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//! Instruction shrinking.
//!
//! Sometimes there are multiple valid encodings for a given instruction. Cranelift often initially
//! chooses the largest one, because this typically provides the register allocator the most
//! flexibility. However, once register allocation is done, this is no longer important, and we
//! can switch to smaller encodings when possible.

use crate::ir::instructions::InstructionData;
use crate::ir::Function;
use crate::isa::TargetIsa;
use crate::regalloc::RegDiversions;
use crate::timing;
use log::debug;

/// Pick the smallest valid encodings for instructions.
pub fn shrink_instructions(func: &mut Function, isa: &dyn TargetIsa) {
    let _tt = timing::shrink_instructions();

    let encinfo = isa.encoding_info();
    let mut divert = RegDiversions::new();

    for ebb in func.layout.ebbs() {
        // Load diversions from predecessors.
        divert.at_ebb(&func.entry_diversions, ebb);

        for inst in func.layout.ebb_insts(ebb) {
            let enc = func.encodings[inst];
            if enc.is_legal() {
                // regmove/regfill/regspill are special instructions with register immediates
                // that represented as normal operands, so the normal predicates below don't
                // handle them correctly.
                //
                // Also, they need to be presented to the `RegDiversions` to update the
                // location tracking.
                //
                // TODO: Eventually, we want the register allocator to avoid leaving these special
                // instructions behind, but for now, just temporarily avoid trying to shrink them.
                let inst_data = &func.dfg[inst];
                match inst_data {
                    InstructionData::RegMove { .. }
                    | InstructionData::RegFill { .. }
                    | InstructionData::RegSpill { .. } => {
                        divert.apply(inst_data);
                        continue;
                    }
                    _ => (),
                }

                let ctrl_type = func.dfg.ctrl_typevar(inst);

                // Pick the last encoding with constraints that are satisfied.
                let best_enc = isa
                    .legal_encodings(func, &func.dfg[inst], ctrl_type)
                    .filter(|e| encinfo.constraints[e.recipe()].satisfied(inst, &divert, &func))
                    .min_by_key(|e| encinfo.byte_size(*e, inst, &divert, &func))
                    .unwrap();

                if best_enc != enc {
                    func.encodings[inst] = best_enc;

                    debug!(
                        "Shrunk [{}] to [{}] in {}, reducing the size from {} to {}",
                        encinfo.display(enc),
                        encinfo.display(best_enc),
                        func.dfg.display_inst(inst, isa),
                        encinfo.byte_size(enc, inst, &divert, &func),
                        encinfo.byte_size(best_enc, inst, &divert, &func)
                    );
                }
            }
        }
    }
}