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| 1 | +use super::{MirPass, MirSource}; |
| 2 | +use rustc_middle::{ |
| 3 | + mir::{ |
| 4 | + interpret::Scalar, BasicBlock, BinOp, Body, Operand, Place, Rvalue, Statement, |
| 5 | + StatementKind, TerminatorKind, |
| 6 | + }, |
| 7 | + ty::{Ty, TyCtxt}, |
| 8 | +}; |
| 9 | + |
| 10 | +/// Pass to convert `if` conditions on integrals into switches on the integral. |
| 11 | +/// For an example, it turns something like |
| 12 | +/// |
| 13 | +/// ``` |
| 14 | +/// _3 = Eq(move _4, const 43i32); |
| 15 | +/// StorageDead(_4); |
| 16 | +/// switchInt(_3) -> [false: bb2, otherwise: bb3]; |
| 17 | +/// ``` |
| 18 | +/// |
| 19 | +/// into: |
| 20 | +/// |
| 21 | +/// ``` |
| 22 | +/// switchInt(_4) -> [43i32: bb3, otherwise: bb2]; |
| 23 | +/// ``` |
| 24 | +pub struct SimplifyComparisonIntegral; |
| 25 | + |
| 26 | +impl<'tcx> MirPass<'tcx> for SimplifyComparisonIntegral { |
| 27 | + fn run_pass(&self, _: TyCtxt<'tcx>, source: MirSource<'tcx>, body: &mut Body<'tcx>) { |
| 28 | + trace!("Running SimplifyComparisonIntegral on {:?}", source); |
| 29 | + |
| 30 | + let helper = OptimizationFinder { body }; |
| 31 | + let opts = helper.find_optimizations(); |
| 32 | + let mut storage_deads_to_insert = vec![]; |
| 33 | + let mut storage_deads_to_remove: Vec<(usize, BasicBlock)> = vec![]; |
| 34 | + for opt in opts { |
| 35 | + trace!("SUCCESS: Applying {:?}", opt); |
| 36 | + // replace terminator with a switchInt that switches on the integer directly |
| 37 | + let bbs = &mut body.basic_blocks_mut(); |
| 38 | + let bb = &mut bbs[opt.bb_idx]; |
| 39 | + // We only use the bits for the untyped, not length checked `values` field. Thus we are |
| 40 | + // not using any of the convenience wrappers here and directly access the bits. |
| 41 | + let new_value = match opt.branch_value_scalar { |
| 42 | + Scalar::Raw { data, .. } => data, |
| 43 | + Scalar::Ptr(_) => continue, |
| 44 | + }; |
| 45 | + const FALSE: u128 = 0; |
| 46 | + let mut new_targets = opt.targets.clone(); |
| 47 | + let first_is_false_target = opt.values[0] == FALSE; |
| 48 | + match opt.op { |
| 49 | + BinOp::Eq => { |
| 50 | + // if the assignment was Eq we want the true case to be first |
| 51 | + if first_is_false_target { |
| 52 | + new_targets.swap(0, 1); |
| 53 | + } |
| 54 | + } |
| 55 | + BinOp::Ne => { |
| 56 | + // if the assignment was Ne we want the false case to be first |
| 57 | + if !first_is_false_target { |
| 58 | + new_targets.swap(0, 1); |
| 59 | + } |
| 60 | + } |
| 61 | + _ => unreachable!(), |
| 62 | + } |
| 63 | + |
| 64 | + let terminator = bb.terminator_mut(); |
| 65 | + |
| 66 | + // add StorageDead for the place switched on at the top of each target |
| 67 | + for bb_idx in new_targets.iter() { |
| 68 | + storage_deads_to_insert.push(( |
| 69 | + *bb_idx, |
| 70 | + Statement { |
| 71 | + source_info: terminator.source_info, |
| 72 | + kind: StatementKind::StorageDead(opt.to_switch_on.local), |
| 73 | + }, |
| 74 | + )); |
| 75 | + } |
| 76 | + |
| 77 | + terminator.kind = TerminatorKind::SwitchInt { |
| 78 | + discr: Operand::Move(opt.to_switch_on), |
| 79 | + switch_ty: opt.branch_value_ty, |
| 80 | + values: vec![new_value].into(), |
| 81 | + targets: new_targets, |
| 82 | + }; |
| 83 | + |
| 84 | + // delete comparison statement if it the value being switched on was moved, which means it can not be user later on |
| 85 | + if opt.can_remove_bin_op_stmt { |
| 86 | + bb.statements[opt.bin_op_stmt_idx].make_nop(); |
| 87 | + } else { |
| 88 | + // if the integer being compared to a const integral is being moved into the comparison, |
| 89 | + // e.g `_2 = Eq(move _3, const 'x');` |
| 90 | + // we want to avoid making a double move later on in the switchInt on _3. |
| 91 | + // So to avoid `switchInt(move _3) -> ['x': bb2, otherwise: bb1];`, |
| 92 | + // we convert the move in the comparison statement to a copy. |
| 93 | + |
| 94 | + // unwrap is safe as we know this statement is an assign |
| 95 | + let box (_, rhs) = bb.statements[opt.bin_op_stmt_idx].kind.as_assign_mut().unwrap(); |
| 96 | + |
| 97 | + use Operand::*; |
| 98 | + match rhs { |
| 99 | + Rvalue::BinaryOp(_, ref mut left @ Move(_), Constant(_)) => { |
| 100 | + *left = Copy(opt.to_switch_on); |
| 101 | + } |
| 102 | + Rvalue::BinaryOp(_, Constant(_), ref mut right @ Move(_)) => { |
| 103 | + *right = Copy(opt.to_switch_on); |
| 104 | + } |
| 105 | + _ => (), |
| 106 | + } |
| 107 | + } |
| 108 | + |
| 109 | + // remove StorageDead (if it exists) being used in the assign of the comparison |
| 110 | + for (stmt_idx, stmt) in bb.statements.iter().enumerate() { |
| 111 | + if !matches!(stmt.kind, StatementKind::StorageDead(local) if local == opt.to_switch_on.local) |
| 112 | + { |
| 113 | + continue; |
| 114 | + } |
| 115 | + storage_deads_to_remove.push((stmt_idx, opt.bb_idx)) |
| 116 | + } |
| 117 | + } |
| 118 | + |
| 119 | + for (idx, bb_idx) in storage_deads_to_remove { |
| 120 | + body.basic_blocks_mut()[bb_idx].statements[idx].make_nop(); |
| 121 | + } |
| 122 | + |
| 123 | + for (idx, stmt) in storage_deads_to_insert { |
| 124 | + body.basic_blocks_mut()[idx].statements.insert(0, stmt); |
| 125 | + } |
| 126 | + } |
| 127 | +} |
| 128 | + |
| 129 | +struct OptimizationFinder<'a, 'tcx> { |
| 130 | + body: &'a Body<'tcx>, |
| 131 | +} |
| 132 | + |
| 133 | +impl<'a, 'tcx> OptimizationFinder<'a, 'tcx> { |
| 134 | + fn find_optimizations(&self) -> Vec<OptimizationInfo<'tcx>> { |
| 135 | + self.body |
| 136 | + .basic_blocks() |
| 137 | + .iter_enumerated() |
| 138 | + .filter_map(|(bb_idx, bb)| { |
| 139 | + // find switch |
| 140 | + let (place_switched_on, values, targets, place_switched_on_moved) = match &bb |
| 141 | + .terminator() |
| 142 | + .kind |
| 143 | + { |
| 144 | + rustc_middle::mir::TerminatorKind::SwitchInt { |
| 145 | + discr, values, targets, .. |
| 146 | + } => Some((discr.place()?, values, targets, discr.is_move())), |
| 147 | + _ => None, |
| 148 | + }?; |
| 149 | + |
| 150 | + // find the statement that assigns the place being switched on |
| 151 | + bb.statements.iter().enumerate().rev().find_map(|(stmt_idx, stmt)| { |
| 152 | + match &stmt.kind { |
| 153 | + rustc_middle::mir::StatementKind::Assign(box (lhs, rhs)) |
| 154 | + if *lhs == place_switched_on => |
| 155 | + { |
| 156 | + match rhs { |
| 157 | + Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), left, right) => { |
| 158 | + let (branch_value_scalar, branch_value_ty, to_switch_on) = |
| 159 | + find_branch_value_info(left, right)?; |
| 160 | + |
| 161 | + Some(OptimizationInfo { |
| 162 | + bin_op_stmt_idx: stmt_idx, |
| 163 | + bb_idx, |
| 164 | + can_remove_bin_op_stmt: place_switched_on_moved, |
| 165 | + to_switch_on, |
| 166 | + branch_value_scalar, |
| 167 | + branch_value_ty, |
| 168 | + op: *op, |
| 169 | + values: values.clone().into_owned(), |
| 170 | + targets: targets.clone(), |
| 171 | + }) |
| 172 | + } |
| 173 | + _ => None, |
| 174 | + } |
| 175 | + } |
| 176 | + _ => None, |
| 177 | + } |
| 178 | + }) |
| 179 | + }) |
| 180 | + .collect() |
| 181 | + } |
| 182 | +} |
| 183 | + |
| 184 | +fn find_branch_value_info<'tcx>( |
| 185 | + left: &Operand<'tcx>, |
| 186 | + right: &Operand<'tcx>, |
| 187 | +) -> Option<(Scalar, Ty<'tcx>, Place<'tcx>)> { |
| 188 | + // check that either left or right is a constant. |
| 189 | + // if any are, we can use the other to switch on, and the constant as a value in a switch |
| 190 | + use Operand::*; |
| 191 | + match (left, right) { |
| 192 | + (Constant(branch_value), Copy(to_switch_on) | Move(to_switch_on)) |
| 193 | + | (Copy(to_switch_on) | Move(to_switch_on), Constant(branch_value)) => { |
| 194 | + let branch_value_ty = branch_value.literal.ty; |
| 195 | + // we only want to apply this optimization if we are matching on integrals (and chars), as it is not possible to switch on floats |
| 196 | + if !branch_value_ty.is_integral() && !branch_value_ty.is_char() { |
| 197 | + return None; |
| 198 | + }; |
| 199 | + let branch_value_scalar = branch_value.literal.val.try_to_scalar()?; |
| 200 | + Some((branch_value_scalar, branch_value_ty, *to_switch_on)) |
| 201 | + } |
| 202 | + _ => None, |
| 203 | + } |
| 204 | +} |
| 205 | + |
| 206 | +#[derive(Debug)] |
| 207 | +struct OptimizationInfo<'tcx> { |
| 208 | + /// Basic block to apply the optimization |
| 209 | + bb_idx: BasicBlock, |
| 210 | + /// Statement index of Eq/Ne assignment that can be removed. None if the assignment can not be removed - i.e the statement is used later on |
| 211 | + bin_op_stmt_idx: usize, |
| 212 | + /// Can remove Eq/Ne assignment |
| 213 | + can_remove_bin_op_stmt: bool, |
| 214 | + /// Place that needs to be switched on. This place is of type integral |
| 215 | + to_switch_on: Place<'tcx>, |
| 216 | + /// Constant to use in switch target value |
| 217 | + branch_value_scalar: Scalar, |
| 218 | + /// Type of the constant value |
| 219 | + branch_value_ty: Ty<'tcx>, |
| 220 | + /// Either Eq or Ne |
| 221 | + op: BinOp, |
| 222 | + /// Current values used in the switch target. This needs to be replaced with the branch_value |
| 223 | + values: Vec<u128>, |
| 224 | + /// Current targets used in the switch |
| 225 | + targets: Vec<BasicBlock>, |
| 226 | +} |
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