RFR: 8358104: Fix ZGC compilation error on GCC 10.2

[Joel Sikström]

On Fri, 30 May 2025 02:40:48 GMT, Qizheng Xing <qxing at openjdk.org> wrote:

> JDK-8350441 introduced Mapped Cache for ZGC. However, the constructor of `ZMappedCache` uses brace-initialization for `_size_class_lists`, i.e. a `ZList` array. `ZList` is a class with a deleted copy constructor and an explicit destructor, and when its array is brace-initialized in the constructor, it triggers [GCC bug 63707](https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63707). A short example to reproduce this bug: https://godbolt.org/z/3397bxc73
> 
> The bug causes compilation error of ZGC on GCC versions 10.1 to 10.2. Considering OpenJDK compilation is still requires GCC 10 or higher, this should be recorded as a bug.
> 
> This patch uses value-initialization for `_size_class_lists` instead of brace-initialization, which should be semantically equivalent and work on all GCC versions.

Marked as reviewed by jsikstro (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/25536#pullrequestreview-2880739474