RFR: 8359378: aarch64: crash when using -XX:+UseFPUForSpilling

[Andrew Dinn]

On Wed, 17 Sep 2025 16:19:12 GMT, Boris Ulasevich <bulasevich at openjdk.org> wrote:

> AArch64 BarrierSetAssembler path assumes only FP/vector ideal regs reach the FP spill/restore encoding. With -XX:+UseFPUForSpilling Register Allocator may allocate scalar values in FP registers. When such values (Op_RegI/Op_RegN/Op_RegL/Op_RegP) hit `BarrierSetAssembler::encode_float_vector_register_size`, we trip ShouldNotReachHere in release build and **"unexpected ideal register"** assertion in debug build.
> 
> Fix: teach the encoder to handle scalar ideal regs when they physically live in FP regs:
> - treat Op_RegI / Op_RegN as 32-bit (single slot) - same class as Op_RegF
> - treat Op_RegL / Op_RegP as 64-bit (two slots) - same class as Op_RegD
> 
> Related:
> - reproduced since #19746
> - spilling logic: 
>   - #18967
>   - #17977
> 
> Testing: tier1-3 with javaoptions -Xcomp -Xbatch -XX:+UseFPUForSpilling on AARCH

I was wondering about that. So, perhaps a better fix is to change the command line ergonomics so that AArch64 either 1) refuses to run with it set to true or 2) prints a warning and resets it to false.

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PR Comment: https://git.openjdk.org/jdk/pull/27350#issuecomment-3307970063