wasmtime/lib/cretonne/src/legalizer.rs

//! Legalize instructions.
//!
//! A legal instruction is one that can be mapped directly to a machine code instruction for the
//! target ISA. The `legalize_function()` function takes as input any function and transforms it
//! into an equivalent function using only legal instructions.
//!
//! The characteristics of legal instructions depend on the target ISA, so any given instruction
//! can be legal for one ISA and illegal for another.
//!
//! Besides transforming instructions, the legalizer also fills out the `function.encodings` map
//! which provides a legal encoding recipe for every instruction.
//!
//! The legalizer does not deal with register allocation constraints. These constraints are derived
//! from the encoding recipes, and solved later by the register allocator.

use abi::{legalize_abi_value, ValueConversion};
use ir::{Function, Cursor, DataFlowGraph, InstructionData, Opcode, InstBuilder, Ebb, Type, Value,
         ArgumentType};
use ir::condcodes::IntCC;
use isa::{TargetIsa, Legalize};

/// Legalize `func` for `isa`.
///
/// - Transform any instructions that don't have a legal representation in `isa`.
/// - Fill out `func.encodings`.
///
pub fn legalize_function(func: &mut Function, isa: &TargetIsa) {
    legalize_signatures(func, isa);

    // TODO: This is very simplified and incomplete.
    func.encodings.resize(func.dfg.num_insts());
    let mut pos = Cursor::new(&mut func.layout);
    while let Some(_ebb) = pos.next_ebb() {
        // Keep track of the cursor position before the instruction being processed, so we can
        // double back when replacing instructions.
        let mut prev_pos = pos.position();

        while let Some(inst) = pos.next_inst() {
            match isa.encode(&func.dfg, &func.dfg[inst]) {
                Ok(encoding) => *func.encodings.ensure(inst) = encoding,
                Err(action) => {
                    // We should transform the instruction into legal equivalents.
                    // Possible strategies are:
                    // 1. Legalize::Expand: Expand instruction into sequence of legal instructions.
                    //    Possibly iteratively. ()
                    // 2. Legalize::Narrow: Split the controlling type variable into high and low
                    //    parts. This applies both to SIMD vector types which can be halved and to
                    //    integer types such as `i64` used on a 32-bit ISA. ().
                    // 3. TODO: Promote the controlling type variable to a larger type. This
                    //    typically means expressing `i8` and `i16` arithmetic in terms if `i32`
                    //    operations on RISC targets. (It may or may not be beneficial to promote
                    //    small vector types versus splitting them.)
                    // 4. TODO: Convert to library calls. For example, floating point operations on
                    //    an ISA with no IEEE 754 support.
                    let changed = match action {
                        Legalize::Expand => expand(&mut pos, &mut func.dfg),
                        Legalize::Narrow => narrow(&mut pos, &mut func.dfg),
                    };
                    // If the current instruction was replaced, we need to double back and revisit
                    // the expanded sequence. This is both to assign encodings and possible to
                    // expand further.
                    // There's a risk of infinite looping here if the legalization patterns are
                    // unsound. Should we attempt to detect that?
                    if changed {
                        pos.set_position(prev_pos);
                    }
                }
            }

            // Remember this position in case we need to double back.
            prev_pos = pos.position();
        }
    }
}

// Include legalization patterns that were generated by `gen_legalizer.py` from the `XForms` in
// `meta/cretonne/legalize.py`.
//
// Concretely, this defines private functions `narrow()`, and `expand()`.
include!(concat!(env!("OUT_DIR"), "/legalizer.rs"));

/// Legalize all the function signatures in `func`.
///
/// This changes all signatures to be ABI-compliant with full `ArgumentLoc` annotations. It doesn't
/// change the entry block arguments, calls, or return instructions, so this can leave the function
/// in a state with type discrepancies.
fn legalize_signatures(func: &mut Function, isa: &TargetIsa) {
    isa.legalize_signature(&mut func.signature);
    for sig in func.dfg.signatures.keys() {
        isa.legalize_signature(&mut func.dfg.signatures[sig]);
    }

    if let Some(entry) = func.layout.entry_block() {
        legalize_entry_arguments(func, entry);
    }
}

/// Legalize the entry block arguments after `func`'s signature has been legalized.
///
/// The legalized signature may contain more arguments than the original signature, and the
/// argument types have been changed. This function goes through the arguments to the entry EBB and
/// replaces them with arguments of the right type for the ABI.
///
/// The original entry EBB arguments are computed from the new ABI arguments by code inserted at
/// the top of the entry block.
fn legalize_entry_arguments(func: &mut Function, entry: Ebb) {
    // Insert position for argument conversion code.
    // We want to insert instructions before the first instruction in the entry block.
    // If the entry block is empty, append instructions to it instead.
    let mut pos = Cursor::new(&mut func.layout);
    pos.goto_top(entry);
    pos.next_inst();

    // Keep track of the argument types in the ABI-legalized signature.
    let abi_types = &func.signature.argument_types;
    let mut abi_arg = 0;

    // Process the EBB arguments one at a time, possibly replacing one argument with multiple new
    // ones. We do this by detaching the entry EBB arguments first.
    let mut next_arg = func.dfg.take_ebb_args(entry);
    while let Some(arg) = next_arg {
        // Get the next argument before we mutate `arg`.
        next_arg = func.dfg.next_ebb_arg(arg);

        let arg_type = func.dfg.value_type(arg);
        if arg_type == abi_types[abi_arg].value_type {
            // No value translation is necessary, this argument matches the ABI type.
            // Just use the original EBB argument value. This is the most common case.
            func.dfg.put_ebb_arg(entry, arg);
            abi_arg += 1;
        } else {
            // Compute the value we want for `arg` from the legalized ABI arguments.
            let converted = convert_from_abi(&mut func.dfg,
                                             &mut pos,
                                             entry,
                                             &mut abi_arg,
                                             abi_types,
                                             arg_type);
            // The old `arg` is no longer an attached EBB argument, but there are probably still
            // uses of the value. Make it an alias to the converted value.
            func.dfg.change_to_alias(arg, converted);
        }
    }
}

/// Compute original value of type `ty` from the legalized ABI arguments beginning at `abi_arg`.
///
/// Update `abi_arg` to reflect the ABI arguments consumed and return the computed value.
fn convert_from_abi(dfg: &mut DataFlowGraph,
                    pos: &mut Cursor,
                    entry: Ebb,
                    abi_arg: &mut usize,
                    abi_types: &[ArgumentType],
                    ty: Type)
                    -> Value {
    // Terminate the recursion when we get the desired type.
    if ty == abi_types[*abi_arg].value_type {
        return dfg.append_ebb_arg(entry, ty);
    }

    // Reconstruct how `ty` was legalized into the argument at `abi_arg`.
    let conversion = legalize_abi_value(ty, &abi_types[*abi_arg]);

    // The conversion describes value to ABI argument. We implement the reverse conversion here.
    match conversion {
        // Construct a `ty` by concatenating two ABI integers.
        ValueConversion::IntSplit => {
            let abi_ty = ty.half_width().expect("Invalid type for conversion");
            let lo = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            let hi = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            dfg.ins(pos).iconcat_lohi(lo, hi)
        }
        // Construct a `ty` by concatenating two halves of a vector.
        ValueConversion::VectorSplit => {
            let abi_ty = ty.half_vector().expect("Invalid type for conversion");
            let lo = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            let hi = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            dfg.ins(pos).vconcat(lo, hi)
        }
        // Construct a `ty` by bit-casting from an integer type.
        ValueConversion::IntBits => {
            assert!(!ty.is_int());
            let abi_ty = Type::int(ty.bits()).expect("Invalid type for conversion");
            let arg = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            dfg.ins(pos).bitcast(ty, arg)
        }
        // ABI argument is a sign-extended version of the value we want.
        ValueConversion::Sext(abi_ty) => {
            let arg = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            // TODO: Currently, we don't take advantage of the ABI argument being sign-extended.
            // We could insert an `assert_sreduce` which would fold with a following `sextend` of
            // this value.
            dfg.ins(pos).ireduce(ty, arg)
        }
        ValueConversion::Uext(abi_ty) => {
            let arg = convert_from_abi(dfg, pos, entry, abi_arg, abi_types, abi_ty);
            // TODO: Currently, we don't take advantage of the ABI argument being sign-extended.
            // We could insert an `assert_ureduce` which would fold with a following `uextend` of
            // this value.
            dfg.ins(pos).ireduce(ty, arg)
        }
    }
}