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

[Coleen Phillimore]

On Tue, 13 Jun 2023 14:37:36 GMT, Coleen Phillimore <coleenp at openjdk.org> wrote:

>> Thanks for the suggested change.  The sign extension for integral promotion was a problem.  The second emit_int8 will still assert for the callers (there are still some) that pass a type that gets sign extended to int to emit_in8.  I can see how many are left once the first function is added.  Maybe it's possible to fix them all.  We'll see.
>
> src/hotspot/cpu/x86/assembler_x86.cpp|4421 col 12| error: no matching function for call to 'Assembler::emit_int8(Assembler::ComparisonPredicate&)'                                                                                                                                          
> ||  4421 |   emit_int8(vcc);
> ||       |   ~~~~~~~~~^~~~~
> || In file included from /scratch/cphillim/hg/21more-conversion/src/hotspot/cpu/x86/assembler_x86.cpp:26:
> 
> But I made ComparisonPredicate enum a uint8_t.  Why didn't it match it?
> 
>   // Comparison predicates for integral types & FP types when using SSE
>   enum ComparisonPredicate : uint8_t {
> 
> There are a bunch of these that don't like the templates.

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;
}```

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PR Review Comment: https://git.openjdk.org/jdk/pull/14396#discussion_r1228362727