rust_for_linux: -Zreg-struct-return commandline flag for X86 (#116973) #130777

azhogin · 2024-09-24T09:40:26Z

Command line flag -Zreg-struct-return for X86 (32-bit) for rust-for-linux.
This flag enables the same behavior as the abi_return_struct_as_int target spec key.

Tracking issue: Tracking issue: -Zreg-struct-return support in rustc #116973

rustbot · 2024-09-24T09:40:34Z

r? @cjgillot

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Darksonn · 2024-09-24T16:44:42Z

cc @ojeda

rustbot · 2024-09-26T17:35:29Z

Some changes occurred in compiler/rustc_codegen_gcc

cc @antoyo, @GuillaumeGomez

These commits modify compiler targets.
(See the Target Tier Policy.)

Darksonn · 2024-09-28T11:32:08Z

@rustbot label A-rust-for-linux
@rustbot label A-ABI

compiler/rustc_target/src/spec/mod.rs

Darksonn · 2024-09-28T12:41:45Z

tests/codegen/reg-struct-return.rs

+    // reg-struct-return doesn't work for "Rust" calling conv
+    // CHECK: { i32, i32 } @f2()
+    #[no_mangle]
+    pub extern "Rust" fn f2() -> Foo {
+        Foo { x: 1, y: 2 }
+    }


Why doesn't it work for the Rust calling convention? Is that what we want? I assume that there's a reason we use this flag, but I don't know whether it's just a question of performance or if it's an important mitigation.

@andyhhp Do you know if this matters?

At technical level, "external" abi's are processed a little in different way (another place in code). It is not a big problem to support the same behavior for "Rust" calling conv, I just need to know if it is required.

Why doesn't it work for the Rust calling convention? Is that what we want? I assume that there's a reason we use this flag, but I don't know whether it's just a question of performance or if it's an important mitigation.

@andyhhp Do you know if this matters?

Rust and C need to agree on how to return >GPR sized values, if this happens to be used in either direction.

Whether that is via some bindings using the Rust calling conventions is something I can't answer.

The use of >GPR sized return values is rare in C because it's irritating to set up. You need a struct/typedef with a fairly precise layout, but the code generation improvements are substantial, hence why they do get used occasionally.

The Rust ABI is not stable. C should not call an extern "Rust" fn. This matters for extern "C" fn and a few others. And reusing this code, tuned for extern "C" and other foreign ABIs, for Rust ABI code? Proved incapable of compiling libcore in #130432.

We're not going to be scope-creeping this PR into extern "Rust". That can be addressed alongside the extensive cleanups of our ABI code that it will require.

ojeda · 2024-10-02T15:41:30Z

Cc @vincenzopalazzo who was interested in this.

bors · 2024-10-10T01:59:50Z

☔ The latest upstream changes (presumably #131458) made this pull request unmergeable. Please resolve the merge conflicts.

cjgillot · 2024-10-11T22:26:23Z

I'm not in capacity to review this.
r? compiler

petrochenkov · 2024-10-12T10:28:41Z

r? @workingjubilee

workingjubilee · 2024-10-22T01:54:31Z

I have mutated #116973 into a tracking issue for this, so we do not need to open another one.

bors · 2024-10-22T09:13:33Z

☔ The latest upstream changes (presumably #132020) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2024-10-24T02:00:45Z

☔ The latest upstream changes (presumably #132079) made this pull request unmergeable. Please resolve the merge conflicts.

workingjubilee · 2024-10-24T02:31:01Z

compiler/rustc_ty_utils/src/abi.rs

+            let is_indirect_not_on_stack = !matches!(arg.layout.abi, Abi::Aggregate { .. })
+                || matches!(arg.mode, PassMode::Indirect { on_stack: false, .. });
+            assert!(is_indirect_not_on_stack, "is_indirect_not_on_stack: {:?}", arg);

            let size = arg.layout.size;
-            if !arg.layout.is_unsized() && size <= Pointer(AddressSpace::DATA).size(cx) {
+            let ptr_size = Pointer(AddressSpace::DATA).size(cx);
+
+            // In x86 we may return 2x pointer sized struct as i64
+            let reg_struct_return_case = tcx.sess.target.arch == "x86"
+                && arg_idx.is_none()
+                && tcx.sess.opts.unstable_opts.reg_struct_return
+                && abi != SpecAbi::RustIntrinsic;
+
+            if !arg.layout.is_unsized()
+                && (size <= ptr_size || (reg_struct_return_case && (size <= 2 * ptr_size)))
+            {


some of this code has moved into rustc_target.

…ng#116973)

azhogin · 2024-10-29T23:12:54Z

Reverted changes for "Rust" calling conv (flag works only for extern "C"-like calling conventions). Added unstable-book record, error about wrong arch and related test.

workingjubilee · 2024-11-03T22:04:23Z

tests/codegen/reg-struct-return.rs

+#![feature(no_core, lang_items)]
+
+#[lang = "sized"]
+trait Sized {}
+#[lang = "copy"]
+trait Copy {}


You can just use //@ add-core-stubs for this now: #130693

workingjubilee · 2024-11-03T22:20:15Z

tests/codegen/reg-struct-return.rs

We should test this slightly more exhaustively. I'd like to see

single-field structs, too

structs with fields that aren't valid for all bitpatterns, like, say, { bool, bool, i16 }... I am not aware of a reason it should differ in handling, mind.

oddly-shaped structs that don't exactly match the sizes we're looking for, but are under the 64-bit limit, like e.g. { char, bool, u8 }

larger-than-64-bits structs, which obviously shouldn't be changed to integers

workingjubilee · 2024-11-03T22:21:10Z

tests/codegen/reg-struct-return.rs

Float handling is consistently weird, especially on x86-32, so I'd like to see additional tests for

floats, so specifically { f32, f32 }, { f64 }, and { f32 }

another test that disables SSE (but leaves x87 on) and enables this feature

another test that uses only soft floats and enables this feature

Feel free to use revisions, additional test files, or both as you prefer.

Obviously this is mostly about the resulting "C" ABI, and you don't need to also test Rust ABIs.

rustbot assigned cjgillot Sep 24, 2024

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. labels Sep 24, 2024

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ojeda mentioned this pull request Sep 24, 2024

rustc wanted features & bugfixes Rust-for-Linux/linux#355

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rust_for_linux: -Zregparm=<N> commandline flag for X86 (#116972) #130432

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workingjubilee added the O-x86_32 Target: x86 processors, 32 bit (like i686-*) label Oct 21, 2024

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Tracking issue: -Zreg-struct-return support in rustc #116973

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rust_for_linux: -Zreg-struct-return commandline flag for X86 (#116973) #130777

rust_for_linux: -Zreg-struct-return commandline flag for X86 (#116973) #130777

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