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, 22 insertions, 0 deletions

diff --git a/src/test/codegen/no-jump-tables.rs b/src/test/codegen/no-jump-tables.rs
new file mode 100644
index 00000000000..8e2cb47566e
--- /dev/null
+++ b/src/test/codegen/no-jump-tables.rs
@@ -0,0 +1,22 @@
+// Test that the `no-jump-tables` function attribute are (not) emitted when
+// the `-Zno-jump-tables` flag is (not) set.
+
+// revisions: unset set
+// needs-llvm-components: x86
+// compile-flags: --target x86_64-unknown-linux-gnu
+// [set] compile-flags: -Zno-jump-tables
+
+#![crate_type = "lib"]
+#![feature(no_core, lang_items)]
+#![no_core]
+
+#[lang = "sized"]
+trait Sized {}
+
+#[no_mangle]
+pub fn foo() {
+    // CHECK: @foo() unnamed_addr #0
+
+    // unset-NOT: attributes #0 = { {{.*}}"no-jump-tables"="true"{{.*}} }
+    // set: attributes #0 = { {{.*}}"no-jump-tables"="true"{{.*}} }
+}