RFR: 8342103: C2 compiler support for Float16 type and associated operations

[John R Rose]

On Mon, 14 Oct 2024 11:40:01 GMT, Jatin Bhateja <jbhateja at openjdk.org> wrote:

> Hi All,
> 
> This patch adds C2 compiler support for various Float16 operations added by [PR#22128](https://github.com/openjdk/jdk/pull/22128)
> 
> Following is the summary of changes included with this patch:-
> 
> 1. Detection of various Float16 operations through inline expansion or pattern folding idealizations.
> 2. Float16 operations like add, sub, mul, div, max, and min are inferred through pattern folding idealization.
> 3. Float16 SQRT and FMA operation are inferred through inline expansion and their corresponding entry points are defined in the newly added Float16Math class.
>       -    These intrinsics receive unwrapped short arguments encoding IEEE 754 binary16 values.
> 5. New specialized IR nodes for Float16 operations, associated idealizations, and constant folding routines.
> 6. New Ideal type for constant and non-constant Float16 IR nodes. Please refer to [FAQs ](https://github.com/openjdk/jdk/pull/21490#issuecomment-2482867818)for more details.
> 7. Since Float16 uses short as its storage type, hence raw FP16 values are always loaded into general purpose register, but FP16 ISA instructions generally operate over floating point registers, therefore compiler injectes reinterpretation IR before and after Float16 operation nodes to move short value to floating point register and vice versa.
> 8. New idealization routines to optimize redundant reinterpretation chains. HF2S + S2HF = HF
> 6. Auto-vectorization of newly supported scalar operations.
> 7. X86 and AARCH64 backend implementation for all supported intrinsics.
> 9. Functional and Performance validation tests.
> 
> **Missing Pieces:-**
> **-  AARCH64 Backend.**
> 
> Kindly review and share your feedback.
> 
> Best Regards,
> Jatin

As I noted on Joe's PR, I like the fact that the intrinsics are decoupled from the box class.

I'm now wondering if there is another simplification possible (as I claimed to Joe!) which is to reduce the number of intrinsics, ideally down to conversions (to and from HF).

For example, `sqrt_float16` is an intrinsic, but I think it could be just an invisible IR node.  After inlining the Java definition, you start with an IR graph that mentions `sqrtD` and is surrounded by conversion nodes.   Then you refactor the IR graph to use `sqrt_float16` directly, presumably with fewer conversions (and/or reinterprets).

Same argument for max, min, add, mul, etc.

I'm not saying the current PR is wrong, but I would like to know if it could be simplified, either now or later.

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PR Comment: https://git.openjdk.org/jdk/pull/21490#issuecomment-2424373685