kbuild: rust: add `CONFIG_RUSTC_CLANG_LLVM_COMPATIBLE`

This config detects if Rust and Clang have matching LLVM major version.
All IR or bitcode operations (e.g. LTO) rely on LLVM major version to be
matching, otherwise it may generate errors, or worse, miscompile
silently due to change of IR semantics.

It's usually suggested to use the exact same LLVM version, but this can
be difficult to guarantee. Rust's suggestion [1] is also major-version
only, so I think this check is sufficient for the kernel.

Link: https://doc.rust-lang.org/rustc/linker-plugin-lto.html
Reviewed-by: Andreas Hindborg <a.hindborg@kernel.org>
Signed-off-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Nicolas Schier <nsc@kernel.org>
Tested-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260203-inline-helpers-v2-1-beb8547a03c9@google.com
[ Fixed typo. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

diff --git a/init/Kconfig b/init/Kconfig
index 444ce811ea674a402db3056bcf3784db78fe13e0..36d32ea445942814f5576a78097621ad6c16f950 100644 (file)
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -82,6 +82,21 @@ config RUSTC_LLVM_VERSION
        int
        default $(rustc-llvm-version)
 
+config RUSTC_LLVM_MAJOR_VERSION
+       int
+       default $(shell,expr $(rustc-llvm-version) / 10000)
+
+config RUSTC_CLANG_LLVM_COMPATIBLE
+       bool
+       default y if CC_IS_CLANG && RUSTC_LLVM_MAJOR_VERSION = $(shell,expr $(cc-version) / 10000)
+       help
+         This indicates whether Rust and Clang use LLVM of the same major
+         version.
+
+         Operations involving handling LLVM IR or bitcode (e.g. cross-language
+         LTO) require the same LLVM major version to work properly. For best
+         compatibility it is recommended that the exact same LLVM is used.
+
 config ARCH_HAS_CC_CAN_LINK
        bool