RFR: 8366193: Add comments about ResolvedFieldEntry::copy_from()

[Francesco Andreuzzi]

On Tue, 26 Aug 2025 16:53:17 GMT, Ioi Lam <iklam at openjdk.org> wrote:

> [JDK-8293980](https://bugs.openjdk.org/browse/JDK-8293980) added `ResolvedFieldEntry::copy_from()` to prevent random bits in the CDS archive. However, I forgot to add a comment to explain why this is necessary.
> 
> This PR adds the missing comment, which will be useful if we consider alternative solutions (using `memset()` in constructor??) in the future.

Hi @iklam, thanks for following up on this. I was wondering, would it be possible to explicitly set padding fields with default value zero? Something like this:

u8 padding1;
#ifdef _LP64
u32 padding2;
#endif

so we could use the default constructor, like I'm doing in #26818.

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PR Comment: https://git.openjdk.org/jdk/pull/26946#issuecomment-3227351623