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Auto merge of #120614 - DianQK:simplify-switch-int, r=cjgillot

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Transforms match into an assignment statement

Fixes #106459.

We should be able to do some similar transformations, like `enum` to `enum`.

r? mir-opt
This commit is contained in:
bors 2024-04-08 18:28:50 +00:00
commit 211518e5fb
16 changed files with 1113 additions and 179 deletions
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594
compiler/rustc_mir_transform/src/match_branches.rs
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@ -1,11 +1,125 @@
use rustc_index::IndexSlice;
use rustc_middle::mir::patch::MirPatch;
use rustc_middle::mir::*;
use rustc_middle::ty::TyCtxt;
use rustc_middle::ty::{ParamEnv, ScalarInt, Ty, TyCtxt};
use rustc_target::abi::Size;
use std::iter;
use super::simplify::simplify_cfg;
pub struct MatchBranchSimplification;
impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 1
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let def_id = body.source.def_id();
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let mut should_cleanup = false;
for i in 0..body.basic_blocks.len() {
let bbs = &*body.basic_blocks;
let bb_idx = BasicBlock::from_usize(i);
if !tcx.consider_optimizing(|| format!("MatchBranchSimplification {def_id:?} ")) {
continue;
}
match bbs[bb_idx].terminator().kind {
TerminatorKind::SwitchInt {
discr: ref _discr @ (Operand::Copy(_) | Operand::Move(_)),
ref targets,
..
// We require that the possible target blocks don't contain this block.
} if !targets.all_targets().contains(&bb_idx) => {}
// Only optimize switch int statements
_ => continue,
};
if SimplifyToIf.simplify(tcx, body, bb_idx, param_env).is_some() {
should_cleanup = true;
continue;
}
if SimplifyToExp::default().simplify(tcx, body, bb_idx, param_env).is_some() {
should_cleanup = true;
continue;
}
}
if should_cleanup {
simplify_cfg(body);
}
}
}
trait SimplifyMatch<'tcx> {
/// Simplifies a match statement, returning true if the simplification succeeds, false otherwise.
/// Generic code is written here, and we generally don't need a custom implementation.
fn simplify(
&mut self,
tcx: TyCtxt<'tcx>,
body: &mut Body<'tcx>,
switch_bb_idx: BasicBlock,
param_env: ParamEnv<'tcx>,
) -> Option<()> {
let bbs = &body.basic_blocks;
let (discr, targets) = match bbs[switch_bb_idx].terminator().kind {
TerminatorKind::SwitchInt { ref discr, ref targets, .. } => (discr, targets),
_ => unreachable!(),
};
let discr_ty = discr.ty(body.local_decls(), tcx);
self.can_simplify(tcx, targets, param_env, bbs, discr_ty)?;
let mut patch = MirPatch::new(body);
// Take ownership of items now that we know we can optimize.
let discr = discr.clone();
// Introduce a temporary for the discriminant value.
let source_info = bbs[switch_bb_idx].terminator().source_info;
let discr_local = patch.new_temp(discr_ty, source_info.span);
let (_, first) = targets.iter().next().unwrap();
let statement_index = bbs[switch_bb_idx].statements.len();
let parent_end = Location { block: switch_bb_idx, statement_index };
patch.add_statement(parent_end, StatementKind::StorageLive(discr_local));
patch.add_assign(parent_end, Place::from(discr_local), Rvalue::Use(discr));
self.new_stmts(tcx, targets, param_env, &mut patch, parent_end, bbs, discr_local, discr_ty);
patch.add_statement(parent_end, StatementKind::StorageDead(discr_local));
patch.patch_terminator(switch_bb_idx, bbs[first].terminator().kind.clone());
patch.apply(body);
Some(())
}
/// Check that the BBs to be simplified satisfies all distinct and
/// that the terminator are the same.
/// There are also conditions for different ways of simplification.
fn can_simplify(
&mut self,
tcx: TyCtxt<'tcx>,
targets: &SwitchTargets,
param_env: ParamEnv<'tcx>,
bbs: &IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
discr_ty: Ty<'tcx>,
) -> Option<()>;
fn new_stmts(
&self,
tcx: TyCtxt<'tcx>,
targets: &SwitchTargets,
param_env: ParamEnv<'tcx>,
patch: &mut MirPatch<'tcx>,
parent_end: Location,
bbs: &IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
discr_local: Local,
discr_ty: Ty<'tcx>,
);
}
struct SimplifyToIf;
/// If a source block is found that switches between two blocks that are exactly
/// the same modulo const bool assignments (e.g., one assigns true another false
/// to the same place), merge a target block statements into the source block,
@ -37,144 +151,350 @@ pub struct MatchBranchSimplification;
/// goto -> bb3;
/// }
/// ```
impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
sess.mir_opt_level() >= 1
}
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let def_id = body.source.def_id();
let param_env = tcx.param_env_reveal_all_normalized(def_id);
let bbs = body.basic_blocks.as_mut();
let mut should_cleanup = false;
'outer: for bb_idx in bbs.indices() {
if !tcx.consider_optimizing(|| format!("MatchBranchSimplification {def_id:?} ")) {
continue;
}
let (discr, val, first, second) = match bbs[bb_idx].terminator().kind {
TerminatorKind::SwitchInt {
discr: ref discr @ (Operand::Copy(_) | Operand::Move(_)),
ref targets,
..
} if targets.iter().len() == 1 => {
let (value, target) = targets.iter().next().unwrap();
// We require that this block and the two possible target blocks all be
// distinct.
if target == targets.otherwise()
|| bb_idx == target
|| bb_idx == targets.otherwise()
{
continue;
}
(discr, value, target, targets.otherwise())
}
// Only optimize switch int statements
_ => continue,
};
// Check that destinations are identical, and if not, then don't optimize this block
if bbs[first].terminator().kind != bbs[second].terminator().kind {
continue;
}
// Check that blocks are assignments of consts to the same place or same statement,
// and match up 1-1, if not don't optimize this block.
let first_stmts = &bbs[first].statements;
let scnd_stmts = &bbs[second].statements;
if first_stmts.len() != scnd_stmts.len() {
continue;
}
for (f, s) in iter::zip(first_stmts, scnd_stmts) {
match (&f.kind, &s.kind) {
// If two statements are exactly the same, we can optimize.
(f_s, s_s) if f_s == s_s => {}
// If two statements are const bool assignments to the same place, we can optimize.
(
StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
) if lhs_f == lhs_s
&& f_c.const_.ty().is_bool()
&& s_c.const_.ty().is_bool()
&& f_c.const_.try_eval_bool(tcx, param_env).is_some()
&& s_c.const_.try_eval_bool(tcx, param_env).is_some() => {}
// Otherwise we cannot optimize. Try another block.
_ => continue 'outer,
}
}
// Take ownership of items now that we know we can optimize.
let discr = discr.clone();
let discr_ty = discr.ty(&body.local_decls, tcx);
// Introduce a temporary for the discriminant value.
let source_info = bbs[bb_idx].terminator().source_info;
let discr_local = body.local_decls.push(LocalDecl::new(discr_ty, source_info.span));
// We already checked that first and second are different blocks,
// and bb_idx has a different terminator from both of them.
let (from, first, second) = bbs.pick3_mut(bb_idx, first, second);
let new_stmts = iter::zip(&first.statements, &second.statements).map(|(f, s)| {
match (&f.kind, &s.kind) {
(f_s, s_s) if f_s == s_s => (*f).clone(),
(
StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
) => {
// From earlier loop we know that we are dealing with bool constants only:
let f_b = f_c.const_.try_eval_bool(tcx, param_env).unwrap();
let s_b = s_c.const_.try_eval_bool(tcx, param_env).unwrap();
if f_b == s_b {
// Same value in both blocks. Use statement as is.
(*f).clone()
} else {
// Different value between blocks. Make value conditional on switch condition.
let size = tcx.layout_of(param_env.and(discr_ty)).unwrap().size;
let const_cmp = Operand::const_from_scalar(
tcx,
discr_ty,
rustc_const_eval::interpret::Scalar::from_uint(val, size),
rustc_span::DUMMY_SP,
);
let op = if f_b { BinOp::Eq } else { BinOp::Ne };
let rhs = Rvalue::BinaryOp(
op,
Box::new((Operand::Copy(Place::from(discr_local)), const_cmp)),
);
Statement {
source_info: f.source_info,
kind: StatementKind::Assign(Box::new((*lhs, rhs))),
}
}
}
_ => unreachable!(),
}
});
from.statements
.push(Statement { source_info, kind: StatementKind::StorageLive(discr_local) });
from.statements.push(Statement {
source_info,
kind: StatementKind::Assign(Box::new((
Place::from(discr_local),
Rvalue::Use(discr),
))),
});
from.statements.extend(new_stmts);
from.statements
.push(Statement { source_info, kind: StatementKind::StorageDead(discr_local) });
from.terminator_mut().kind = first.terminator().kind.clone();
should_cleanup = true;
impl<'tcx> SimplifyMatch<'tcx> for SimplifyToIf {
fn can_simplify(
&mut self,
tcx: TyCtxt<'tcx>,
targets: &SwitchTargets,
param_env: ParamEnv<'tcx>,
bbs: &IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
_discr_ty: Ty<'tcx>,
) -> Option<()> {
if targets.iter().len() != 1 {
return None;
}
// We require that the possible target blocks all be distinct.
let (_, first) = targets.iter().next().unwrap();
let second = targets.otherwise();
if first == second {
return None;
}
// Check that destinations are identical, and if not, then don't optimize this block
if bbs[first].terminator().kind != bbs[second].terminator().kind {
return None;
}
if should_cleanup {
simplify_cfg(body);
// Check that blocks are assignments of consts to the same place or same statement,
// and match up 1-1, if not don't optimize this block.
let first_stmts = &bbs[first].statements;
let second_stmts = &bbs[second].statements;
if first_stmts.len() != second_stmts.len() {
return None;
}
for (f, s) in iter::zip(first_stmts, second_stmts) {
match (&f.kind, &s.kind) {
// If two statements are exactly the same, we can optimize.
(f_s, s_s) if f_s == s_s => {}
// If two statements are const bool assignments to the same place, we can optimize.
(
StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
) if lhs_f == lhs_s
&& f_c.const_.ty().is_bool()
&& s_c.const_.ty().is_bool()
&& f_c.const_.try_eval_bool(tcx, param_env).is_some()
&& s_c.const_.try_eval_bool(tcx, param_env).is_some() => {}
// Otherwise we cannot optimize. Try another block.
_ => return None,
}
}
Some(())
}
fn new_stmts(
&self,
tcx: TyCtxt<'tcx>,
targets: &SwitchTargets,
param_env: ParamEnv<'tcx>,
patch: &mut MirPatch<'tcx>,
parent_end: Location,
bbs: &IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
discr_local: Local,
discr_ty: Ty<'tcx>,
) {
let (val, first) = targets.iter().next().unwrap();
let second = targets.otherwise();
// We already checked that first and second are different blocks,
// and bb_idx has a different terminator from both of them.
let first = &bbs[first];
let second = &bbs[second];
for (f, s) in iter::zip(&first.statements, &second.statements) {
match (&f.kind, &s.kind) {
(f_s, s_s) if f_s == s_s => {
patch.add_statement(parent_end, f.kind.clone());
}
(
StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (_, Rvalue::Use(Operand::Constant(s_c)))),
) => {
// From earlier loop we know that we are dealing with bool constants only:
let f_b = f_c.const_.try_eval_bool(tcx, param_env).unwrap();
let s_b = s_c.const_.try_eval_bool(tcx, param_env).unwrap();
if f_b == s_b {
// Same value in both blocks. Use statement as is.
patch.add_statement(parent_end, f.kind.clone());
} else {
// Different value between blocks. Make value conditional on switch condition.
let size = tcx.layout_of(param_env.and(discr_ty)).unwrap().size;
let const_cmp = Operand::const_from_scalar(
tcx,
discr_ty,
rustc_const_eval::interpret::Scalar::from_uint(val, size),
rustc_span::DUMMY_SP,
);
let op = if f_b { BinOp::Eq } else { BinOp::Ne };
let rhs = Rvalue::BinaryOp(
op,
Box::new((Operand::Copy(Place::from(discr_local)), const_cmp)),
);
patch.add_assign(parent_end, *lhs, rhs);
}
}
_ => unreachable!(),
}
}
}
}
#[derive(Default)]
struct SimplifyToExp {
transfrom_types: Vec<TransfromType>,
}
#[derive(Clone, Copy)]
enum CompareType<'tcx, 'a> {
/// Identical statements.
Same(&'a StatementKind<'tcx>),
/// Assignment statements have the same value.
Eq(&'a Place<'tcx>, Ty<'tcx>, ScalarInt),
/// Enum variant comparison type.
Discr { place: &'a Place<'tcx>, ty: Ty<'tcx>, is_signed: bool },
}
enum TransfromType {
Same,
Eq,
Discr,
}
impl From<CompareType<'_, '_>> for TransfromType {
fn from(compare_type: CompareType<'_, '_>) -> Self {
match compare_type {
CompareType::Same(_) => TransfromType::Same,
CompareType::Eq(_, _, _) => TransfromType::Eq,
CompareType::Discr { .. } => TransfromType::Discr,
}
}
}
/// If we find that the value of match is the same as the assignment,
/// merge a target block statements into the source block,
/// using cast to transform different integer types.
///
/// For example:
///
/// ```ignore (MIR)
/// bb0: {
/// switchInt(_1) -> [1: bb2, 2: bb3, 3: bb4, otherwise: bb1];
/// }
///
/// bb1: {
/// unreachable;
/// }
///
/// bb2: {
/// _0 = const 1_i16;
/// goto -> bb5;
/// }
///
/// bb3: {
/// _0 = const 2_i16;
/// goto -> bb5;
/// }
///
/// bb4: {
/// _0 = const 3_i16;
/// goto -> bb5;
/// }
/// ```
///
/// into:
///
/// ```ignore (MIR)
/// bb0: {
/// _0 = _3 as i16 (IntToInt);
/// goto -> bb5;
/// }
/// ```
impl<'tcx> SimplifyMatch<'tcx> for SimplifyToExp {
fn can_simplify(
&mut self,
tcx: TyCtxt<'tcx>,
targets: &SwitchTargets,
param_env: ParamEnv<'tcx>,
bbs: &IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
discr_ty: Ty<'tcx>,
) -> Option<()> {
if targets.iter().len() < 2 || targets.iter().len() > 64 {
return None;
}
// We require that the possible target blocks all be distinct.
if !targets.is_distinct() {
return None;
}
if !bbs[targets.otherwise()].is_empty_unreachable() {
return None;
}
let mut target_iter = targets.iter();
let (first_val, first_target) = target_iter.next().unwrap();
let first_terminator_kind = &bbs[first_target].terminator().kind;
// Check that destinations are identical, and if not, then don't optimize this block
if !targets
.iter()
.all(|(_, other_target)| first_terminator_kind == &bbs[other_target].terminator().kind)
{
return None;
}
let discr_size = tcx.layout_of(param_env.and(discr_ty)).unwrap().size;
let first_stmts = &bbs[first_target].statements;
let (second_val, second_target) = target_iter.next().unwrap();
let second_stmts = &bbs[second_target].statements;
if first_stmts.len() != second_stmts.len() {
return None;
}
fn int_equal(l: ScalarInt, r: impl Into<u128>, size: Size) -> bool {
l.try_to_int(l.size()).unwrap()
== ScalarInt::try_from_uint(r, size).unwrap().try_to_int(size).unwrap()
}
// We first compare the two branches, and then the other branches need to fulfill the same conditions.
let mut compare_types = Vec::new();
for (f, s) in iter::zip(first_stmts, second_stmts) {
let compare_type = match (&f.kind, &s.kind) {
// If two statements are exactly the same, we can optimize.
(f_s, s_s) if f_s == s_s => CompareType::Same(f_s),
// If two statements are assignments with the match values to the same place, we can optimize.
(
StatementKind::Assign(box (lhs_f, Rvalue::Use(Operand::Constant(f_c)))),
StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
) if lhs_f == lhs_s
&& f_c.const_.ty() == s_c.const_.ty()
&& f_c.const_.ty().is_integral() =>
{
match (
f_c.const_.try_eval_scalar_int(tcx, param_env),
s_c.const_.try_eval_scalar_int(tcx, param_env),
) {
(Some(f), Some(s)) if f == s => CompareType::Eq(lhs_f, f_c.const_.ty(), f),
// Enum variants can also be simplified to an assignment statement if their values are equal.
// We need to consider both unsigned and signed scenarios here.
(Some(f), Some(s))
if ((f_c.const_.ty().is_signed() || discr_ty.is_signed())
&& int_equal(f, first_val, discr_size)
&& int_equal(s, second_val, discr_size))
|| (Some(f) == ScalarInt::try_from_uint(first_val, f.size())
&& Some(s)
== ScalarInt::try_from_uint(second_val, s.size())) =>
{
CompareType::Discr {
place: lhs_f,
ty: f_c.const_.ty(),
is_signed: f_c.const_.ty().is_signed() || discr_ty.is_signed(),
}
}
_ => {
return None;
}
}
}
// Otherwise we cannot optimize. Try another block.
_ => return None,
};
compare_types.push(compare_type);
}
// All remaining BBs need to fulfill the same pattern as the two BBs from the previous step.
for (other_val, other_target) in target_iter {
let other_stmts = &bbs[other_target].statements;
if compare_types.len() != other_stmts.len() {
return None;
}
for (f, s) in iter::zip(&compare_types, other_stmts) {
match (*f, &s.kind) {
(CompareType::Same(f_s), s_s) if f_s == s_s => {}
(
CompareType::Eq(lhs_f, f_ty, val),
StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
) if lhs_f == lhs_s
&& s_c.const_.ty() == f_ty
&& s_c.const_.try_eval_scalar_int(tcx, param_env) == Some(val) => {}
(
CompareType::Discr { place: lhs_f, ty: f_ty, is_signed },
StatementKind::Assign(box (lhs_s, Rvalue::Use(Operand::Constant(s_c)))),
) if lhs_f == lhs_s && s_c.const_.ty() == f_ty => {
let Some(f) = s_c.const_.try_eval_scalar_int(tcx, param_env) else {
return None;
};
if is_signed
&& s_c.const_.ty().is_signed()
&& int_equal(f, other_val, discr_size)
{
continue;
}
if Some(f) == ScalarInt::try_from_uint(other_val, f.size()) {
continue;
}
return None;
}
_ => return None,
}
}
}
self.transfrom_types = compare_types.into_iter().map(|c| c.into()).collect();
Some(())
}
fn new_stmts(
&self,
_tcx: TyCtxt<'tcx>,
targets: &SwitchTargets,
_param_env: ParamEnv<'tcx>,
patch: &mut MirPatch<'tcx>,
parent_end: Location,
bbs: &IndexSlice<BasicBlock, BasicBlockData<'tcx>>,
discr_local: Local,
discr_ty: Ty<'tcx>,
) {
let (_, first) = targets.iter().next().unwrap();
let first = &bbs[first];
for (t, s) in iter::zip(&self.transfrom_types, &first.statements) {
match (t, &s.kind) {
(TransfromType::Same, _) | (TransfromType::Eq, _) => {
patch.add_statement(parent_end, s.kind.clone());
}
(
TransfromType::Discr,
StatementKind::Assign(box (lhs, Rvalue::Use(Operand::Constant(f_c)))),
) => {
let operand = Operand::Copy(Place::from(discr_local));
let r_val = if f_c.const_.ty() == discr_ty {
Rvalue::Use(operand)
} else {
Rvalue::Cast(CastKind::IntToInt, operand, f_c.const_.ty())
};
patch.add_assign(parent_end, *lhs, r_val);
}
_ => unreachable!(),
}
}
}
}