[foreign-memaccess+abi] RFR: 8268230: Foreign Linker API & Windows user32/kernel32: String conversion seems broken

[Jorn Vernee]

On Thu, 10 Jun 2021 12:33:23 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> The problem is that we only add a single 0 byte as a null terminator, regardless of the charset used. For wider char sets, more 0 bytes need to be added. For instance, for UTF_16LE two 0 bytes need to be added.
>> 
>> This patch fixes the issue by adding the null terminator to the Java string, and only then encoding it as a `byte[]`.
>
> src/jdk.incubator.foreign/share/classes/jdk/incubator/foreign/CLinker.java line 302:
> 
>> 300:     }
>> 301: 
>> 302:     private static String addNullTerminator(String str) {
> 
> This is gonna allocate another string in an already allocation-heavy code. Wouldn't it be better to just add the correct termination sequence in the segment? I suggest running StrLen benchmark before/after to make sure string conversion performance isn't negatively impacted.

I'll check the benchmark.

I looked at the CharSet API, and AFAICS the only way to get the number of bytes for the null terminator would be to create a String with the value `"\0"` then encode that and check the resulting `byte[]`. I thought going the string concat route would have better chances of being optimized.

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PR: https://git.openjdk.java.net/panama-foreign/pull/554