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Stabilize -Zno-jump-tables into -Cjump-tables=bool #145974

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Stabilize -Zno-jump-tables into -Cjump-tables=bool #145974

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e642e09
Rename -Zno-jump-tables to -Zjump-tables=<bool>
pmur Aug 28, 2025
68bfda9
Stabilize -Zjump-tables=<bool> into -Cjump-table=<bool>
pmur Aug 28, 2025
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2 changes: 1 addition & 1 deletion compiler/rustc_codegen_llvm/src/attributes.rs
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Question [GUARANTEE 1/3]: is this intended to be a hint, or a guarantee?

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Question:

This option enables the -fno-jump-tables flag for LLVM, which makes the codegen backend avoid generating jump tables when lowering switches.

This option adds the LLVM no-jump-tables=true attribute to every function.

The option can be used to help provide protection against jump-oriented-programming (JOP) attacks, such as with the linux kernel's IBT.

How does this interact with pre-compiled std? I.e. can you mix downstream user crates compiled with -Cjump-tables=no versus a pre-compiled std compiled and distributed with -Cjump-tables=yes?

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Are there objections renaming -Zno-jump-tables to -Zjump-tables=?

Not having double-negatives is very nice 👍

Is it desirable to keep -Zno-jump-tables for a period of time?

No, I rather not have to keep around an unstable flag as an alias to a stable flag, that's very weird.

Original file line number Diff line number Diff line change
Expand Up @@ -208,7 +208,7 @@ fn instrument_function_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> SmallVec<[&'ll Attr
}

fn nojumptables_attr<'ll>(cx: &CodegenCx<'ll, '_>) -> Option<&'ll Attribute> {
if !cx.sess().opts.unstable_opts.no_jump_tables {
if cx.sess().opts.cg.jump_tables {
return None;
}

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2 changes: 1 addition & 1 deletion compiler/rustc_interface/src/tests.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -620,6 +620,7 @@ fn test_codegen_options_tracking_hash() {
tracked!(force_frame_pointers, FramePointer::Always);
tracked!(force_unwind_tables, Some(true));
tracked!(instrument_coverage, InstrumentCoverage::Yes);
tracked!(jump_tables, false);
tracked!(link_dead_code, Some(true));
tracked!(linker_plugin_lto, LinkerPluginLto::LinkerPluginAuto);
tracked!(llvm_args, vec![String::from("1"), String::from("2")]);
Expand Down Expand Up @@ -830,7 +831,6 @@ fn test_unstable_options_tracking_hash() {
tracked!(mutable_noalias, false);
tracked!(next_solver, NextSolverConfig { coherence: true, globally: true });
tracked!(no_generate_arange_section, true);
tracked!(no_jump_tables, true);
tracked!(no_link, true);
tracked!(no_profiler_runtime, true);
tracked!(no_trait_vptr, true);
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4 changes: 2 additions & 2 deletions compiler/rustc_session/src/options.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2022,6 +2022,8 @@ options! {
"instrument the generated code to support LLVM source-based code coverage reports \
(note, the compiler build config must include `profiler = true`); \
implies `-C symbol-mangling-version=v0`"),
jump_tables: bool = (true, parse_bool, [TRACKED],
"Allow jump table and lookup table generation from switch case lowering (default: yes)"),
link_arg: (/* redirected to link_args */) = ((), parse_string_push, [UNTRACKED],
"a single extra argument to append to the linker invocation (can be used several times)"),
link_args: Vec<String> = (Vec::new(), parse_list, [UNTRACKED],
Expand Down Expand Up @@ -2404,8 +2406,6 @@ options! {
"omit DWARF address ranges that give faster lookups"),
no_implied_bounds_compat: bool = (false, parse_bool, [TRACKED],
"disable the compatibility version of the `implied_bounds_ty` query"),
no_jump_tables: bool = (false, parse_no_value, [TRACKED],
"disable the jump tables and lookup tables that can be generated from a switch case lowering"),
no_leak_check: bool = (false, parse_no_value, [UNTRACKED],
"disable the 'leak check' for subtyping; unsound, but useful for tests"),
no_link: bool = (false, parse_no_value, [TRACKED],
Expand Down
13 changes: 13 additions & 0 deletions src/doc/rustc/src/codegen-options/index.md
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Original file line number Diff line number Diff line change
Expand Up @@ -209,6 +209,19 @@ Note that while the `-C instrument-coverage` option is stable, the profile data
format produced by the resulting instrumentation may change, and may not work
with coverage tools other than those built and shipped with the compiler.

## jump-tables

This option is used to allow or prevent the codegen backend from creating
jump tables when lowering switches.
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Discussion: Hm, what happens if a different cg backend is selected?

cc @GuillaumeGomez @antoyo

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Question [GUARANTEE 2/3]: This wording reads like a guarantee -- but is it? Can we make that guarantee?


* `y`, `yes`, `on`, `true` or no value: allow jump tables (the default).
* `n`, `no`, `off` or `false`: disable jump tables.

Disabling jump tables can be used to help provide protection against
jump-oriented-programming (JOP) attacks, such as with the linux kernel's [IBT].
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Discussion [GUARANTEE 3/3]: If the flag is intended to be a hint, then this sentence can be a bit misleading, because we may not always guarantee it. We may want to slightly caveat this wording to not convey a "false promise" so to speak.

Or, if a user do want such protection, then do they need to enforce it over the whole crate graph?


[IBT]: https://www.phoronix.com/news/Linux-IBT-By-Default-Tip

## link-arg

This flag lets you append a single extra argument to the linker invocation.
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19 changes: 0 additions & 19 deletions src/doc/unstable-book/src/compiler-flags/no-jump-tables.md
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4 changes: 2 additions & 2 deletions tests/assembly-llvm/x86_64-no-jump-tables.rs
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Question: I'm assuming that if you inspect the assembly of an actual hello world binary that uses std in some way, then you might see jump stables still? 🤔

Original file line number Diff line number Diff line change
@@ -1,10 +1,10 @@
// Test that jump tables are (not) emitted when the `-Zno-jump-tables`
// Test that jump tables are (not) emitted when the `-Cjump-tables=no`
// flag is (not) set.

//@ revisions: unset set
//@ assembly-output: emit-asm
//@ compile-flags: -Copt-level=3
//@ [set] compile-flags: -Zno-jump-tables
//@ [set] compile-flags: -Cjump-tables=no
//@ only-x86_64
//@ ignore-sgx

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10 changes: 6 additions & 4 deletions tests/codegen-llvm/no-jump-tables.rs
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Original file line number Diff line number Diff line change
@@ -1,11 +1,12 @@
// Test that the `no-jump-tables` function attribute are (not) emitted when
// the `-Zno-jump-tables` flag is (not) set.
// the `-Cjump-tables=no` flag is (not) set.

//@ add-core-stubs
//@ revisions: unset set
//@ revisions: unset set_no set_yes
//@ needs-llvm-components: x86
//@ compile-flags: --target x86_64-unknown-linux-gnu
//@ [set] compile-flags: -Zno-jump-tables
//@ [set_no] compile-flags: -Cjump-tables=no
//@ [set_yes] compile-flags: -Cjump-tables=yes

#![crate_type = "lib"]
#![feature(no_core, lang_items)]
Expand All @@ -19,5 +20,6 @@ pub fn foo() {
// CHECK: @foo() unnamed_addr #0

// unset-NOT: attributes #0 = { {{.*}}"no-jump-tables"="true"{{.*}} }
// set: attributes #0 = { {{.*}}"no-jump-tables"="true"{{.*}} }
// set_yes-NOT: attributes #0 = { {{.*}}"no-jump-tables"="true"{{.*}} }
// set_no: attributes #0 = { {{.*}}"no-jump-tables"="true"{{.*}} }
}
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