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8319316: Clarify text around which layouts a linker supports

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Reviewed-by: mcimadamore
This commit is contained in:
Jorn Vernee 2023-11-06 15:09:26 +00:00
parent 1696603ccd
commit cdf337357a
1 changed files with 18 additions and 2 deletions
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20
src/java.base/share/classes/java/lang/foreign/Linker.java
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@ -224,8 +224,8 @@ import java.util.stream.Stream;
* </tbody>
* </table></blockquote>
* <p>
* All native linker implementations operate on a subset of memory layouts. More formally, a layout {@code L}
* is supported by a native linker {@code NL} if:
* All native linker implementations support a well-defined subset of layouts. More formally,
* a layout {@code L} is supported by a native linker {@code NL} if:
* <ul>
* <li>{@code L} is a value layout {@code V} and {@code V.withoutName()} is a canonical layout</li>
* <li>{@code L} is a sequence layout {@code S} and all the following conditions hold:
@ -244,6 +244,22 @@ import java.util.stream.Stream;
* </li>
* </ul>
*
* Linker implementations may optionally support additional layouts, such as <em>packed</em> struct layouts.
* A packed struct is a struct in which there is at least one member layout {@code L} that has an alignment
* constraint less strict than its natural alignment. This allows avoiding padding between member layouts,
* as well as avoiding padding at the end of the struct layout. For example:
* {@snippet lang = java:
* // No padding between the 2 element layouts:
* MemoryLayout noFieldPadding = MemoryLayout.structLayout(
* ValueLayout.JAVA_INT,
* ValueLayout.JAVA_DOUBLE.withByteAlignment(4));
*
* // No padding at the end of the struct:
* MemoryLayout noTrailingPadding = MemoryLayout.structLayout(
* ValueLayout.JAVA_DOUBLE.withByteAlignment(4),
* ValueLayout.JAVA_INT);
* }
* <p>
* A native linker only supports function descriptors whose argument/return layouts are layouts supported by that linker
* and are not sequence layouts.
*