RFR: 8320353: Reenable stringop-overflow warnings

[Julian Waters]

On Tue, 1 Jul 2025 12:25:04 GMT, Anton Artemov <duke at openjdk.org> wrote:

> Hi, please consider the following changes:
> 
> this PR addresses the issue of stringop-overflow warnings produced by GCC. The compiler does think that the thread pointer returned by `JavaThread::current()` can be null, though it cant. The thread pointer ends up being an argument in `__atomic_load`, and the compiler reports the warning related to argument of that method. 
> 
> The patch adds a hint to the compiler by means of `__builtin_unreachable()` intrinsic, which tells the compiler that certain piece of code will never be reached (case of thread pointer being null). This solves the issue. 
> 
> Tested in tiers 1-3 and GHA.

src/hotspot/share/runtime/javaThread.hpp line 1070:

> 1068:   // Returns the running thread as a JavaThread
> 1069:   static JavaThread* current() {
> 1070:     auto result = JavaThread::cast(Thread::current());

Why auto when the type is known?

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PR Review Comment: https://git.openjdk.org/jdk/pull/26067#discussion_r2179528150