RFR: 8377658: G1: Cleanup g1ConcurrentMark.cpp for stricter warning flags

[Leo Korinth]

On Thu, 12 Feb 2026 20:30:31 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Cleanup `g1ConcurrentMark.cpp` and included header files to remove implicit narrowing conversions.
>> 
>> For gcc: `-Wconversion -Wno-float-conversion`
>> For clang: `-Wimplicit-int-conversion`
>> 
>> `sizeof_auto` is created so that we can reduce the amount of casting and thus make the code more type safe. The normal `sizeof` will return a `size_t` although the size of most stuff can be represented in a `uint8_t`. `sizeof_auto`  will return the size in an as small unsigned type as is possible. The result is that expressions that uses `sizeof_auto` can convert to most integral types, and no explicit narrowing cast will be needed.
>
> src/hotspot/share/oops/fieldInfo.hpp line 305:
> 
>> 303:   // boilerplate:
>> 304:   u1 _flags;
>> 305:   static constexpr u1 flag_mask(FieldStatusBitPosition pos) { return static_cast<u1>(1 << pos); }
> 
> Shouldn't this use `checked_cast` or Kim's proposed `integer_cast` to make sure that a future change to `FieldStatusBitPosition` doesn't make this overflow an `u1`?

Yes, that is better.

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PR Review Comment: https://git.openjdk.org/jdk/pull/29701#discussion_r2803635880