On Tue, 20 Aug 2024 15:05:24 GMT, Maurizio Cimadamore <mcimadam...@openjdk.org> wrote:
> This PR reduces the amount of lambda forms (LFs) which are created when > generating var handles for simple struct field accessors. This contributes to > the startup regression seen in > [JDK-8337505](https://bugs.openjdk.org/browse/JDK-8337505). > > There are essentially three sources of excessive var handle adaptation: > > 1. `LayoutPath::dereferenceHandle` has to do some very complex adaptation > (including a permute) in order to inject alignment and size checks (against > the enclosing layout) on the generated var handle. > 2. Even in simple cases (e.g. when there's no dynamic coordinate), the offset > of the accessed field is added to the var handle via an expensive collect > adapter. > 3. When we adapt a `long` var handle to work on `MemorySegment` using an > `AddressLayout`, we make no distinction on whether the address layout has a > target layout or not. In the latter case (common!) we can adapt more simply. > > The meat of this PR is to address (1) by changing the shape of the generated > helpers in the `X-VarHandleSegmentView.java.template` class. That is, the > method for doing a plain get will now have the following shape: > > > T get(MemorySegment segment, MemoryLayout enclosing, long base, long offset) > > > Where: > * `segment` is the segment being accessed > * `enclosing` is the enclosing layout (the root of the selected layout path) > against which to check size and alignment > * `base` is the public-facing offset passed by the user when calling `get` on > the var handle > * `offset` is the offset at which the selected layout element can be found > from the root (this can be replaced with an expression that takes several > dynamic indices and turn them into a single offset) > > With this organization, it is easy to see how, in order to create a memory > access var handle for a struct field `S.f` we only need to: > * inject the enclosing layout `S` into the var handle (into the `enclosing` > coordinate) > * inject the offset of `S.f` into the var handle (into the `offset` > coordinate) > > This way, we get our plain old memory access var handle featuring only two > coordinates: a segment and an offset. Note how, to get there, we only needed > very simple adaptations (e.g. `MethodHandles::insertCoordinates`). > > #### Evaluation > > I did some tests using the benchmark in > [JDK-8337505](https://bugs.openjdk.org/browse/JDK-8337505) to assess the > impact of this change on startup. To evaluate startup, I ran the benchmark 50 > times and then took some stats. Here's what the numbers look before this > change (AVG = average, MED = med... src/java.base/share/classes/jdk/internal/foreign/Utils.java line 245: > 243: > 244: @ForceInline > 245: public static void checkEnclosingLayout(MemorySegment segment, long > offset, MemoryLayout enclosing, boolean readOnly) { Can't the first argument be `AbstractMemorySegmentImpl`? The new call site already has an `AbstractMemorySegmentImpl` and the private static method site can do the cast instead. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/20647#discussion_r1723770255
