Rollup merge of #140740 - ojeda:indirect-branch-cs-prefix, r=davidtwco

Add `-Zindirect-branch-cs-prefix`

Cc: ``@azhogin`` ``@Darksonn``

This goes on top of https://github.com/rust-lang/rust/pull/135927, i.e. please skip the first commit here. Please feel free to inherit it there.

In fact, I am not sure if there is any use case for the flag without `-Zretpoline*`. GCC and Clang allow it, though.

There is a `FIXME` for two `ignore`s in the test that I took from another test I did in the past -- they may be needed or not here since I didn't run the full CI. Either way, it is not critical.

Tracking issue: https://github.com/rust-lang/rust/issues/116852.
MCP: https://github.com/rust-lang/compiler-team/issues/868.

1 files changed, 19 insertions, 0 deletions

diff --git a/src/doc/unstable-book/src/compiler-flags/indirect-branch-cs-prefix.md b/src/doc/unstable-book/src/compiler-flags/indirect-branch-cs-prefix.md
new file mode 100644
index 00000000000..040e2d41701
--- /dev/null
+++ b/src/doc/unstable-book/src/compiler-flags/indirect-branch-cs-prefix.md
@@ -0,0 +1,19 @@
+# `indirect-branch-cs-prefix`
+
+The tracking issue for this feature is: https://github.com/rust-lang/rust/issues/116852.
+
+------------------------
+
+Option `-Zindirect-branch-cs-prefix` controls whether a `cs` prefix is added to
+`call` and `jmp` to indirect thunks.
+
+It is equivalent to [Clang]'s and [GCC]'s `-mindirect-branch-cs-prefix`. The
+Linux kernel uses it for RETPOLINE builds. For details, see
+[LLVM commit 6f867f910283] ("[X86] Support ``-mindirect-branch-cs-prefix`` for
+call and jmp to indirect thunk") which introduces the feature.
+
+Only x86 and x86_64 are supported.
+
+[Clang]: https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-mindirect-branch-cs-prefix
+[GCC]: https://gcc.gnu.org/onlinedocs/gcc/x86-Options.html#index-mindirect-branch-cs-prefix
+[LLVM commit 6f867f910283]: https://github.com/llvm/llvm-project/commit/6f867f9102838ebe314c1f3661fdf95700386e5a