On Tue, 16 May 2023 13:53:32 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
> As explained in [1], memory layouts and memory segments describe sizes using > different units. Memory layouts use bits, whereas memory segments use bytes. > This is historical: memory layouts were designed after the Minimal LDL [2], > which allowed layout strings to be used to describe both memory *and* > register. In practice that ship has sailed, and the FFM API uses layouts to > firmly model the contents of regions of memory. While it would be possible, > in principle, to tweak segments to work on bits, changing that would have > implications on how easily code that is currently using ByteBuffer can > migrate to use MemorySegment. > > For these reasons, this patch fixes the asymmetry so that layouts also model > sizes in term of bytes. > > The patch is straightforward, although a bit tedious (as you can imagine), as > various uses of bit sizes had to be turned in byte sizes. In practice, the > migration had not been too hard, for the following reasons: > > * the `withBitAlignment` and `bitSize` methods are no longer in the API, it > is easy to fix any code (mainly tests) using it; > * most code already uses ready-made constants such as `JAVA_INT` - such code > continues to work unchanged; > * the layout API de facto already required clients to work with bit sizes > that are a multiple of 8. > > The only problematic case is the presence of the > `MemoryLayout::paddingLayout(long size)` factory. As this factory is changed > to deal in bytes instead of bits, all constants passed to this factory will > need to be updated. This is not a problem for code using jextract (as > jextract will take care of generating padding) but will be an issue for code > using the layout API directly. > > [1] - https://mail.openjdk.org/pipermail/panama-dev/2023-May/019059.html javadoc: https://cr.openjdk.org/~mcimadamore/jdk/8308276/8308276/v1/javadoc/java.base/java/lang/foreign/package-summary.html specdiff: https://cr.openjdk.org/~mcimadamore/jdk/8308276/8308276/v1/specdiff_out/overview-summary.html ------------- PR Comment: https://git.openjdk.org/jdk/pull/14013#issuecomment-1551184388
