Add `-Zno-jump-tables`

This flag mimics GCC/Clang's `-fno-jump-tables` [1][2], which makes
the codegen backend avoid generating jump tables when lowering switches.

In the case of LLVM, the `"no-jump-tables"="true"` function attribute is
added to every function.

The kernel currently needs it for x86 when enabling IBT [3], as well
as for Alpha (plus VDSO objects in MIPS/LoongArch).

[1] https://gcc.gnu.org/onlinedocs/gcc/Code-Gen-Options.html#index-fno-jump-tables
[2] https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-fjump-tables
[3] https://github.com/torvalds/linux/blob/v6.1/arch/x86/Makefile#L75-L83

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/attributes.rs b/compiler/rustc_codegen_llvm/src/attributes.rs
index f3bdacf6085..487eead22b8 100644
--- a/compiler/rustc_codegen_llvm/src/attributes.rs
+++ b/compiler/rustc_codegen_llvm/src/attributes.rs
@@ -137,6 +137,14 @@ fn instrument_function_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribu
     }
 }
 
+fn nojumptables_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
+    if !cx.sess().opts.unstable_opts.no_jump_tables {
+        return None;
+    }
+
+    Some(llvm::CreateAttrStringValue(cx.llcx, "no-jump-tables", "true"))
+}
+
 fn probestack_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
     // Currently stack probes seem somewhat incompatible with the address
     // sanitizer and thread sanitizer. With asan we're already protected from
@@ -293,6 +301,7 @@ pub fn from_fn_attrs<'ll, 'tcx>(
     // FIXME: none of these three functions interact with source level attributes.
     to_add.extend(frame_pointer_type_attr(cx));
     to_add.extend(instrument_function_attr(cx));
+    to_add.extend(nojumptables_attr(cx));
     to_add.extend(probestack_attr(cx));
     to_add.extend(stackprotector_attr(cx));