RFR: 8345067: C2: enable implicit null checks for ZGC reads

[Roberto Castañeda Lozano]

On Wed, 11 Dec 2024 09:59:44 GMT, Roberto Castañeda Lozano <rcastanedalo at openjdk.org> wrote:

> Currently, C2 cannot exploit late-expanded GC memory accesses as implicit null checks because of their use of temporary operands, which prevents `PhaseCFG::implicit_null_check` from [hoisting the memory accesses to the test basic block](https://github.com/openjdk/jdk/blob/f88c1c6ff86b8f29a71647e46136b6432bb67619/src/hotspot/share/opto/lcm.cpp#L319-L335).
> 
> This changeset extends the scope of the implicit null check optimization so that it can exploit ZGC object loads. It introduces a platform-dependent predicate (`MachNode::has_initial_implicit_null_check_candidate`) to mark late-expanded instructions that emit a suitable memory access as a first instruction as candidates, and extends the optimization to recognize and hoist candidate memory accesses that use temporary operands:
> 
> ![example](https://github.com/user-attachments/assets/b5f9bbc8-d75d-4cf3-841e-73db3dbae753)
> 
> Exploiting ZGC loads increases the effectiveness of the implicit null check optimization (measured in percent of explicit null checks turned into implicit ones at compile time) by around 10% in the DaCapo chopin benchmarks:
> 
> ![C2-inc-hit-rate-jdk-25+1-vs-jdk-25+1-with-8345067](https://github.com/user-attachments/assets/8d114058-c6b2-4254-a374-0d0b220af718)
> 
> The larger number of implicit null checks results in slight performance improvements (in the 1-2% range) in a few DaCapo and SPECjvm2008 benchmarks and an overall slight improvement across Renaissance benchmarks.
> 
> A further extension of the optimization to arbitrary memory access instructions (including e.g. G1 object stores, which emit multiple memory accesses at arbitrary address offsets) will be investigated separately as part of [JDK-8344627](https://bugs.openjdk.org/browse/JDK-8344627).
> 
> #### Testing
> - tier1-5, compiler stress test (linux-x64, macosx-x64, windows-x64, linux-aarch64, macosx-aarch64; release and debug mode).

Putting this PR on hold until the related RFE [JDK-8341611](https://bugs.openjdk.org/browse/JDK-8341611) (with PR https://github.com/openjdk/jdk/pull/22862 under review) is resolved.

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PR Comment: https://git.openjdk.org/jdk/pull/22678#issuecomment-2586601599