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Feature Name: isa_attribute
Start Date: 2020-02-16
RFC PR: rust-lang/rfcs#2867
Rust Issue: rust-lang/rust#74727

Summary

This RFC proposes a new function attribute, #[instruction_set(set)] which allows you to declare the instruction set to be used when compiling the function. It also proposes two initial allowed values for the ARM arch (arm::a32 and arm::t32). Other allowed values could be added to the language later.

Motivation

Starting with ARMv4T, many ARM CPUs support two separate instruction sets. At the time they were called "ARM code" and "Thumb code", but with the development of AArch64, they're now called a32 and t32. Unlike with the x86_64 architecture, where the CPU can run both x86 and x86_64 code, but a single program still uses just one of the two instruction sets, on ARM you can have a single program that intersperses both a32 and t32 code. A particular form of branch instruction allows for the CPU to change between the two modes any time it branches, and so code can be designated as being either a32 or t32 on a per-function basis.

In LLVM, selecting that code should be a32 or t32 is done by either disabling (for a32) or enabling (for t32) the thumb-mode target feature. Previously, Rust was able to do this using the target_feature attribute because it was able to either add or subtract an LLVM target feature during a function. However, when RFC 2045 was accepted, its final form did not allow for the subtraction of target features. Its final form is primarily designed around always opting in to additional features, and it's no longer the correct tool for an "either A or B, but not both" situation like a32/t32 is.

Guide-level explanation

Some platforms support having more than one instruction set used within a single program. Generally, each one will be better for specific parts of a program. Every target has a default instruction set, based on the target triple. If you would like to set a specific function to use an alternate instruction set you use the #[instruction_set(set)] attribute.

Currently this is only of use on ARM family CPUs, which support both the arm::a32 and arm::t32 instruction sets. Targets starting with arm (eg: arm-linux-androideabi) default to arm::a32 and targets starting with thumb (eg: thumbv7neon-linux-androideabi) default to arm::t32.

// this uses the default instruction set for your target
fn add_one(x: i32) -> i32 {
    x + 1
}

// This will compile as `a32` code on both `arm` and `thumb` targets
#[instruction_set(arm::a32)]
fn add_five(x: i32) -> i32 {
    x + 5
}

It is a compile time error to specify an instruction set that is not available on the target you're compiling for. Users wishing for their code to be as portable as possible should use cfg_attr to only enable the attribute when using the appropriate targets.

// This will fail to build if `arm::a32` isn't available
#[instruction_set(arm::a32)]
fn add_five(x: i32) -> i32 {
    x + 5
}

// This will build on all platforms, and apply the `instruction_set` attribute
// only on ARM targets.
#[cfg_attr(target_cpu="arm", instruction_set(arm::a32))]
fn add_six(x: i32) -> i32 {
    x + 6
}

As you can see it can get a little verbose, so projects which plan to use the instruction_set attribute might want to consider writing a proc-macro with a shorter name.

The specifics of when you should specify a non-default instruction set on a function are platform specific. Unless a piece of platform documentation has indicated a specific requirement, you do not need to think about adding this attribute at all.

Reference-level explanation

Every target is now considered to have one default instruction set (for functions that lack the instruction_set attribute), as well as possibly supporting specific additional instruction sets:

The targets with names that start with arm default to arm::a32, but can also use arm::t32.
The targets with names that start with thumb default to arm::t32, but can also use arm::a32.
The instruction_set attribute is not currently defined for use with any other arch.
To avoid possible name clashes, the convention for this attribute is that the name of the instruction set itself (eg: a32) is prefixed with the name of the arch it goes with (eg: arm).

Where can this attribute be used:

This attribute can be used on any fn item that has a body: Free functions, inherent methods, trait default methods, and trait impl methods.
This attribute cannot be used on closures or within extern block declarations.
(Allowing this on trait prototypes is a Future Possibility.)

What is a Compile Error:

If an alternate instruction set is designated that doesn't exist (eg: "unicorn") then that is a compiler error. Later versions of the compiler/language are free to add additional allowed instruction set values.
Specifying an alternate instruction set attribute more than once with each usage being for a different arch it is allowed.

Guarantees:

If an alternate instruction set is designated on a function then the compiler must respect that. It is not a hint, it is a guarantee.
The exact details of an instruction_set guarantee vary by target.
Notably, the instruction_set attribute is most likely to interact (in a target specific way) with function inlining and use of inline assembly.

ARM

(this portion is a little extra technical, and very platform specific)

On ARM, there are two different instruction encodings. In textual/assembly form, Thumb assembly is written as a subset of ARM assembly, but the actual bit patterns produced when the text is assembled are entirely different. The CPU has a bit within the Program Status Register that indicates if the CPU should read 4 bytes at the Program Counter address and interpret them as an a32 opcode, or if it should read 2 bytes at the Program Counter address and interpret them as a t32 opcode. Because the amount of data read and the interpretation of the data is totally dissimilar, attempting to read one form of code while the CPU's flag is set for the other form of code is Undefined Behavior.

The outside world can tell what type of code a given function is based on the address of the function: a32 code has an even address, and t32 code has an odd address. The Program Counter ignores the actual value of the low bit, so t32 code is still considered to be "aligned to 2". When a branch-exchange (bx) or branch-link-exchange (blx) instruction is used then the target address's lowest bit is used to determine the CPU's new code state. When a branch (b) or branch-link (bl) instruction are used, the CPU's code state is not changed.

Thus, what we have to ensure with a32 and t32 is that the code generated for the marked function has the right encoding and also that the address is correctly even or odd:

It is Guaranteed that the address of the function will be correctly even or odd, and also that the start of the function's body will be in the correct encoding.
It is Hinted for the entire function body to generate with a single encoding.
If necessary, it is considered conforming for a compiler to insert only a stub of the correct encoding and address, which then jumps to a function body using another encoding. This should be considered a fallback strategy, but it would technically satisfy the requirements.

Backend support:

In LLVM this corresponds to enabling or disabling the thumb-mode target feature on a particular function.
Other future backends (eg: Cranelift) would presumably support this in some similar way. A "quick and dirty" version of a32/t32 interworking can be achieved simply by simply placing all a32 code in one translation unit, all t32 code in another, and then telling the linker to sort it out. Currently, Cranelift does not support ARM chips at all, but they can easily work towards this over time.
Because Miri operates on Rust's MIR stage, this attribute doesn't affect the operation of Miri. If Miri were to some day support inline assembly this attribute would need to be taken into account for that to work right, but Miri could also simply choose to not support this attribute in combination with inline assembly.
Assemblers and Linkers for ARM platforms have flags to enable the "interwork" of a32 and t32 code. If a user is writing their own assembly and then linking that with Rust code manually they might have to adjust their flags appropriately. This is mostly an implementation detail, though we can do our best to document that in the reference, and to provide any "good defaults" on our end.

Inlining:

If a function call is inlined, there's no longer an actual branch to another address, so if an entirely rust function with the instruction_set attribute is inlined into the caller, there's no further effect for the attribute to have.
If a function with an instruction_set attribute also contains an inline assembly block things are complicated. Even if the assembly text were valid within the instruction set it was inlined into, checking if that's the case or not would involve inspecting the assembly string and then making decisions based on that, which is explicitly against the design intent of the inline assembly feature (that the compiler should generally not inspect the assembly string).
Unfortunately, it's also not always clear to the programmer when inlining happens because sometimes a function might be inlined up through several layers of the call stack.
How to resolve this is an Unresolved Question (see below).

Drawbacks

Adding another attribute complicates Rust's design.

Rationale and alternatives

Rationale

Here's a simple but complete-enough program of how this would be used in practice. In this example, the program is for the Game Boy Advance (GBA). I have attempted to limit it to the essentials, so all the MMIO definitions, as well as the assembly runtime you'd need to boot and call main, are still omitted from the example.

// The GBA's BIOS provides some functionality available via software
// interrupt. We expose them to Rust in our assumed assembly "runtime".
extern "C" fn {
    /// Puts the CPU into a low-power state until a vblank interrupt,
    /// and then returns after the interrupt handler completes.
    VBlankInterWait(isize, isize);
}

// We assume that the MMIO stuff is imported from somewhere.
// The exact addresses and constant values aren't important.
mod all_the_gba_mmio_definitions;
use all_the_gba_mmio_definitions::*;

fn main() {
    // All of the `write_volatile` calls here refer to
    // the method of the `*mut T` type. Proper safe abstractions
    // for all of this would complicate the example, so we
    // simply use raw pointers and one large `unsafe` block.
    unsafe {
        // set the interrupt function to be our handler
        INTR_FN_ADDR.write_volatile(core::transmute(my_inter_fn));

        // enable vblank interrupts
        DISPSTAT.write_volatile(DISPSTAT_VBLANK);
        IME.write_volatile(IME_VBLANK);
        IE.write_volatile(true);
        
        // set the device for a basic display mode.
        DISPCNT.write_volatile(MODE3_BG2);
        let mut x = 0;
        loop {
            // wait in a low-power state for the vertical blank to start.
            VBlankInterWait(0, 0);
            // draw one new red pixel per frame along the top.
            VRAM_MODE3.row(0).col(x).write(RED);
            x += 1;
            // loop our position as necessary so that we don't
            // go out of bounds.
            if x >= VRAM_MODE3::WIDTH { x = 0 }
        }
    }
}

/// Responds to any interrupt by clearing all interrupt flags
/// and then immediately returning with no other effect.
#[instruction_set(arm::a32)]
fn my_inter_fn() {
    INTER_BIOS_FLAGS.write_volatile(ALL_INTER_FLAGS);
    INTER_STANDARD_FLAGS.write_volatile(ALL_INTER_FLAGS);
}

We setup the device with our interrupt handler.
We set the device to have an interrupt every time the vertical blank starts.
We set the display to use a basic bitmap mode and begin our loop.
Each pass of the loop we wait for vertical blank, then draw a single pixel to video memory.

In the case of this particular device, the hardware interrupts go to the device's BIOS, which then calls your interrupt handler function. However, because the BIOS is a32 code and uses a b branch instead of a bx branch-exchange, it jumps to the handler with the CPU in an a32 state. If the handler were written as t32 code it would immediately trigger UB.

Alternatives

Extending target_feature to allow #[target_feature(disable = "...")] and adding thumb-mode to the whitelist would support this functionality without adding another distinct attribute; however, this does not fit with the target_feature attribute's current focus on features such as AVX and SSE whose absence is not necessarily compensated for by the presence of something else.
Doing nothing is an option; it is currently possible to incorporate code using other instruction sets through means such as external assembly and build scripts. However, this has greatly reduced ergonomics.
Of note is the fact that this is a feature that mostly improves Rust's support for the more legacy end of ARM devices. Newer devices, with much larger amounts of memory (relatively), don't usually benefit as much. They could simply compile the entire program as a32, without needing to gain the space savings of t32 code.

Prior art

In C you can use __attribute__((target("arm"))) and __attribute__((target("thumb"))) to access similar functionality. It's a compiler-specific extension, but it's supported by both GCC and Clang (this PR appears to be the one that added this feature to LLVM/clang).

Unresolved questions

How do we ensure that instruction_set and inline assembly always interact correctly? This isn't an implementation blocker but needs to be resolved before Stabilization of the attribute.
- Currently, LLVM will not inline a32 functions into t32 functions and vice versa, because they count as different code targets. However, this is not necessarily a guarantee from LLVM, it could just be the current implementation, so more investigation is needed.

Future possibilities

If Rust gains support for the 65C816, the #[instruction_set(?)] attribute might be extended to allow shifting into its 65C02 compatibility mode and back again.
MIPS has a 16-bit encoding which uses a similar scheme as ARM, where the low bit of a function's address is set when the 16-bit encoding is in use for that function.
It might become possible to apply this attribute to trait prototypes in a future versions, in which case all impls of the method would take on the attribute. The main problems are properly specifying it and also that it would add additional compiler complexity for very minimal gain.
- Even without this change, a particular impl of the trait can use the attribute on its methods.
LLVM might eventually gain support for inter-instruction-set calls that allow calls between two arches (eg: a hybrid PowerPC/RISC-V).

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