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>

Author: ojeda

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));
 

compiler/rustc_interface/src/tests.rs

@@ -754,6 +754,7 @@ fn test_unstable_options_tracking_hash() {
     tracked!(move_size_limit, Some(4096));
     tracked!(mutable_noalias, Some(true));
     tracked!(no_generate_arange_section, true);
+    tracked!(no_jump_tables, true);
     tracked!(no_link, true);
     tracked!(no_profiler_runtime, true);
     tracked!(no_unique_section_names, true);

compiler/rustc_session/src/options.rs

@@ -1421,6 +1421,8 @@ options! {
         "run all passes except codegen; no output"),
     no_generate_arange_section: bool = (false, parse_no_flag, [TRACKED],
         "omit DWARF address ranges that give faster lookups"),
+    no_jump_tables: bool = (false, parse_no_flag, [TRACKED],
+        "disable the jump tables and lookup tables that can be generated from a switch case lowering"),
     no_leak_check: bool = (false, parse_no_flag, [UNTRACKED],
         "disable the 'leak check' for subtyping; unsound, but useful for tests"),
     no_link: bool = (false, parse_no_flag, [TRACKED],

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)