RFR: 8307375: Alignment check on layouts used as sequence element is not correct

[Jorn Vernee]

On Wed, 3 May 2023 17:44:55 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> This patch fixes `Utils::checkElementAlignment` to do the right thing for _all_ layouts.
> 
> The current implementation is broken, as it only works correctly when the input layout is a value layout.
> Since value layouts have a size that is a power of two (and size all layouts have alignment that is also a power of two), then verifying that `size > alignment` works well.
> 
> But if the input layout is some other layout (e.g. a `StructLayout`), this "power of two" assumption no longer holds. E.g. we can have a layout whose size is 48, and whose alignment is 32. While 48 is clearly bigger than 32, such a layout is still not suitable to be used as an element layout in a sequence.
> 
> The fix is to provide two overloads for `Utils::checkElementAlignment` - one which works on `ValueLayout` and another which works on any `MemoryLayout`. The `ValueLayout` version works as before (so performance is not affected).
> The `MemoryLayout` variant would perform a full check using the `%` operator. Currently we only use this when creating a new sequence layout and when creating a stream out of a memory segment, so I'm not worried about potential performance regressions.
> 
> I've fixed the javadoc so that the various `@throws` clauses in the affected methods reflect the correct behavior.
> 
> Finally, I've made the existing alignment/layout tests a bit more robust, by also adding pair-wise combinations of layouts, wrapped in a struct/union. This does generate illegal layout cases which would not have been detected w/o this patch.

Marked as reviewed by jvernee (Reviewer).

src/java.base/share/classes/jdk/internal/foreign/Utils.java line 179:

> 177:     public static void checkElementAlignment(ValueLayout layout, String msg) {
> 178:         // Fast-path: if both size and alignment are powers of two, we can just
> 179:         // check if one is greater than the other.

Maybe we could add an `assert` here to check that the size is actually a power of two as well. Theoretically there are some value layouts for which is not the case (`long double` is 12 bytes on certain systems).

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PR Review: https://git.openjdk.org/jdk/pull/13784#pullrequestreview-1413047960
PR Review Comment: https://git.openjdk.org/jdk/pull/13784#discussion_r1184986370