RFR: 8377895: Create sizeof_auto, to reduce narrowing conversions [v6]

[Kim Barrett]

On Thu, 19 Feb 2026 11:13:26 GMT, Leo Korinth <lkorinth at openjdk.org> wrote:

>> `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.
>> 
>> The change in `cpCache.hpp` is an example where we would get an implicit narrowing warning (if we had those enabled) using normal `sizeof`, but where `sizeof_auto` will not make the expression a `size_t` .
>
> Leo Korinth has updated the pull request incrementally with one additional commit since the last revision:
> 
>   suggestion from Kim

Just a couple of optional comment suggestions at this point.  Otherwise looks good.

src/hotspot/share/utilities/globalDefinitions.hpp line 189:

> 187: // Example of correct usage of sizeof/sizeof_auto:
> 188: // size_t size = std::numeric_limits<uint32_t>::max() * sizeof(uint16_t); // this will wrap using sizeof_auto, use sizeof to ensure computation using size_t
> 189: // int count = 42 / sizeof_auto(uint16_t);                                // implicit narrowing conversion or compiler warning/error using stricter compiler flags when using sizeof

Could you shorten these (very) long lines?  Just move the description of each snippet
before the snippet, rather than as a same-line comment.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/29716#pullrequestreview-3828418197
PR Review Comment: https://git.openjdk.org/jdk/pull/29716#discussion_r2830069466