RFR: 8309685: Fix -Wconversion warnings in assembler and register code [v2]

[Coleen Phillimore]

On Tue, 13 Jun 2023 18:35:20 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Taking out the ENABLE_IF<is_integral> at least allows it to compile but there are lots of crashes because of sign extension.
>> 
>> Maybe we can make checked_cast<> tolerant of that somehow?
>> 
>> ```template <typename T2, typename T1>
>> constexpr T2 checked_cast(T1 thing) {
>>   T2 result = static_cast<T2>(thing);
>>   assert(static_cast<T1>(result) == thing, "must be");
>>   return result;
>> }```
>
> How about something like this?
> 
>   template<typename T, ENABLE_IF(std::is_same<T, uint8_t>::value)>
>   void emit_int8(T v) { code_section()->emit_int8(v); }
> 
>   template<typename T, ENABLE_IF(!std::is_same<T, uint8_t>::value)>
>   void emit_int8(T v) {
>     if (v < 0) {
>       int8_t tmp = static_cast<int8_t>(v);
>       assert(static_cast<T>(v) == tmp, "signed cast preserves value");
>       emit_int8(static_cast<uint8_t>(v));
>     } else {
>       emit_int8(checked_cast<uint8_t>(v)); 
>     }
>   }

I pushed something that I think is better.

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PR Review Comment: https://git.openjdk.org/jdk/pull/14396#discussion_r1229526028