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.

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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azhogin marked this pull request as ready for review September 26, 2024 17:35

rustbot added A-rust-for-linux Relevant for the Rust-for-Linux project A-ABI Area: Concerning the application binary interface (ABI) labels Sep 28, 2024

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rustbot assigned petrochenkov and unassigned cjgillot Oct 11, 2024

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

Merged

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

workingjubilee mentioned this pull request Oct 22, 2024

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

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

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