RFR: 8318446: C2: optimize stores into primitive arrays by combining values into larger store

[Yi Yang]

On Wed, 18 Oct 2023 13:04:35 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

> This is a feature requiested by @RogerRiggs and @cl4es .
> 
> **Idea**
> 
> Merging multiple consecutive small stores (e.g. 8 byte stores) into larger stores (e.g. one long store) can lead to speedup. Recently, @cl4es and @RogerRiggs had to review a few PR's where people would try to get speedups by using Unsafe (e.g. `Unsafe.putLongUnaligned`), or ByteArrayLittleEndian (e.g. `ByteArrayLittleEndian.setLong`). They have asked if we can do such an optimization in C2, rather than in the Java library code, or even user code.
> 
> This patch here supports a few simple use-cases, like these:
> 
> Merge consecutive array stores, with constants. We can combine the separate constants into a larger constant:
> https://github.com/openjdk/jdk/blob/adca9e220822884d95d73c7f070adeee2632130d/test/hotspot/jtreg/compiler/c2/TestMergeStores.java#L383-L395
> 
> Merge consecutive array stores, with a variable that was split (using shifts). We can essentially undo the splitting (i.e. shifting and truncation), and directly store the variable:
> https://github.com/openjdk/jdk/blob/adca9e220822884d95d73c7f070adeee2632130d/test/hotspot/jtreg/compiler/c2/TestMergeStores.java#L444-L456
> 
> The idea is that this would allow the introduction of a very simple API, without any "heavy" dependencies (Unsafe or ByteArrayLittleEndian):
> 
> https://github.com/openjdk/jdk/blob/adca9e220822884d95d73c7f070adeee2632130d/test/hotspot/jtreg/compiler/c2/TestMergeStores.java#L327-L338
> https://github.com/openjdk/jdk/blob/adca9e220822884d95d73c7f070adeee2632130d/test/hotspot/jtreg/compiler/c2/TestMergeStores.java#L467-L472
> 
> **Details**
> 
> This draft currently implements the optimization in an additional special IGVN phase:
> https://github.com/openjdk/jdk/blob/adca9e220822884d95d73c7f070adeee2632130d/src/hotspot/share/opto/compile.cpp#L2479-L2485
> 
> We first collect all `StoreB|C|I`, and put them in the IGVN worklist (see `Compile::gather_nodes_for_merge_stores`). During IGVN, we call `StoreNode::Ideal_merge_stores` at the end (after all other optimizations) of `StoreNode::Ideal`. We essentially try to establish a chain of mergable stores:
> 
> https://github.com/openjdk/jdk/blob/adca9e220822884d95d73c7f070adeee2632130d/src/hotspot/share/opto/memnode.cpp#L2802-L2806
> 
> Mergable stores must have the same Opcode (implies they have the same element type and hence size). Further, mergable stores must have the same control (or be separated by only a RangeCheck). Further, they must either both store constants, or adjacent segments of a larger value ...

Good idea, how many times can we touch such idealization during building jdk itself? This could be an indicator to some extent of whether the pattern is common.

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PR Comment: https://git.openjdk.org/jdk/pull/16245#issuecomment-1902992549