.java.net/~shade/8150180/notes.txt
>>
>> While VM change looks like a workaround, Vladimir I. and me concluded
>> that @Stable folding code should just reject folding mismatched loads to
>> begin with. So, getChar intrinsic change is the actual fix. Vladimir I.
>> ought to add more assertions in @Stable folding code separately:
>> https://bugs.openjdk.java.net/browse/JDK-8150186
>>
>> Since this issue might trigger compiler bugs, I would like to push
>> through hs-comp to pass compiler nightlies.
>>
>> Cheers,
>> -Aleksey
>>
>
