RFR: 8288992: AArch64: CMN should be handled the same way as CMP

[Andrew Haley]

On Fri, 24 Jun 2022 12:27:48 GMT, Evgeny Astigeevich <duke at openjdk.org> wrote:

>> At present, `cmp(r8, -1)` fails at compile time, but `cmn(r8, -1)` fails at runtime. We should fix cmn() to be the same as `cmp()`. 
>> 
>> After this change, it's much less likely that we'll be surprised by immediate overflows in `cmn()`.
>
> src/hotspot/cpu/aarch64/macroAssembler_aarch64.hpp line 200:
> 
>> 198:   inline void cmp(Register Rd, unsigned imm) = delete;
>> 199: 
>> 200:   template<class T>
> 
> Why do we need it to be a template?

If we made it `unsigned int`, we'd risk a 64-bit signed value (from C2, for example) being truncated before being passed to `addsw(Register Rd, Register Rn, uint64_t imm)`. As we now have a 64-bit-clean path through all this code, that would be a Bad Thing.

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PR: https://git.openjdk.org/jdk/pull/9246