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// `#![no_std]`: these arrive with the standard prelude and name no path, so a `std::`
// search cannot see them - and a `#[derive]` can use them without the name appearing
// in this file at all, which is why they are not trimmed by inspection.
use alloc::borrow::ToOwned;
use alloc::boxed::Box;
use alloc::format;
use alloc::string::{String, ToString};
use alloc::vec;
use alloc::vec::Vec;
use core::iter;
use crate::bug;
use crate::rustc_middle::mir::interpret::Scalar;
use crate::rustc_middle::mir::{
BasicBlock, BinOp, Body, Operand, Place, Rvalue, StatementKind, SwitchTargets, TerminatorKind,
};
use crate::rustc_middle::ty::{Ty, TyCtxt};
use tracing::trace;
use crate::rustc_mir_transform::PassPolicy;
use crate::rustc_mir_transform::ssa::SsaLocals;
/// Pass to convert `if` conditions on integrals into switches on the integral.
/// For an example, it turns something like
///
/// ```ignore (MIR)
/// _3 = Eq(move _4, const 43i32);
/// switchInt(_3) -> [false: bb2, otherwise: bb3];
/// ```
///
/// into:
///
/// ```ignore (MIR)
/// switchInt(_4) -> [43i32: bb3, otherwise: bb2];
/// ```
pub(super) struct SimplifyComparisonIntegral;
impl<'tcx> crate::rustc_mir_transform::MirPass<'tcx> for SimplifyComparisonIntegral {
fn policy(&self, sess: &crate::rustc_session::Session) -> PassPolicy {
PassPolicy::optimization(sess.mir_opt_level() > 1)
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
trace!("Running SimplifyComparisonIntegral on {:?}", body.source);
let typing_env = body.typing_env(tcx);
let ssa = SsaLocals::new(tcx, body, typing_env);
let helper = OptimizationFinder { body };
let opts = helper.find_optimizations(&ssa);
for opt in opts {
trace!("SUCCESS: Applying {:?}", opt);
// replace terminator with a switchInt that switches on the integer directly
let bbs = &mut body.basic_blocks_mut();
let bb = &mut bbs[opt.bb_idx];
let new_value = match opt.branch_value_scalar {
Scalar::Int(int) => {
let layout = tcx
.layout_of(typing_env.as_query_input(opt.branch_value_ty))
.expect("if we have an evaluated constant we must know the layout");
int.to_bits(layout.size)
}
Scalar::Ptr(..) => continue,
};
const FALSE: u128 = 0;
let mut new_targets = opt.targets;
let first_value = new_targets.iter().next().unwrap().0;
let first_is_false_target = first_value == FALSE;
match opt.op {
BinOp::Eq => {
// if the assignment was Eq we want the true case to be first
if first_is_false_target {
new_targets.all_targets_mut().swap(0, 1);
}
}
BinOp::Ne => {
// if the assignment was Ne we want the false case to be first
if !first_is_false_target {
new_targets.all_targets_mut().swap(0, 1);
}
}
_ => unreachable!(),
}
// if the integer being compared to a const integral is being moved into the
// comparison, e.g `_2 = Eq(move _3, const 'x');`
// we want to avoid making a double move later on in the switchInt on _3.
// So to avoid `switchInt(move _3) -> ['x': bb2, otherwise: bb1];`,
// we convert the move in the comparison statement to a copy.
// unwrap is safe as we know this statement is an assign
let (_, rhs) = bb.statements[opt.bin_op_stmt_idx].kind.as_assign_mut().unwrap();
use Operand::*;
if let Rvalue::BinaryOp(_, operands) = rhs {
match &mut **operands {
(left @ Move(_), Constant(_)) => {
*left = Copy(opt.to_switch_on);
}
(Constant(_), right @ Move(_)) => {
*right = Copy(opt.to_switch_on);
}
_ => (),
}
}
let [bb_cond, bb_otherwise] = match new_targets.all_targets() {
[a, b] => [*a, *b],
e => bug!("expected 2 switch targets, got: {:?}", e),
};
let targets = SwitchTargets::new(iter::once((new_value, bb_cond)), bb_otherwise);
let terminator = bb.terminator_mut();
terminator.kind =
TerminatorKind::SwitchInt { discr: Operand::Copy(opt.to_switch_on), targets };
}
}
}
struct OptimizationFinder<'a, 'tcx> {
body: &'a Body<'tcx>,
}
impl<'tcx> OptimizationFinder<'_, 'tcx> {
fn find_optimizations(&self, ssa: &SsaLocals) -> Vec<OptimizationInfo<'tcx>> {
self.body
.basic_blocks
.iter_enumerated()
.filter_map(|(bb_idx, bb)| {
// find switch
let (discr, targets) = bb.terminator().kind.as_switch()?;
let place_switched_on = discr.place()?;
// Make sure that the place is not modified.
if !ssa.is_ssa(place_switched_on.local) || !place_switched_on.is_stable_offset() {
return None;
}
// find the statement that assigns the place being switched on
bb.statements.iter().enumerate().rev().find_map(|(stmt_idx, stmt)| {
match &stmt.kind {
crate::rustc_middle::mir::StatementKind::Assign(assign)
if assign.0 == place_switched_on =>
{
let (_, rhs) = &**assign;
match rhs {
Rvalue::BinaryOp(op @ (BinOp::Eq | BinOp::Ne), operands) => {
let (left, right) = &**operands;
let (branch_value_scalar, branch_value_ty, to_switch_on) =
find_branch_value_info(left, right, ssa)?;
// The transformation adds a use of `to_switch_on` at the
// terminator. Both storage markers make the local uninitialized,
// so either invalidates the value used by the comparison.
if bb.statements[stmt_idx + 1..].iter().any(|stmt| {
matches!(
stmt.kind,
StatementKind::StorageLive(local)
| StatementKind::StorageDead(local)
if local == to_switch_on.local
)
}) {
return None;
}
Some(OptimizationInfo {
bin_op_stmt_idx: stmt_idx,
bb_idx,
to_switch_on,
branch_value_scalar,
branch_value_ty,
op: *op,
targets: targets.clone(),
})
}
_ => None,
}
}
_ => None,
}
})
})
.collect()
}
}
fn find_branch_value_info<'tcx>(
left: &Operand<'tcx>,
right: &Operand<'tcx>,
ssa: &SsaLocals,
) -> Option<(Scalar, Ty<'tcx>, Place<'tcx>)> {
// check that either left or right is a constant.
// if any are, we can use the other to switch on, and the constant as a value in a switch
use Operand::*;
match (left, right) {
(Constant(branch_value), Copy(to_switch_on) | Move(to_switch_on))
| (Copy(to_switch_on) | Move(to_switch_on), Constant(branch_value)) => {
// Make sure that the place is not modified.
if !ssa.is_ssa(to_switch_on.local) || !to_switch_on.is_stable_offset() {
return None;
}
let branch_value_ty = branch_value.const_.ty();
// we only want to apply this optimization if we are matching on integrals (and chars),
// as it is not possible to switch on floats
if !branch_value_ty.is_integral() && !branch_value_ty.is_char() {
return None;
};
let branch_value_scalar = branch_value.const_.try_to_scalar()?;
Some((branch_value_scalar, branch_value_ty, *to_switch_on))
}
_ => None,
}
}
#[derive(Debug)]
struct OptimizationInfo<'tcx> {
/// Basic block to apply the optimization
bb_idx: BasicBlock,
/// Statement index of Eq/Ne assignment
bin_op_stmt_idx: usize,
/// Place that needs to be switched on. This place is of type integral
to_switch_on: Place<'tcx>,
/// Constant to use in switch target value
branch_value_scalar: Scalar,
/// Type of the constant value
branch_value_ty: Ty<'tcx>,
/// Either Eq or Ne
op: BinOp,
/// Current targets used in the switch
targets: SwitchTargets,
}