intrinsic: Add black_box intrinsic

[Pasta-coder]

This patch implements the black_box intrinsic, which is used to prevent the compiler from optimizing away a piece of code. It maps to an empty volatile inline assembly block with a memory clobber and a general input constraint, matching the behavior expected by the standard library.

Addresses #3372

gcc/rust/ChangeLog:

* backend/rust-compile-intrinsic.cc (black_box_handler): New handler.

gcc/testsuite/ChangeLog:

* rust/compile/intrinsic-black-box.rs: New test.
* rust/execute/intrinsic-black-box.rs: New test.

[P-E-P]

I don't think this was already available with rust 1.49 ?

[Pasta-coder]

While black_box was stabilized in std::hint in 1.66, it has existed as a core compiler-internal intrinsic since 2019 (see RFC #2360 and tracking issue #64102).

This implementation follows the same "infrastructure parity" logic as the existing assume and likely handlers in rust-compile-intrinsic.cc. Although their stable library wrappers (like assert_unchecked in 1.81) appeared in much later Rust versions, the underlying intrinsics were 1.49-era requirements for the test crate and core library optimization barriers.

[P-E-P]

I'm not sure I'm following you here, black box was indeed available in 1.49 but what I meant really was "did it exist as an intrinsic yet ?".
As you can see here black box was then implemented using inline assembly https://github.com/rust-lang/rust/blob/1.49.0/library/core/src/hint.rs#L159

[Pasta-coder]

yeah , you are right . i just gazed over that file . disregard my comment .