static field record holding downcall MethodHandle

[Per-Ake Minborg]

Hi Duncan,

As components in a record are trusted by the VM, this will give similar performance.

If you always use LOOKUP, then you could perhaps create an even more easy-to-use constructor where you just provide the name of the native function (e.g. "qsort") and the FunctionDescription.

Best, Per
________________________________
From: panama-dev <panama-dev-retn at openjdk.org> on behalf of Duncan Gittins <duncan.gittins at gmail.com>
Sent: Sunday, March 30, 2025 9:55 AM
To: panama-dev <panama-dev at openjdk.org>
Subject: static field record holding downcall MethodHandle

Sorry if this is a silly question. I wanted to check my understanding of method handles in downcalls.  The example below is from jextract, though I have similar handwritten sections in my own non-jextract examples. For best performance, the MethodHandle is declared in static field:


public static final FunctionDescriptor DESC = FunctionDescriptor.ofVoid(ADDRESS, JAVA_LONG, JAVA_LONG, ADDRESS);

public static final MemorySegment ADDR = LOOKUP.findOrThrow("qsort");

public static final MethodHandle MH = LINKER.downcallHandle(ADDR, DESC);


Would I get similar performance benefit for qsort MH.invokeExact if I changed above to use a record for Download fields instead?


record Downcall(MemorySegment address, FunctionDescriptor descriptor, MethodHandle handle) {

public Downcall(MemorySegment addr, FunctionDescriptor desc) {

this(addr, desc, Linker.nativeLinker().downcallHandle(addr, desc));

}

}

This means the definition is now a one liner:


private static final Downcall CALL = new Downcall(LOOKUP.findOrThrow("qsort"), FunctionDescriptor.ofVoid(ADDRESS, JAVA_LONG, JAVA_LONG, ADDRESS));

// invoke with CALL.handle.invokeExact(base, nitems, size, compar)

Kind regards


Duncan Gittins






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