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;
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
