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

[Roberto Castañeda Lozano]

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 suggest handling this in two steps:
> 
> * In JDK 25 we fix the crash when UseFPUForSpilling is enabled.
> * In the next release we prohibit the option softly: if it is set on the command line, the VM prints a warning and resets it to false. Proposed change for the latter:
> 
> ```diff
> diff --git a/src/hotspot/cpu/aarch64/vm_version_aarch64.cpp b/src/hotspot/cpu/aarch64/vm_version_aarch64.cpp
> index 308deeaf5e2..7702988c11c 100644
> --- a/src/hotspot/cpu/aarch64/vm_version_aarch64.cpp
> +++ b/src/hotspot/cpu/aarch64/vm_version_aarch64.cpp
> @@ -621,4 +621,9 @@ void VM_Version::initialize() {
>      FLAG_SET_DEFAULT(UseVectorizedHashCodeIntrinsic, true);
>    }
> +
> +  if (UseFPUForSpilling) {
> +    warning("UseFPUForSpilling is known to degrade performance on this platform and will be ignored.");
> +    FLAG_SET_DEFAULT(UseFPUForSpilling, false);
> +  }
>  #endif
>  
> ```

This seems reasonable to me as well.

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

PR Comment: https://git.openjdk.org/jdk/pull/27350#issuecomment-3337898957