RFR: 8260355: AArch64: deoptimization stub should save vector registers

[Vladimir Ivanov]

On Thu, 28 Jan 2021 08:27:22 GMT, Nick Gasson <ngasson at openjdk.org> wrote:

> This is an AArch64 port of the fix for JDK-8256056 "Deoptimization stub
> doesn't save vector registers on x86". The problem is that a vector
> produced by the Vector API may be stored in a register when the deopt
> blob is called. Because the deopt blob only stores the lower half of
> vector registers, the full vector object cannot be rematerialized during
> deoptimization. So the following will crash on AArch64 with current JDK:
> 
>   make test TEST="jdk/incubator/vector" \
>     JTREG="VM_OPTIONS=-XX:+DeoptimizeALot -XX:DeoptimizeALotInterval=0"
> 
> The fix is to store the full vector registers by passing
> save_vectors=true to save_live_registers() in the deopt blob. Because
> save_live_registers() places the integer registers above the floating
> registers in the stack frame, RegisterSaver::r0_offset_in_bytes() needs
> to calculate the SP offset based on whether full vectors were saved, and
> whether those vectors were NEON or SVE, rather than using a static
> offset as it does currently.
> 
> The change to VectorSupport::allocate_vector_payload_helper() is
> required because we only store the lowest VMReg slot in the oop map.
> However unlike x86 the vector registers are always saved in a contiguous
> region of memory, so we can calculate the address of each vector element
> as an offset from the address of the first slot. X86 handles this in
> RegisterMap::pd_location() but that won't work on AArch64 because with
> SVE there isn't a unique VMReg corresponding to each four-byte physical
> slot in the vector (there are always exactly eight logical VMRegs
> regardless of the actual vector length).
> 
> Tested hotspot_all_no_apps and jdk_core.

src/hotspot/share/prims/vectorSupport.cpp line 138:

> 136: 
> 137:     for (int i = 0; i < num_elem; i++) {
> 138:       bool contiguous = X86_ONLY(false) NOT_X86(true);

I don't like this change. It's not x86-specific, but SVE-specific code. What is broken here is `VMReg::next()` doesn't work properly for `VecA` registers. And, as a result, it makes `RegisterMap::location(VMReg)` unusable as well. 

So, a proper fix should address that instead. If there's no way to save `VMReg::next()` and `RegisterMap::location(VMReg)`, then new cross-platform API should be introduced and `VectorSupport::allocate_vector_payload_helper()` migrated to it.

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PR: https://git.openjdk.java.net/jdk/pull/2279