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Tracking Issue for asm_goto #119364

Open
4 tasks
Tracked by #1
nbdd0121 opened this issue Dec 27, 2023 · 10 comments
Open
4 tasks
Tracked by #1

Tracking Issue for asm_goto #119364

nbdd0121 opened this issue Dec 27, 2023 · 10 comments
Labels
A-inline-assembly Area: Inline assembly (`asm!(…)`) C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. F-asm `#![feature(asm)]` (not `llvm_asm`)

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@nbdd0121
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nbdd0121 commented Dec 27, 2023

The feature gate for the issue is #![feature(asm_goto)].

Summary

This feature adds a label<block> operand type to asm!.

Example:

unsafe {
    asm!(
        "jmp {}",
        label {
            println!("Jumped from asm!");
        }
    );
}

The block must have unit type.

Steps

Unresolved Questions

@rustbot labels: +A-inline-assembly +F-asm

@nbdd0121 nbdd0121 added the C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. label Dec 27, 2023
@rustbot rustbot added A-inline-assembly Area: Inline assembly (`asm!(…)`) F-asm `#![feature(asm)]` (not `llvm_asm`) labels Dec 27, 2023
@Noratrieb
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Where is this coming from? Is there a lang MCP or anything like that?

@nbdd0121
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This is outlined as a future possibility in RFC2873 which I am implementing in #119365.

bors added a commit to rust-lang-ci/rust that referenced this issue Feb 25, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? `@Amanieu`
cc `@ojeda`
bors added a commit to rust-lang-ci/rust that referenced this issue Feb 26, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? `@Amanieu`
cc `@ojeda`
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Mar 8, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? `@Amanieu`
cc `@ojeda`
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Mar 8, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Mar 8, 2024
Rollup merge of rust-lang#119365 - nbdd0121:asm-goto, r=Amanieu

Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``
bjorn3 pushed a commit to bjorn3/rust that referenced this issue Mar 16, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Mar 21, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``
ojeda added a commit to ojeda/linux that referenced this issue Apr 1, 2024
This is the next upgrade to the Rust toolchain, from 1.77.1 to 1.78.0
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4 ("rust: upgrade to Rust 1.68.2").

# Unstable features

There have been no changes to the set of unstable features used in
our own code. Therefore, the only unstable features allowed to be used
outside the `kernel` crate is still `new_uninit`.

However, since we are finally dropping our `alloc` fork [3], all the
unstable features used by `alloc` (~30 language ones, ~60 library ones)
are not a concern anymore. This reduces the maintanance burden, increases
the chances of new compiler versions working without changes and gets
us closer to the goal of supporting several compiler versions.

It also means that, ignoring non-language/library features, we are
currently left with just the few language features needed to implement the
kernel `Arc`, the `new_uninit` library feature, the `compiler_builtins`
marker and the few `no_*` `cfg`s we pass when compiling `core`/`alloc`.

Please see [4] for details.

# Required changes

## LLVM's data layout

Rust 1.77.0 (i.e. the previous upgrade) introduced a check for matching
LLVM data layouts [5]. Then, Rust 1.78.0 upgraded LLVM's bundled major
version from 17 to 18 [6], which changed the data layout in x86 [7]. Thus
update the data layout in our custom target specification for x86 so
that the compiler does not complain about the mismatch:

    error: data-layout for target `target-5559158138856098584`,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128`,
    differs from LLVM target's `x86_64-linux-gnu` default layout,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128`

In the future, the goal is to drop the custom target specification
files. Meanwhile, if we want to support other LLVM versions used
in `rustc` (e.g. for LTO), we will need to add some extra logic
(e.g. conditional on LLVM's version, or extracting the data layout from
an existing built-in target specification).

## `unused_imports`

Rust's `unused_imports` lint covers both unused and redudant imports.
Now, in 1.78.0, the lint detects more cases of redundant imports [8].
Thus the previous commit cleaned them up.

## Clippy's `new_without_default`

Clippy now suggests to implement `Default` even when `new()` is `const`,
since `Default::default()` may call `const` functions even if it is not
`const` itself [9]. Thus the previous commit added the implementation.

# Other changes in Rust

Rust 1.78.0 introduces `feature(asm_goto)` [10] [11]. This feature was
discussed in the past [12].

Rust 1.78.0 introduced support for mutable pointers to Rust statics,
including a test case for the Linux kernel's `VTABLE` use case [13].

Rust 1.78.0 with debug assertions enabled (i.e. `-Cdebug-assertions=y`,
kernel's `CONFIG_RUST_DEBUG_ASSERTIONS=y`) will now always check all
unsafe preconditions, without a way to opt-out for particular cases [14].

Rust 1.78.0 also improved a couple issues we reported when giving feedback
for the new `--check-cfg` feature [15] [16].

# `alloc` upgrade and reviewing

As mentioned above, compiler upgrades will not update `alloc` anymore,
since we are dropping our `alloc` fork [3].

As a bonus, even if that series is not applied, the new compiler release
happens to build cleanly the existing `alloc` too (i.e. the previous
version's).

Link: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1770-2024-03-21 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://lore.kernel.org/rust-for-linux/20240328013603.206764-1-wedsonaf@gmail.com/ [3]
Link: Rust-for-Linux#2 [4]
Link: rust-lang/rust#120062 [5]
Link: rust-lang/rust#120055 [6]
Link: https://reviews.llvm.org/D86310 [7]
Link: rust-lang/rust#117772 [8]
Link: rust-lang/rust-clippy#10903 [9]
Link: rust-lang/rust#119365 [10]
Link: rust-lang/rust#119364 [11]
Link: https://lore.kernel.org/rust-for-linux/ZWipTZysC2YL7qsq@Boquns-Mac-mini.home/ [12]
Link: rust-lang/rust#120932 [13]
Link: rust-lang/rust#120969 [14]
Link: rust-lang/rust#121202 [15]
Link: rust-lang/rust#121237 [16]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
ojeda added a commit to ojeda/linux that referenced this issue Apr 1, 2024
This is the next upgrade to the Rust toolchain, from 1.77.1 to 1.78.0
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4 ("rust: upgrade to Rust 1.68.2").

# Unstable features

There have been no changes to the set of unstable features used in
our own code. Therefore, the only unstable features allowed to be used
outside the `kernel` crate is still `new_uninit`.

However, since we are finally dropping our `alloc` fork [3], all the
unstable features used by `alloc` (~30 language ones, ~60 library ones)
are not a concern anymore. This reduces the maintenance burden, increases
the chances of new compiler versions working without changes and gets
us closer to the goal of supporting several compiler versions.

It also means that, ignoring non-language/library features, we are
currently left with just the few language features needed to implement the
kernel `Arc`, the `new_uninit` library feature, the `compiler_builtins`
marker and the few `no_*` `cfg`s we pass when compiling `core`/`alloc`.

Please see [4] for details.

# Required changes

## LLVM's data layout

Rust 1.77.0 (i.e. the previous upgrade) introduced a check for matching
LLVM data layouts [5]. Then, Rust 1.78.0 upgraded LLVM's bundled major
version from 17 to 18 [6], which changed the data layout in x86 [7]. Thus
update the data layout in our custom target specification for x86 so
that the compiler does not complain about the mismatch:

    error: data-layout for target `target-5559158138856098584`,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128`,
    differs from LLVM target's `x86_64-linux-gnu` default layout,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128`

In the future, the goal is to drop the custom target specification
files. Meanwhile, if we want to support other LLVM versions used
in `rustc` (e.g. for LTO), we will need to add some extra logic
(e.g. conditional on LLVM's version, or extracting the data layout from
an existing built-in target specification).

## `unused_imports`

Rust's `unused_imports` lint covers both unused and redundant imports.
Now, in 1.78.0, the lint detects more cases of redundant imports [8].
Thus one of the previous patches cleaned them up.

## Clippy's `new_without_default`

Clippy now suggests to implement `Default` even when `new()` is `const`,
since `Default::default()` may call `const` functions even if it is not
`const` itself [9]. Thus one of the previous patches implemented it.

# Other changes in Rust

Rust 1.78.0 introduced `feature(asm_goto)` [10] [11]. This feature was
discussed in the past [12].

Rust 1.78.0 introduced support for mutable pointers to Rust statics,
including a test case for the Linux kernel's `VTABLE` use case [13].

Rust 1.78.0 with debug assertions enabled (i.e. `-Cdebug-assertions=y`,
kernel's `CONFIG_RUST_DEBUG_ASSERTIONS=y`) now always checks all unsafe
preconditions, without a way to opt-out for particular cases [14].

Rust 1.78.0 also improved a couple issues we reported when giving feedback
for the new `--check-cfg` feature [15] [16].

# `alloc` upgrade and reviewing

As mentioned above, compiler upgrades will not update `alloc` anymore,
since we are dropping our `alloc` fork [3].

Link: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1780-2024-05-02 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://lore.kernel.org/rust-for-linux/20240328013603.206764-1-wedsonaf@gmail.com/ [3]
Link: Rust-for-Linux#2 [4]
Link: rust-lang/rust#120062 [5]
Link: rust-lang/rust#120055 [6]
Link: https://reviews.llvm.org/D86310 [7]
Link: rust-lang/rust#117772 [8]
Link: rust-lang/rust-clippy#10903 [9]
Link: rust-lang/rust#119365 [10]
Link: rust-lang/rust#119364 [11]
Link: https://lore.kernel.org/rust-for-linux/ZWipTZysC2YL7qsq@Boquns-Mac-mini.home/ [12]
Link: rust-lang/rust#120932 [13]
Link: rust-lang/rust#120969 [14]
Link: rust-lang/rust#121202 [15]
Link: rust-lang/rust#121237 [16]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
ojeda added a commit to ojeda/linux that referenced this issue Apr 1, 2024
This is the next upgrade to the Rust toolchain, from 1.77.1 to 1.78.0
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4 ("rust: upgrade to Rust 1.68.2").

# Unstable features

There have been no changes to the set of unstable features used in
our own code. Therefore, the only unstable features allowed to be used
outside the `kernel` crate is still `new_uninit`.

However, since we are finally dropping our `alloc` fork [3], all the
unstable features used by `alloc` (~30 language ones, ~60 library ones)
are not a concern anymore. This reduces the maintenance burden, increases
the chances of new compiler versions working without changes and gets
us closer to the goal of supporting several compiler versions.

It also means that, ignoring non-language/library features, we are
currently left with just the few language features needed to implement the
kernel `Arc`, the `new_uninit` library feature, the `compiler_builtins`
marker and the few `no_*` `cfg`s we pass when compiling `core`/`alloc`.

Please see [4] for details.

# Required changes

## LLVM's data layout

Rust 1.77.0 (i.e. the previous upgrade) introduced a check for matching
LLVM data layouts [5]. Then, Rust 1.78.0 upgraded LLVM's bundled major
version from 17 to 18 [6], which changed the data layout in x86 [7]. Thus
update the data layout in our custom target specification for x86 so
that the compiler does not complain about the mismatch:

    error: data-layout for target `target-5559158138856098584`,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128`,
    differs from LLVM target's `x86_64-linux-gnu` default layout,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128`

In the future, the goal is to drop the custom target specifications.
Meanwhile, if we want to support other LLVM versions used in `rustc`
(e.g. for LTO), we will need to add some extra logic (e.g. conditional on
LLVM's version, or extracting the data layout from an existing built-in
target specification).

## `unused_imports`

Rust's `unused_imports` lint covers both unused and redundant imports.
Now, in 1.78.0, the lint detects more cases of redundant imports [8].
Thus one of the previous patches cleaned them up.

## Clippy's `new_without_default`

Clippy now suggests to implement `Default` even when `new()` is `const`,
since `Default::default()` may call `const` functions even if it is not
`const` itself [9]. Thus one of the previous patches implemented it.

# Other changes in Rust

Rust 1.78.0 introduced `feature(asm_goto)` [10] [11]. This feature was
discussed in the past [12].

Rust 1.78.0 introduced support for mutable pointers to Rust statics,
including a test case for the Linux kernel's `VTABLE` use case [13].

Rust 1.78.0 with debug assertions enabled (i.e. `-Cdebug-assertions=y`,
kernel's `CONFIG_RUST_DEBUG_ASSERTIONS=y`) now always checks all unsafe
preconditions, without a way to opt-out for particular cases [14].

Rust 1.78.0 also improved a couple issues we reported when giving feedback
for the new `--check-cfg` feature [15] [16].

# `alloc` upgrade and reviewing

As mentioned above, compiler upgrades will not update `alloc` anymore,
since we are dropping our `alloc` fork [3].

Link: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1780-2024-05-02 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://lore.kernel.org/rust-for-linux/20240328013603.206764-1-wedsonaf@gmail.com/ [3]
Link: Rust-for-Linux#2 [4]
Link: rust-lang/rust#120062 [5]
Link: rust-lang/rust#120055 [6]
Link: https://reviews.llvm.org/D86310 [7]
Link: rust-lang/rust#117772 [8]
Link: rust-lang/rust-clippy#10903 [9]
Link: rust-lang/rust#119365 [10]
Link: rust-lang/rust#119364 [11]
Link: https://lore.kernel.org/rust-for-linux/ZWipTZysC2YL7qsq@Boquns-Mac-mini.home/ [12]
Link: rust-lang/rust#120932 [13]
Link: rust-lang/rust#120969 [14]
Link: rust-lang/rust#121202 [15]
Link: rust-lang/rust#121237 [16]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
ojeda added a commit to ojeda/linux that referenced this issue May 5, 2024
This is the next upgrade to the Rust toolchain, from 1.77.1 to 1.78.0
(i.e. the latest) [1].

See the upgrade policy [2] and the comments on the first upgrade in
commit 3ed03f4 ("rust: upgrade to Rust 1.68.2").

It is much smaller than previous upgrades, since the `alloc` fork was
dropped in commit 9d0441b ("rust: alloc: remove our fork of the
`alloc` crate") [3].

# Unstable features

There have been no changes to the set of unstable features used in
our own code. Therefore, the only unstable features allowed to be used
outside the `kernel` crate is still `new_uninit`.

However, since we finally dropped our `alloc` fork [3], all the unstable
features used by `alloc` (~30 language ones, ~60 library ones) are not
a concern anymore. This reduces the maintenance burden, increases the
chances of new compiler versions working without changes and gets us
closer to the goal of supporting several compiler versions.

It also means that, ignoring non-language/library features, we are
currently left with just the few language features needed to implement the
kernel `Arc`, the `new_uninit` library feature, the `compiler_builtins`
marker and the few `no_*` `cfg`s we pass when compiling `core`/`alloc`.

Please see [4] for details.

# Required changes

## LLVM's data layout

Rust 1.77.0 (i.e. the previous upgrade) introduced a check for matching
LLVM data layouts [5]. Then, Rust 1.78.0 upgraded LLVM's bundled major
version from 17 to 18 [6], which changed the data layout in x86 [7]. Thus
update the data layout in our custom target specification for x86 so
that the compiler does not complain about the mismatch:

    error: data-layout for target `target-5559158138856098584`,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128`,
    differs from LLVM target's `x86_64-linux-gnu` default layout,
    `e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128`

In the future, the goal is to drop the custom target specifications.
Meanwhile, if we want to support other LLVM versions used in `rustc`
(e.g. for LTO), we will need to add some extra logic (e.g. conditional on
LLVM's version, or extracting the data layout from an existing built-in
target specification).

## `unused_imports`

Rust's `unused_imports` lint covers both unused and redundant imports.
Now, in 1.78.0, the lint detects more cases of redundant imports [8].
Thus one of the previous patches cleaned them up.

## Clippy's `new_without_default`

Clippy now suggests to implement `Default` even when `new()` is `const`,
since `Default::default()` may call `const` functions even if it is not
`const` itself [9]. Thus one of the previous patches implemented it.

# Other changes in Rust

Rust 1.78.0 introduced `feature(asm_goto)` [10] [11]. This feature was
discussed in the past [12].

Rust 1.78.0 introduced `feature(const_refs_to_static)` [13] to allow
referencing statics in constants and extended `feature(const_mut_refs)`
to allow raw mutable pointers in constants. Together, this should cover
the kernel's `VTABLE` use case. In fact, the implementation [14] in
upstream Rust added a test case for it [15].

Rust 1.78.0 with debug assertions enabled (i.e. `-Cdebug-assertions=y`,
kernel's `CONFIG_RUST_DEBUG_ASSERTIONS=y`) now always checks all unsafe
preconditions, though without a way to opt-out for particular cases [16].
It would be ideal to have a way to selectively disable certain checks
per-call site for this one (i.e. not just per check but for particular
instances of a check), even if the vast majority of the checks remain
in place [17].

Rust 1.78.0 also improved a couple issues we reported when giving feedback
for the new `--check-cfg` feature [18] [19].

# `alloc` upgrade and reviewing

As mentioned above, compiler upgrades will not update `alloc` anymore,
since we dropped our `alloc` fork [3].

Link: https://github.com/rust-lang/rust/blob/stable/RELEASES.md#version-1780-2024-05-02 [1]
Link: https://rust-for-linux.com/rust-version-policy [2]
Link: https://lore.kernel.org/rust-for-linux/20240328013603.206764-1-wedsonaf@gmail.com/ [3]
Link: Rust-for-Linux#2 [4]
Link: rust-lang/rust#120062 [5]
Link: rust-lang/rust#120055 [6]
Link: https://reviews.llvm.org/D86310 [7]
Link: rust-lang/rust#117772 [8]
Link: rust-lang/rust-clippy#10903 [9]
Link: rust-lang/rust#119365 [10]
Link: rust-lang/rust#119364 [11]
Link: https://lore.kernel.org/rust-for-linux/ZWipTZysC2YL7qsq@Boquns-Mac-mini.home/ [12]
Link: rust-lang/rust#119618 [13]
Link: rust-lang/rust#120932 [14]
Link: https://github.com/rust-lang/rust/pull/120932/files#diff-e6fc1622c46054cd46b1d225c5386c5554564b3b0fa8a03c2dc2d8627a1079d9 [15]
Link: rust-lang/rust#120969 [16]
Link: Rust-for-Linux#354 [17]
Link: rust-lang/rust#121202 [18]
Link: rust-lang/rust#121237 [19]
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Link: https://lore.kernel.org/r/20240401212303.537355-4-ojeda@kernel.org
[ Added a few more details and links I mentioned in the list. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
@tgross35
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tgross35 commented May 22, 2024

Could we support a named label rather than being only positional? Maybe

unsafe { asm!(
    "jmp {foo}",
    label foo {
        println!("Jumped from asm!");
    }
);}

(I mean that foo would be the alias for use in the format string, not that the label in assembly would wind up as foo:)

@bjorn3
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Member

bjorn3 commented May 22, 2024

unsafe { asm!(
    "jmp {foo}",
    foo = label {
        println!("Jumped from asm!");
    }
);}

is closer to the syntax used by other operands.

@tgross35
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oh, that does actually work already. Awesome

GuillaumeGomez pushed a commit to GuillaumeGomez/rust that referenced this issue Jul 10, 2024
Add asm goto support to `asm!`

Tracking issue: rust-lang#119364

This PR implements asm-goto support, using the syntax described in "future possibilities" section of [RFC2873](https://rust-lang.github.io/rfcs/2873-inline-asm.html#asm-goto).

Currently I have only implemented the `label` part, not the `fallthrough` part (i.e. fallthrough is implicit). This doesn't reduce the expressive though, since you can use label-break to get arbitrary control flow or simply set a value and rely on jump threading optimisation to get the desired control flow. I can add that later if deemed necessary.

r? ``@Amanieu``
cc ``@ojeda``
@joshtriplett
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In today's Rust-for-Linux meeting, we discussed this syntax. We realized a potential issue that wasn't considered during the design:

Because asm! always has to be in an unsafe block, that makes all the code in the labels be unsafe, too. That isn't necessarily what people want. If this integrated with existing if conditions people might write if unsafe { asm!(...) } { ... } else { ... } and those branches wouldn't be unsafe, but doing that with the current construct requires returning a bool (which currently requires label-break-value) and using that in an if, and hoping the optimizer does the right thing to have the asm branches go to the if branches. When we considered the label-based design as a syntactically simpler construct, we didn't consider that that would make the label blocks unsafe code.

We talked about a few possible ways to solve this. One that seemed appealing in the RfL meeting:

We could change the label blocks to be safe code by default. (There's some precedent for switching back to safe in unsafe code: if you define an item in an unsafe block, the contents of the item are safe.)

And, to make it less painful to use unsafe if you do need it, we can syntactically allow ident = label unsafe { /* unsafe code */ } in addition to ident = label { /* safe code */ }.

@Lokathor
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Do you mean a proposed future different extension to asm!, separate from asm_goto, would make asm! in some cases be an expression that could evaluate to a boolean? Because currently it can't be used in if like that so I'm a little confused.

@programmerjake
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We could change the label blocks to be safe code by default. (There's some precedent for switching back to safe in unsafe code: if you define an item in an unsafe block, the contents of the item are safe.)

I think we also need a more general safe { ... } block, since this is far from the only case where you want known-safe code inside an unsafe block, e.g. macros that expand to code like unsafe { unsafe_macro_helper(safe { safe_user_code() }, ...) }

@joshtriplett
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Do you mean a proposed future different extension to asm!, separate from asm_goto, would make asm! in some cases be an expression that could evaluate to a boolean? Because currently it can't be used in if like that so I'm a little confused.

When I said:

returning a bool (which currently requires label-break-value)

I mean this:

{'bar: { asm!("...", foo = label { break 'bar true; }); false }}

@nikomatsakis
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@nbdd0121 wrote up this document on advanced assembly usage in the Linux kernel.

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A-inline-assembly Area: Inline assembly (`asm!(…)`) C-tracking-issue Category: A tracking issue for an RFC or an unstable feature. F-asm `#![feature(asm)]` (not `llvm_asm`)
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