1 use crate::transform::{MirPass, MirSource};
2 use rustc_middle::mir::*;
3 use rustc_middle::ty::TyCtxt;
5 pub struct MatchBranchSimplification;
7 /// If a source block is found that switches between two blocks that are exactly
8 /// the same modulo const bool assignments (e.g., one assigns true another false
9 /// to the same place), merge a target block statements into the source block,
10 /// using Eq / Ne comparison with switch value where const bools value differ.
16 /// switchInt(move _3) -> [42_isize: bb1, otherwise: bb2];
34 /// _2 = Eq(move _3, const 42_isize);
39 impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
40 fn run_pass(&self, tcx: TyCtxt<'tcx>, src: MirSource<'tcx>, body: &mut Body<'tcx>) {
41 let param_env = tcx.param_env(src.def_id());
42 let bbs = body.basic_blocks_mut();
43 'outer: for bb_idx in bbs.indices() {
44 let (discr, val, switch_ty, first, second) = match bbs[bb_idx].terminator().kind {
45 TerminatorKind::SwitchInt {
46 discr: Operand::Copy(ref place) | Operand::Move(ref place),
51 } if targets.len() == 2 && values.len() == 1 && targets[0] != targets[1] => {
52 (place, values[0], switch_ty, targets[0], targets[1])
54 // Only optimize switch int statements
58 // Check that destinations are identical, and if not, then don't optimize this block
59 if &bbs[first].terminator().kind != &bbs[second].terminator().kind {
63 // Check that blocks are assignments of consts to the same place or same statement,
64 // and match up 1-1, if not don't optimize this block.
65 let first_stmts = &bbs[first].statements;
66 let scnd_stmts = &bbs[second].statements;
67 if first_stmts.len() != scnd_stmts.len() {
70 for (f, s) in first_stmts.iter().zip(scnd_stmts.iter()) {
71 match (&f.kind, &s.kind) {
72 // If two statements are exactly the same, we can optimize.
73 (f_s, s_s) if f_s == s_s => {}
75 // If two statements are const bool assignments to the same place, we can optimize.
77 StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
78 StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
80 && f_c.literal.ty.is_bool()
81 && s_c.literal.ty.is_bool()
82 && f_c.literal.try_eval_bool(tcx, param_env).is_some()
83 && s_c.literal.try_eval_bool(tcx, param_env).is_some() => {}
85 // Otherwise we cannot optimize. Try another block.
89 // Take ownership of items now that we know we can optimize.
90 let discr = discr.clone();
92 // We already checked that first and second are different blocks,
93 // and bb_idx has a different terminator from both of them.
94 let (from, first, second) = bbs.pick3_mut(bb_idx, first, second);
96 let new_stmts = first.statements.iter().zip(second.statements.iter()).map(|(f, s)| {
97 match (&f.kind, &s.kind) {
98 (f_s, s_s) if f_s == s_s => (*f).clone(),
101 StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
102 StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
104 // From earlier loop we know that we are dealing with bool constants only:
105 let f_b = f_c.literal.try_eval_bool(tcx, param_env).unwrap();
106 let s_b = s_c.literal.try_eval_bool(tcx, param_env).unwrap();
108 // Same value in both blocks. Use statement as is.
111 // Different value between blocks. Make value conditional on switch condition.
112 let size = tcx.layout_of(param_env.and(switch_ty)).unwrap().size;
113 let const_cmp = Operand::const_from_scalar(
116 crate::interpret::Scalar::from_uint(val, size),
117 rustc_span::DUMMY_SP,
119 let op = if f_b { BinOp::Eq } else { BinOp::Ne };
120 let rhs = Rvalue::BinaryOp(op, Operand::Copy(discr.clone()), const_cmp);
122 source_info: f.source_info,
123 kind: StatementKind::Assign(box (*lhs, rhs)),
131 from.statements.extend(new_stmts);
132 from.terminator_mut().kind = first.terminator().kind.clone();