RFR: 8333893: Optimization for StringBuilder append boolean & null [v4]

[Emanuel Peter]

On Wed, 12 Jun 2024 14:16:50 GMT, Shaojin Wen <duke at openjdk.org> wrote:

>> Shaojin Wen has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   revert
>
> _putCharStringU will perform MergeStore but with bounds checking. Can the C2 optimizer perform MergeStore without bounds checking in this case?
> 
> StringUTF16.putChar does not have a bounds check, so there should be no bounds check after MergeStore?
> 
> 
> class AbstractStringBuilder {
>     private AbstractStringBuilder appendNull() {
>         // ...
>         StringUTF16.putCharsAt(val, count, 'n', 'u', 'l', 'l');
>         // ...
>     }
> }
> 
> class StringUTF16 {
>   	public static void putCharsAt(byte[] value, int i, char c1, char c2, char c3, char c4) {
>         putChar(value, i    , c1);
>         putChar(value, i + 1, c2);
>         putChar(value, i + 2, c3);
>         putChar(value, i + 3, c4);
>     }
> 
>  	@IntrinsicCandidate
>     // intrinsic performs no bounds checks
>     static void putChar(byte[] val, int index, int c) {
>         assert index >= 0 && index < length(val) : "Trusted caller missed bounds check";
>         index <<= 1;
>         val[index++] = (byte)(c >> HI_BYTE_SHIFT);
>         val[index]   = (byte)(c >> LO_BYTE_SHIFT);
>     }
> }

@wenshao If you look at the tests in https://github.com/openjdk/jdk/pull/16245, you can see examples like this:
https://github.com/eme64/jdk/blob/93bf2ddc9b7f584724034aec6a4f8b9fe1b2dfda/test/hotspot/jtreg/compiler/c2/TestMergeStores.java#L501-L514
So yes, we can merge them.

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PR Comment: https://git.openjdk.org/jdk/pull/19626#issuecomment-2163150153