RFR: 8342156: C2: Compilation failure with fewer arguments after JDK-8329032

[Dean Long]

On Mon, 21 Oct 2024 14:05:54 GMT, Daniel Lundén <dlunden at openjdk.org> wrote:

> Adding C2 register allocation support for APX EGPRs ([JDK-8329032](https://bugs.openjdk.org/browse/JDK-8329032)) reduced, due to unfortunate rounding in the register mask size computation, the available space for incoming/outgoing method arguments in register masks.
> 
> ### Changeset
> 
> - Bump the number of 32-bit words dedicated to incoming/outgoing arguments in register masks from 3 to 4.
> - Add a regression test.
> 
> ### Testing
> 
> - [GitHub Actions](https://github.com/dlunde/jdk/actions/runs/11436050131)
> - `tier1` to `tier4` (and additional Oracle-internal testing) on Windows x64, Linux x64, Linux aarch64, macOS x64, and macOS aarch64.
> - C2 compilation time benchmarking for DaCapo on Windows x64, Linux x64, Linux aarch64, macOS x64, and macOS aarch64. There is no observable difference in C2 compilation time.

Is this a problem when UseAPX is false?  Where exactly is the rounding problem?  If it's because number_of_registers != available_gp_registers then it feels like we are not fixing the problem but only masking its side-effects.

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PR Comment: https://git.openjdk.org/jdk/pull/21612#issuecomment-2427674557