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, 34 insertions, 0 deletions

diff --git a/src/test/assembly/x86_64-no-jump-tables.rs b/src/test/assembly/x86_64-no-jump-tables.rs
new file mode 100644
index 00000000000..007c3591a4a
--- /dev/null
+++ b/src/test/assembly/x86_64-no-jump-tables.rs
@@ -0,0 +1,34 @@
+// Test that jump tables are (not) emitted when the `-Zno-jump-tables`
+// flag is (not) set.
+
+// revisions: unset set
+// assembly-output: emit-asm
+// compile-flags: -O
+// [set] compile-flags: -Zno-jump-tables
+// only-x86_64
+
+#![crate_type = "lib"]
+
+extern "C" {
+    fn bar1();
+    fn bar2();
+    fn bar3();
+    fn bar4();
+    fn bar5();
+    fn bar6();
+}
+
+// CHECK-LABEL: foo:
+#[no_mangle]
+pub unsafe fn foo(x: i32) {
+    // unset: LJTI0_0
+    // set-NOT: LJTI0_0
+    match x {
+        1 => bar1(),
+        2 => bar2(),
+        3 => bar3(),
+        4 => bar4(),
+        5 => bar5(),
+        _ => bar6(),
+    }
+}