RFR: 8342103: C2 compiler support for Float16 type and associated scalar operations [v8]

[Jatin Bhateja]

> 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/22754#issuecomment-2543982577)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 generally operates over floating point registers, thus the compiler injects 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
> 9. X86  backend implementation for all supported intrinsics.
> 10. Functional and Performance validation tests.
> 
> Kindly review the patch and share your feedback.
> 
> Best Regards,
> Jatin

Jatin Bhateja has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains nine additional commits since the last revision:

 - Updating copyright year of modified files.
 - Merge branch 'master' of http://github.com/openjdk/jdk into JDK-8342103
 - Review suggestions incorporated.
 - Review comments resolutions
 - Addressing review comments
 - Fixing obfuscation due to intrinsic entries
 - Adding more test points
 - Adding missed check in container type detection.
 - C2 compiler support for float16 scalar operations.

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/22754/files
  - new: https://git.openjdk.org/jdk/pull/22754/files/dd444c44..d3cbf2c4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=22754&range=07
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=22754&range=06-07

  Stats: 17820 lines in 567 files changed: 13308 ins; 2583 del; 1929 mod
  Patch: https://git.openjdk.org/jdk/pull/22754.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22754/head:pull/22754

PR: https://git.openjdk.org/jdk/pull/22754