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//! Stack slots.
//!
//! The `StackSlotData` struct keeps track of a single stack slot in a function.
//!

use crate::entity::PrimaryMap;
use crate::ir::StackSlot;
use core::fmt;
use core::str::FromStr;

#[cfg(feature = "enable-serde")]
use serde::{Deserialize, Serialize};

/// The size of an object on the stack, or the size of a stack frame.
///
/// We don't use `usize` to represent object sizes on the target platform because Cranelift supports
/// cross-compilation, and `usize` is a type that depends on the host platform, not the target
/// platform.
pub type StackSize = u32;

/// The kind of a stack slot.
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
pub enum StackSlotKind {
    /// An explicit stack slot. This is a chunk of stack memory for use by the `stack_load`
    /// and `stack_store` instructions.
    ExplicitSlot,
}

impl FromStr for StackSlotKind {
    type Err = ();

    fn from_str(s: &str) -> Result<Self, ()> {
        use self::StackSlotKind::*;
        match s {
            "explicit_slot" => Ok(ExplicitSlot),
            _ => Err(()),
        }
    }
}

impl fmt::Display for StackSlotKind {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        use self::StackSlotKind::*;
        f.write_str(match *self {
            ExplicitSlot => "explicit_slot",
        })
    }
}

/// Contents of a stack slot.
#[derive(Clone, Debug, PartialEq, Eq)]
#[cfg_attr(feature = "enable-serde", derive(Serialize, Deserialize))]
pub struct StackSlotData {
    /// The kind of stack slot.
    pub kind: StackSlotKind,

    /// Size of stack slot in bytes.
    pub size: StackSize,
}

impl StackSlotData {
    /// Create a stack slot with the specified byte size.
    pub fn new(kind: StackSlotKind, size: StackSize) -> Self {
        Self { kind, size }
    }

    /// Get the alignment in bytes of this stack slot given the stack pointer alignment.
    pub fn alignment(&self, max_align: StackSize) -> StackSize {
        debug_assert!(max_align.is_power_of_two());
        // We want to find the largest power of two that divides both `self.size` and `max_align`.
        // That is the same as isolating the rightmost bit in `x`.
        let x = self.size | max_align;
        // C.f. Hacker's delight.
        x & x.wrapping_neg()
    }
}

impl fmt::Display for StackSlotData {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        write!(f, "{} {}", self.kind, self.size)
    }
}

/// All allocated stack slots.
pub type StackSlots = PrimaryMap<StackSlot, StackSlotData>;

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ir::Function;
    use alloc::string::ToString;

    #[test]
    fn stack_slot() {
        let mut func = Function::new();

        let ss0 = func.create_stack_slot(StackSlotData::new(StackSlotKind::ExplicitSlot, 4));
        let ss1 = func.create_stack_slot(StackSlotData::new(StackSlotKind::ExplicitSlot, 8));
        assert_eq!(ss0.to_string(), "ss0");
        assert_eq!(ss1.to_string(), "ss1");

        assert_eq!(func.stack_slots[ss0].size, 4);
        assert_eq!(func.stack_slots[ss1].size, 8);

        assert_eq!(func.stack_slots[ss0].to_string(), "explicit_slot 4");
        assert_eq!(func.stack_slots[ss1].to_string(), "explicit_slot 8");
    }

    #[test]
    fn alignment() {
        let slot = StackSlotData::new(StackSlotKind::ExplicitSlot, 8);

        assert_eq!(slot.alignment(4), 4);
        assert_eq!(slot.alignment(8), 8);
        assert_eq!(slot.alignment(16), 8);

        let slot2 = StackSlotData::new(StackSlotKind::ExplicitSlot, 24);

        assert_eq!(slot2.alignment(4), 4);
        assert_eq!(slot2.alignment(8), 8);
        assert_eq!(slot2.alignment(16), 8);
        assert_eq!(slot2.alignment(32), 8);
    }
}