Auto merge of #105812 - ojeda:no-jump-tables, r=nikic

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

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(),
+    }
+}

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"{{.*}} }
+}

src/test/rustdoc-ui/z-help.stdout

@@ -92,6 +92,7 @@
     -Z                             no-analysis=val -- parse and expand the source, but run no analysis
     -Z                              no-codegen=val -- run all passes except codegen; no output
     -Z              no-generate-arange-section=val -- omit DWARF address ranges that give faster lookups
+    -Z                          no-jump-tables=val -- disable the jump tables and lookup tables that can be generated from a switch case lowering
     -Z                           no-leak-check=val -- disable the 'leak check' for subtyping; unsound, but useful for tests
     -Z                                 no-link=val -- compile without linking
     -Z                        no-parallel-llvm=val -- run LLVM in non-parallel mode (while keeping codegen-units and ThinLTO)