compiler/rustc_target/src/callconv/x86_64.rs

// The classification code for the x86_64 ABI is taken from the clay language
// https://github.com/jckarter/clay/blob/db0bd2702ab0b6e48965cd85f8859bbd5f60e48e/compiler/externals.cpp

use rustc_abi::{BackendRepr, HasDataLayout, Size, TyAbiInterface, TyAndLayout};

use crate::abi;
use crate::abi::call::{ArgAbi, CastTarget, FnAbi, Reg, RegKind};

/// Classification of "eightbyte" components.
// N.B., the order of the variants is from general to specific,
// such that `unify(a, b)` is the "smaller" of `a` and `b`.
#[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Debug)]
enum Class {
    Int,
    Sse,
    SseUp,
}

#[derive(Clone, Copy, Debug)]
struct Memory;

// Currently supported vector size (AVX-512).
const LARGEST_VECTOR_SIZE: usize = 512;
const MAX_EIGHTBYTES: usize = LARGEST_VECTOR_SIZE / 64;

fn classify_arg<'a, Ty, C>(
    cx: &C,
    arg: &ArgAbi<'a, Ty>,
) -> Result<[Option<Class>; MAX_EIGHTBYTES], Memory>
where
    Ty: TyAbiInterface<'a, C> + Copy,
    C: HasDataLayout,
{
    fn classify<'a, Ty, C>(
        cx: &C,
        layout: TyAndLayout<'a, Ty>,
        cls: &mut [Option<Class>],
        off: Size,
    ) -> Result<(), Memory>
    where
        Ty: TyAbiInterface<'a, C> + Copy,
        C: HasDataLayout,
    {
        if !off.is_aligned(layout.align.abi) {
            if !layout.is_zst() {
                return Err(Memory);
            }
            return Ok(());
        }

        let mut c = match layout.backend_repr {
            BackendRepr::Uninhabited => return Ok(()),

            BackendRepr::Scalar(scalar) => match scalar.primitive() {
                abi::Int(..) | abi::Pointer(_) => Class::Int,
                abi::Float(_) => Class::Sse,
            },

            BackendRepr::Vector { .. } => Class::Sse,

            BackendRepr::ScalarPair(..) | BackendRepr::Memory { .. } => {
                for i in 0..layout.fields.count() {
                    let field_off = off + layout.fields.offset(i);
                    classify(cx, layout.field(cx, i), cls, field_off)?;
                }

                match &layout.variants {
                    abi::Variants::Single { .. } => {}
                    abi::Variants::Multiple { variants, .. } => {
                        // Treat enum variants like union members.
                        for variant_idx in variants.indices() {
                            classify(cx, layout.for_variant(cx, variant_idx), cls, off)?;
                        }
                    }
                }

                return Ok(());
            }
        };

        // Fill in `cls` for scalars (Int/Sse) and vectors (Sse).
        let first = (off.bytes() / 8) as usize;
        let last = ((off.bytes() + layout.size.bytes() - 1) / 8) as usize;
        for cls in &mut cls[first..=last] {
            *cls = Some(cls.map_or(c, |old| old.min(c)));

            // Everything after the first Sse "eightbyte"
            // component is the upper half of a register.
            if c == Class::Sse {
                c = Class::SseUp;
            }
        }

        Ok(())
    }

    let n = ((arg.layout.size.bytes() + 7) / 8) as usize;
    if n > MAX_EIGHTBYTES {
        return Err(Memory);
    }

    let mut cls = [None; MAX_EIGHTBYTES];
    classify(cx, arg.layout, &mut cls, Size::ZERO)?;
    if n > 2 {
        if cls[0] != Some(Class::Sse) {
            return Err(Memory);
        }
        if cls[1..n].iter().any(|&c| c != Some(Class::SseUp)) {
            return Err(Memory);
        }
    } else {
        let mut i = 0;
        while i < n {
            if cls[i] == Some(Class::SseUp) {
                cls[i] = Some(Class::Sse);
            } else if cls[i] == Some(Class::Sse) {
                i += 1;
                while i != n && cls[i] == Some(Class::SseUp) {
                    i += 1;
                }
            } else {
                i += 1;
            }
        }
    }

    Ok(cls)
}

fn reg_component(cls: &[Option<Class>], i: &mut usize, size: Size) -> Option<Reg> {
    if *i >= cls.len() {
        return None;
    }

    match cls[*i] {
        None => None,
        Some(Class::Int) => {
            *i += 1;
            Some(if size.bytes() < 8 { Reg { kind: RegKind::Integer, size } } else { Reg::i64() })
        }
        Some(Class::Sse) => {
            let vec_len =
                1 + cls[*i + 1..].iter().take_while(|&&c| c == Some(Class::SseUp)).count();
            *i += vec_len;
            Some(if vec_len == 1 {
                match size.bytes() {
                    4 => Reg::f32(),
                    _ => Reg::f64(),
                }
            } else {
                Reg { kind: RegKind::Vector, size: Size::from_bytes(8) * (vec_len as u64) }
            })
        }
        Some(c) => unreachable!("reg_component: unhandled class {:?}", c),
    }
}

fn cast_target(cls: &[Option<Class>], size: Size) -> CastTarget {
    let mut i = 0;
    let lo = reg_component(cls, &mut i, size).unwrap();
    let offset = Size::from_bytes(8) * (i as u64);
    let mut target = CastTarget::from(lo);
    if size > offset {
        if let Some(hi) = reg_component(cls, &mut i, size - offset) {
            target = CastTarget::pair(lo, hi);
        }
    }
    assert_eq!(reg_component(cls, &mut i, Size::ZERO), None);
    target
}

const MAX_INT_REGS: usize = 6; // RDI, RSI, RDX, RCX, R8, R9
const MAX_SSE_REGS: usize = 8; // XMM0-7

pub(crate) fn compute_abi_info<'a, Ty, C>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
where
    Ty: TyAbiInterface<'a, C> + Copy,
    C: HasDataLayout,
{
    let mut int_regs = MAX_INT_REGS;
    let mut sse_regs = MAX_SSE_REGS;

    let mut x86_64_arg_or_ret = |arg: &mut ArgAbi<'a, Ty>, is_arg: bool| {
        if !arg.layout.is_sized() {
            // Not touching this...
            return;
        }
        let mut cls_or_mem = classify_arg(cx, arg);

        if is_arg {
            if let Ok(cls) = cls_or_mem {
                let mut needed_int = 0;
                let mut needed_sse = 0;
                for c in cls {
                    match c {
                        Some(Class::Int) => needed_int += 1,
                        Some(Class::Sse) => needed_sse += 1,
                        _ => {}
                    }
                }
                match (int_regs.checked_sub(needed_int), sse_regs.checked_sub(needed_sse)) {
                    (Some(left_int), Some(left_sse)) => {
                        int_regs = left_int;
                        sse_regs = left_sse;
                    }
                    _ => {
                        // Not enough registers for this argument, so it will be
                        // passed on the stack, but we only mark aggregates
                        // explicitly as indirect `byval` arguments, as LLVM will
                        // automatically put immediates on the stack itself.
                        if arg.layout.is_aggregate() {
                            cls_or_mem = Err(Memory);
                        }
                    }
                }
            }
        }

        match cls_or_mem {
            Err(Memory) => {
                if is_arg {
                    // The x86_64 ABI doesn't have any special requirements for `byval` alignment,
                    // the type's alignment is always used.
                    arg.pass_by_stack_offset(None);
                } else {
                    // `sret` parameter thus one less integer register available
                    arg.make_indirect();
                    // NOTE(eddyb) return is handled first, so no registers
                    // should've been used yet.
                    assert_eq!(int_regs, MAX_INT_REGS);
                    int_regs -= 1;
                }
            }
            Ok(ref cls) => {
                // split into sized chunks passed individually
                if arg.layout.is_aggregate() {
                    let size = arg.layout.size;
                    arg.cast_to(cast_target(cls, size));
                } else {
                    arg.extend_integer_width_to(32);
                }
            }
        }
    };

    if !fn_abi.ret.is_ignore() {
        x86_64_arg_or_ret(&mut fn_abi.ret, false);
    }

    for arg in fn_abi.args.iter_mut() {
        if arg.is_ignore() {
            continue;
        }
        x86_64_arg_or_ret(arg, true);
    }
}