RFR: 8345067: C2: enable implicit null checks for ZGC reads [v6]

[Martin Doerr]

On Tue, 27 May 2025 07:46:43 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 (`MachTemp`), 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::is_late_expanded_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)
>> 
>> ZGC object loads are marked as late-expanded null-check candidates unconditionally on all ZGC-supported platforms except on aarch64, where only loads that do not require an initial `lea` instruction (due to [address legitimization](https://github.com/openjdk/jdk/blob/ddd07b107e814ec846579a66d4f2005b7db9bb2f/src/hotspot/cpu/aarch64/macroAssembler_aarch64.hpp#L132-L144)) are marked as candidates. Fortunately, most aarch64 loads seen in practice use small offsets and can be marked as candidates.
>> 
>> Exploiting ZGC loads increases the effectiveness of the implicit null check optimization (percent of explicit null checks turned into implicit ones at compile time) by around 10% in the DaCapo23 benchmarks. This 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.
>> 
>> #### Testing
>> - tier1-5, compiler stress test (linux-x64, macosx-x64, windows-x64, linux-aarch64, macosx-aarch64; release and debug mode).
>
> Roberto Castañeda Lozano has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Include address mode test in 'legitimize_address'
>  - Excluded IR checks for testLoadVolatile on PPC64

> > I guess it's not worth stepping over the memory barrier. Disabling this rule for PPC64 should be ok, too.
> 
> Thanks for testing and reporting @TheRealMDoerr, I agree that it would be too much complexity for little return. I disabled the rule for PPC64 (commit [fdf34f9](https://github.com/openjdk/jdk/commit/fdf34f905fd1ee4dde27374d66b1b7fb251e1622)). Please let me know if that works as expected.

Thanks! TestImplicitNullChecks has passed on PPC64.

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

PR Comment: https://git.openjdk.org/jdk/pull/25066#issuecomment-2911772562