retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features #135927

azhogin · 2025-01-23T10:46:14Z

-Zretpoline and -Zretpoline-external-thunk flags are target modifiers (tracked to be equal in linked crates).

Enables target features for -Zretpoline-external-thunk:
+retpoline-external-thunk, +retpoline-indirect-branches, +retpoline-indirect-calls.
Enables target features for -Zretpoline:
+retpoline-indirect-branches, +retpoline-indirect-calls.

It corresponds to clang -mretpoline & -mretpoline-external-thunk flags.

Also this PR forbids to specify those target features manually (warning).

rustbot · 2025-01-23T10:46:22Z

rustbot has assigned @compiler-errors.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

rustbot · 2025-01-23T10:46:25Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

compiler-errors · 2025-01-23T18:12:36Z

r? compiler

bors · 2025-01-26T10:22:32Z

☔ The latest upstream changes (presumably #136085) made this pull request unmergeable. Please resolve the merge conflicts.

chenyukang · 2025-02-05T02:30:16Z

r? compiler

fee1-dead · 2025-02-08T01:29:07Z

r? compiler

lcnr · 2025-02-09T18:12:07Z

r? @davidtwco maybe 😅

azhogin · 2025-03-25T15:38:59Z

-Zretpoline-external-thunk flag enables:
+retpoline-external-thunk, +retpoline-indirect-branches, +retpoline-indirect-calls.
-Zretpoline flag enables:
+retpoline-indirect-branches, +retpoline-indirect-calls

-Zretpoline-external-thunk also turns on -Zretpoline.

Looks like it corresponds to clang flags.

rustbot · 2025-03-26T06:48:43Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

WaffleLapkin · 2025-04-09T16:16:21Z

compiler/rustc_target/src/target_features.rs

        }
    }

    /// Returns whether the feature may be toggled via `#[target_feature]` or `-Ctarget-feature`.
    /// (It might still be nightly-only even if this returns `true`, so make sure to also check
    /// `requires_nightly`.)
-    pub fn toggle_allowed(&self) -> Result<(), &'static str> {


this API seems confusing, at the very least change the doc comment.

davidtwco

Per the team documentation, adding a new flag with the intent to later stabilise requires an MCP - https://forge.rust-lang.org/compiler/proposals-and-stabilization.html#compiler-flags

davidtwco · 2025-04-16T12:42:07Z

compiler/rustc_target/src/target_features.rs

        }
    }

    /// Returns whether the feature may be toggled via `#[target_feature]` or `-Ctarget-feature`.
    /// (It might still be nightly-only even if this returns `true`, so make sure to also check
    /// `requires_nightly`.)
-    pub fn toggle_allowed(&self) -> Result<(), &'static str> {
+    pub fn toggle_allowed(&self, flag_enabled: impl Fn(&str) -> bool) -> Result<(), &'static str> {


pre-existing: this name is a little ambiguous, is it toggling something named allowed, or is toggling allowed? maybe is_toggle_permitted instead or just is_toggle_allowed?

davidtwco · 2025-04-16T12:55:17Z

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -733,7 +733,9 @@ pub(crate) fn global_llvm_features(
                        sess.dcx().emit_warn(unknown_feature);
                    }
                    Some((_, stability, _)) => {
-                        if let Err(reason) = stability.toggle_allowed() {


There's a bunch of duplication in the code around this between codegen backends and rustc_codegen_ssa, it's pre-existing, you could sort it if you wanted, but someone ought to.

davidtwco · 2025-04-16T12:59:32Z

compiler/rustc_session/src/options.rs

+                }
+            }
+
+            pub fn fill_target_features_by_flags(


This doesn't feel like the right place to do this, ideally there would be a query or something that we could add this logic to that would mean that codegen backends just get the features they need to enable and we'd insert this logic into that.

davidtwco · 2025-04-16T13:00:38Z

compiler/rustc_target/src/target_features.rs

        }
    }

    /// Returns whether the feature may be toggled via `#[target_feature]` or `-Ctarget-feature`.
    /// (It might still be nightly-only even if this returns `true`, so make sure to also check
    /// `requires_nightly`.)
-    pub fn toggle_allowed(&self) -> Result<(), &'static str> {
+    pub fn toggle_allowed(&self, flag_enabled: impl Fn(&str) -> bool) -> Result<(), &'static str> {


Could we add a StabilityExt trait in another crate that just takes a Session and calls this with the appropriate closure, given its the same closure everywhere?

davidtwco · 2025-04-16T13:01:15Z

compiler/rustc_target/src/target_features.rs

@@ -34,6 +34,9 @@ pub enum Stability {
    /// particular for features are actually ABI configuration flags (not all targets are as nice as
    /// RISC-V and have an explicit way to set the ABI separate from target features).
    Forbidden { reason: &'static str },
+    /// This feature can not be set via `-Ctarget-feature` or `#[target_feature]`, it can only be set
+    /// by target modifier flag. Target modifier flags are tracked to be consistent in linked modules.
+    EnabledByTargetModifierFlag { reason: &'static str, flag: &'static str },


Suggested change

EnabledByTargetModifierFlag { reason: &'static str, flag: &'static str },

TargetModifierOnly { reason: &'static str, flag: &'static str },

Darksonn · 2025-04-16T13:19:18Z

@rustbot label F-target_modifiers A-rust-for-linux

Darksonn · 2025-04-28T12:01:41Z

I filed an MCP: rust-lang/compiler-team#868

ojeda · 2025-05-07T18:59:33Z

@azhogin If you would like to pick up here the commits from the -Zindirect-branch-cs-prefix one, please feel free to do so: #140740 (as far as I can tell, it should be clean when this one is clean).

@davidtwco confirmed it sounded OK to reuse the MCP for -Zretpoline* for -Zindirect-branch-cs-prefix too.

traviscross · 2025-05-21T18:42:13Z

@rustbot labels -S-waiting-on-MCP

Looks like we're no longer waiting on the MCP here:

rust-lang/compiler-team#868

traviscross · 2025-05-21T18:44:27Z

@azhogin: Are you ready for this to be reviewed? If so, mark it with @rustbot ready.

bors · 2025-05-23T03:50:11Z

☔ The latest upstream changes (presumably #135160) made this pull request unmergeable. Please resolve the merge conflicts.

…o enable retpoline-related target features

rustbot assigned compiler-errors Jan 23, 2025

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-rustdoc Relevant to the rustdoc team, which will review and decide on the PR/issue. labels Jan 23, 2025

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azhogin marked this pull request as draft January 30, 2025 08:48

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azhogin force-pushed the azhogin/retpoline branch from 0ea46d2 to 386c90e Compare February 8, 2025 11:41

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azhogin force-pushed the azhogin/retpoline branch from 386c90e to 597eb26 Compare March 25, 2025 15:09

azhogin changed the title ~~x86-retpoline flag (target modifier) to enable retpoline-related target features~~ retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features Mar 25, 2025

azhogin marked this pull request as ready for review March 26, 2025 06:48

WaffleLapkin reviewed Apr 9, 2025

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davidtwco requested changes Apr 16, 2025

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Apr 16, 2025

rustbot added A-rust-for-linux Relevant for the Rust-for-Linux project F-target_modifiers `#![feature(target_modifiers)]` labels Apr 16, 2025

jieyouxu added the S-waiting-on-MCP Status: PR has a compiler MCP and is waiting for the compiler MCP to complete. label Apr 28, 2025

This was referenced May 7, 2025

-Zindirect-branch-cs-prefix on top of -Zretpoline* #140740

Draft

Add -Zindirect-branch-cs-prefix #140733

Draft

azhogin force-pushed the azhogin/retpoline branch from 597eb26 to d70fbca Compare May 10, 2025 18:38

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ojeda mentioned this pull request May 20, 2025

Stabilize tooling needed by Rust for Linux rust-lang/rust-project-goals#116

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rustbot removed the S-waiting-on-MCP Status: PR has a compiler MCP and is waiting for the compiler MCP to complete. label May 21, 2025

-Zretpoline and -Zretpoline-external-thunk flags (target modifiers) t…

8d6ef02

…o enable retpoline-related target features

azhogin force-pushed the azhogin/retpoline branch from 203cb50 to 8d6ef02 Compare May 25, 2025 12:23

	EnabledByTargetModifierFlag { reason: &'static str, flag: &'static str },
	TargetModifierOnly { reason: &'static str, flag: &'static str },

retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features #135927

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retpoline and retpoline-external-thunk flags (target modifiers) to enable retpoline-related target features #135927

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