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Add optimization to avoid load of address #76683

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Sep 22, 2020
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Add optimization to avoid load of address #76683

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2bb3844
Add optimization to avoid load of address
simonvandel Sep 13, 2020
dfc469d
Run the test with explicit -O such that Add is generated instead of C…
simonvandel Sep 21, 2020
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117 changes: 115 additions & 2 deletions compiler/rustc_mir/src/transform/instcombine.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -4,9 +4,14 @@ use crate::transform::{MirPass, MirSource};
use rustc_data_structures::fx::{FxHashMap, FxHashSet};
use rustc_hir::Mutability;
use rustc_index::vec::Idx;
use rustc_middle::mir::visit::{MutVisitor, Visitor};
use rustc_middle::mir::{
BinOp, Body, Constant, Local, Location, Operand, Place, PlaceRef, ProjectionElem, Rvalue,
visit::PlaceContext,
visit::{MutVisitor, Visitor},
Statement,
};
use rustc_middle::mir::{
BinOp, Body, BorrowKind, Constant, Local, Location, Operand, Place, PlaceRef, ProjectionElem,
Rvalue,
};
use rustc_middle::ty::{self, TyCtxt};
use std::mem;
Expand Down Expand Up @@ -71,10 +76,36 @@ impl<'tcx> MutVisitor<'tcx> for InstCombineVisitor<'tcx> {
*rvalue = Rvalue::Use(operand);
}

if let Some(place) = self.optimizations.unneeded_deref.remove(&location) {
debug!("unneeded_deref: replacing {:?} with {:?}", rvalue, place);
*rvalue = Rvalue::Use(Operand::Copy(place));
}

self.super_rvalue(rvalue, location)
}
}

struct MutatingUseVisitor {
has_mutating_use: bool,
local_to_look_for: Local,
}

impl MutatingUseVisitor {
fn has_mutating_use_in_stmt(local: Local, stmt: &Statement<'tcx>, location: Location) -> bool {
let mut _self = Self { has_mutating_use: false, local_to_look_for: local };
_self.visit_statement(stmt, location);
_self.has_mutating_use
}
}

impl<'tcx> Visitor<'tcx> for MutatingUseVisitor {
fn visit_local(&mut self, local: &Local, context: PlaceContext, _: Location) {
if *local == self.local_to_look_for {
self.has_mutating_use |= context.is_mutating_use();
}
}
}

/// Finds optimization opportunities on the MIR.
struct OptimizationFinder<'b, 'tcx> {
body: &'b Body<'tcx>,
Expand All @@ -87,6 +118,85 @@ impl OptimizationFinder<'b, 'tcx> {
OptimizationFinder { body, tcx, optimizations: OptimizationList::default() }
}

fn find_deref_of_address(&mut self, rvalue: &Rvalue<'tcx>, location: Location) -> Option<()> {
// Look for the sequence
//
// _2 = &_1;
// ...
// _5 = (*_2);
//
// which we can replace the last statement with `_5 = _1;` to avoid the load of `_2`.
if let Rvalue::Use(op) = rvalue {
let local_being_derefed = match op.place()?.as_ref() {
PlaceRef { local, projection: [ProjectionElem::Deref] } => Some(local),
_ => None,
}?;

let stmt_index = location.statement_index;
// Look behind for statement that assigns the local from a address of operator.
// 6 is chosen as a heuristic determined by seeing the number of times
// the optimization kicked in compiling rust std.
let lower_index = stmt_index.saturating_sub(6);
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let statements_to_look_in = self.body.basic_blocks()[location.block].statements
[lower_index..stmt_index]
.iter()
.rev();
for stmt in statements_to_look_in {
match &stmt.kind {
// Exhaustive match on statements to detect conditions that warrant we bail out of the optimization.
rustc_middle::mir::StatementKind::Assign(box (l, r))
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if l.local == local_being_derefed =>
{
match r {
// Looking for immutable reference e.g _local_being_deref = &_1;
Rvalue::Ref(
_,
// Only apply the optimization if it is an immutable borrow.
BorrowKind::Shared,
place_taken_address_of,
) => {
self.optimizations
.unneeded_deref
.insert(location, *place_taken_address_of);
return Some(());
}

// We found an assignment of `local_being_deref` that is not an immutable ref, e.g the following sequence
// _2 = &_1;
// _3 = &5
// _2 = _3; <-- this means it is no longer valid to replace the last statement with `_5 = _1;`
// _5 = (*_2);
_ => return None,
}
}

// Inline asm can do anything, so bail out of the optimization.
rustc_middle::mir::StatementKind::LlvmInlineAsm(_) => return None,

// Check that `local_being_deref` is not being used in a mutating way which can cause misoptimization.
rustc_middle::mir::StatementKind::Assign(box (_, _))
| rustc_middle::mir::StatementKind::Coverage(_)
| rustc_middle::mir::StatementKind::Nop
| rustc_middle::mir::StatementKind::FakeRead(_, _)
| rustc_middle::mir::StatementKind::StorageLive(_)
| rustc_middle::mir::StatementKind::StorageDead(_)
| rustc_middle::mir::StatementKind::Retag(_, _)
| rustc_middle::mir::StatementKind::AscribeUserType(_, _)
| rustc_middle::mir::StatementKind::SetDiscriminant { .. } => {
if MutatingUseVisitor::has_mutating_use_in_stmt(
local_being_derefed,
stmt,
location,
) {
return None;
}
}
}
}
}
Some(())
}

fn find_unneeded_equality_comparison(&mut self, rvalue: &Rvalue<'tcx>, location: Location) {
// find Ne(_place, false) or Ne(false, _place)
// or Eq(_place, true) or Eq(true, _place)
Expand Down Expand Up @@ -153,6 +263,8 @@ impl Visitor<'tcx> for OptimizationFinder<'b, 'tcx> {
}
}

let _ = self.find_deref_of_address(rvalue, location);

self.find_unneeded_equality_comparison(rvalue, location);

self.super_rvalue(rvalue, location)
Expand All @@ -164,4 +276,5 @@ struct OptimizationList<'tcx> {
and_stars: FxHashSet<Location>,
arrays_lengths: FxHashMap<Location, Constant<'tcx>>,
unneeded_equality_comparison: FxHashMap<Location, Operand<'tcx>>,
unneeded_deref: FxHashMap<Location, Place<'tcx>>,
}
2 changes: 1 addition & 1 deletion src/test/mir-opt/const_prop/ref_deref.main.ConstProp.diff
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Expand Up @@ -19,7 +19,7 @@
// + span: $DIR/ref_deref.rs:5:6: 5:10
// + literal: Const { ty: &i32, val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref[317d]::main[0]), const_param_did: None }, [], Some(promoted[0])) }
_2 = _4; // scope 0 at $DIR/ref_deref.rs:5:6: 5:10
- _1 = (*_2); // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
- _1 = (*_4); // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
+ _1 = const 4_i32; // scope 0 at $DIR/ref_deref.rs:5:5: 5:10
StorageDead(_2); // scope 0 at $DIR/ref_deref.rs:5:10: 5:11
StorageDead(_1); // scope 0 at $DIR/ref_deref.rs:5:10: 5:11
Expand Down
2 changes: 1 addition & 1 deletion src/test/mir-opt/const_prop/ref_deref_project.main.ConstProp.diff
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Expand Up @@ -19,7 +19,7 @@
// + span: $DIR/ref_deref_project.rs:5:6: 5:17
// + literal: Const { ty: &(i32, i32), val: Unevaluated(WithOptConstParam { did: DefId(0:3 ~ ref_deref_project[317d]::main[0]), const_param_did: None }, [], Some(promoted[0])) }
_2 = &((*_4).1: i32); // scope 0 at $DIR/ref_deref_project.rs:5:6: 5:17
_1 = (*_2); // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
_1 = ((*_4).1: i32); // scope 0 at $DIR/ref_deref_project.rs:5:5: 5:17
StorageDead(_2); // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
StorageDead(_1); // scope 0 at $DIR/ref_deref_project.rs:5:17: 5:18
_0 = const (); // scope 0 at $DIR/ref_deref_project.rs:4:11: 6:2
Expand Down
20 changes: 12 additions & 8 deletions src/test/mir-opt/inline/inline_closure_borrows_arg.foo.Inline.after.mir
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Expand Up @@ -9,13 +9,14 @@ fn foo(_1: T, _2: &i32) -> i32 {
let mut _5: (&i32, &i32); // in scope 0 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
let mut _6: &i32; // in scope 0 at $DIR/inline-closure-borrows-arg.rs:16:7: 16:8
let mut _7: &i32; // in scope 0 at $DIR/inline-closure-borrows-arg.rs:16:10: 16:11
let mut _8: &i32; // in scope 0 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
let mut _9: &i32; // in scope 0 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
let mut _10: &i32; // in scope 0 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
scope 1 {
debug x => _3; // in scope 1 at $DIR/inline-closure-borrows-arg.rs:12:9: 12:10
scope 2 {
debug r => _8; // in scope 2 at $DIR/inline-closure-borrows-arg.rs:12:14: 12:15
debug _s => _9; // in scope 2 at $DIR/inline-closure-borrows-arg.rs:12:23: 12:25
debug r => _9; // in scope 2 at $DIR/inline-closure-borrows-arg.rs:12:14: 12:15
debug _s => _10; // in scope 2 at $DIR/inline-closure-borrows-arg.rs:12:23: 12:25
let _8: &i32; // in scope 2 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
}
}
scope 3 {
Expand All @@ -33,13 +34,16 @@ fn foo(_1: T, _2: &i32) -> i32 {
_7 = &(*_2); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:10: 16:11
(_5.0: &i32) = move _6; // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
(_5.1: &i32) = move _7; // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageLive(_8); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
_8 = move (_5.0: &i32); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageLive(_9); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
_9 = move (_5.1: &i32); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
_0 = (*_8); // scope 3 at $DIR/inline-closure-borrows-arg.rs:14:9: 14:18
_9 = move (_5.0: &i32); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageLive(_10); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
_10 = move (_5.1: &i32); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageLive(_8); // scope 2 at $DIR/inline-closure-borrows-arg.rs:13:13: 13:21
_8 = _9; // scope 2 at $DIR/inline-closure-borrows-arg.rs:13:24: 13:27
_0 = (*_9); // scope 3 at $DIR/inline-closure-borrows-arg.rs:14:9: 14:18
StorageDead(_8); // scope 2 at $DIR/inline-closure-borrows-arg.rs:15:5: 15:6
StorageDead(_10); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageDead(_9); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageDead(_8); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:5: 16:12
StorageDead(_7); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:11: 16:12
StorageDead(_6); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:11: 16:12
StorageDead(_5); // scope 1 at $DIR/inline-closure-borrows-arg.rs:16:11: 16:12
Expand Down
92 changes: 92 additions & 0 deletions src/test/mir-opt/inst_combine_deref.deep_opt.InstCombine.diff
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@@ -0,0 +1,92 @@
- // MIR for `deep_opt` before InstCombine
+ // MIR for `deep_opt` after InstCombine

fn deep_opt() -> (u64, u64, u64) {
let mut _0: (u64, u64, u64); // return place in scope 0 at $DIR/inst_combine_deref.rs:11:18: 11:33
let _1: u64; // in scope 0 at $DIR/inst_combine_deref.rs:12:9: 12:11
let mut _10: u64; // in scope 0 at $DIR/inst_combine_deref.rs:21:6: 21:8
let mut _11: u64; // in scope 0 at $DIR/inst_combine_deref.rs:21:10: 21:12
let mut _12: u64; // in scope 0 at $DIR/inst_combine_deref.rs:21:14: 21:16
scope 1 {
debug x1 => _1; // in scope 1 at $DIR/inst_combine_deref.rs:12:9: 12:11
let _2: u64; // in scope 1 at $DIR/inst_combine_deref.rs:13:9: 13:11
scope 2 {
debug x2 => _2; // in scope 2 at $DIR/inst_combine_deref.rs:13:9: 13:11
let _3: u64; // in scope 2 at $DIR/inst_combine_deref.rs:14:9: 14:11
scope 3 {
debug x3 => _3; // in scope 3 at $DIR/inst_combine_deref.rs:14:9: 14:11
let _4: &u64; // in scope 3 at $DIR/inst_combine_deref.rs:15:9: 15:11
scope 4 {
debug y1 => _4; // in scope 4 at $DIR/inst_combine_deref.rs:15:9: 15:11
let _5: &u64; // in scope 4 at $DIR/inst_combine_deref.rs:16:9: 16:11
scope 5 {
debug y2 => _5; // in scope 5 at $DIR/inst_combine_deref.rs:16:9: 16:11
let _6: &u64; // in scope 5 at $DIR/inst_combine_deref.rs:17:9: 17:11
scope 6 {
debug y3 => _6; // in scope 6 at $DIR/inst_combine_deref.rs:17:9: 17:11
let _7: u64; // in scope 6 at $DIR/inst_combine_deref.rs:18:9: 18:11
scope 7 {
debug z1 => _7; // in scope 7 at $DIR/inst_combine_deref.rs:18:9: 18:11
let _8: u64; // in scope 7 at $DIR/inst_combine_deref.rs:19:9: 19:11
scope 8 {
debug z2 => _8; // in scope 8 at $DIR/inst_combine_deref.rs:19:9: 19:11
let _9: u64; // in scope 8 at $DIR/inst_combine_deref.rs:20:9: 20:11
scope 9 {
debug z3 => _9; // in scope 9 at $DIR/inst_combine_deref.rs:20:9: 20:11
}
}
}
}
}
}
}
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/inst_combine_deref.rs:12:9: 12:11
_1 = const 1_u64; // scope 0 at $DIR/inst_combine_deref.rs:12:14: 12:15
StorageLive(_2); // scope 1 at $DIR/inst_combine_deref.rs:13:9: 13:11
_2 = const 2_u64; // scope 1 at $DIR/inst_combine_deref.rs:13:14: 13:15
StorageLive(_3); // scope 2 at $DIR/inst_combine_deref.rs:14:9: 14:11
_3 = const 3_u64; // scope 2 at $DIR/inst_combine_deref.rs:14:14: 14:15
StorageLive(_4); // scope 3 at $DIR/inst_combine_deref.rs:15:9: 15:11
_4 = &_1; // scope 3 at $DIR/inst_combine_deref.rs:15:14: 15:17
StorageLive(_5); // scope 4 at $DIR/inst_combine_deref.rs:16:9: 16:11
_5 = &_2; // scope 4 at $DIR/inst_combine_deref.rs:16:14: 16:17
StorageLive(_6); // scope 5 at $DIR/inst_combine_deref.rs:17:9: 17:11
_6 = &_3; // scope 5 at $DIR/inst_combine_deref.rs:17:14: 17:17
StorageLive(_7); // scope 6 at $DIR/inst_combine_deref.rs:18:9: 18:11
- _7 = (*_4); // scope 6 at $DIR/inst_combine_deref.rs:18:14: 18:17
+ _7 = _1; // scope 6 at $DIR/inst_combine_deref.rs:18:14: 18:17
StorageLive(_8); // scope 7 at $DIR/inst_combine_deref.rs:19:9: 19:11
- _8 = (*_5); // scope 7 at $DIR/inst_combine_deref.rs:19:14: 19:17
+ _8 = _2; // scope 7 at $DIR/inst_combine_deref.rs:19:14: 19:17
StorageLive(_9); // scope 8 at $DIR/inst_combine_deref.rs:20:9: 20:11
- _9 = (*_6); // scope 8 at $DIR/inst_combine_deref.rs:20:14: 20:17
+ _9 = _3; // scope 8 at $DIR/inst_combine_deref.rs:20:14: 20:17
StorageLive(_10); // scope 9 at $DIR/inst_combine_deref.rs:21:6: 21:8
_10 = _7; // scope 9 at $DIR/inst_combine_deref.rs:21:6: 21:8
StorageLive(_11); // scope 9 at $DIR/inst_combine_deref.rs:21:10: 21:12
_11 = _8; // scope 9 at $DIR/inst_combine_deref.rs:21:10: 21:12
StorageLive(_12); // scope 9 at $DIR/inst_combine_deref.rs:21:14: 21:16
_12 = _9; // scope 9 at $DIR/inst_combine_deref.rs:21:14: 21:16
(_0.0: u64) = move _10; // scope 9 at $DIR/inst_combine_deref.rs:21:5: 21:17
(_0.1: u64) = move _11; // scope 9 at $DIR/inst_combine_deref.rs:21:5: 21:17
(_0.2: u64) = move _12; // scope 9 at $DIR/inst_combine_deref.rs:21:5: 21:17
StorageDead(_12); // scope 9 at $DIR/inst_combine_deref.rs:21:16: 21:17
StorageDead(_11); // scope 9 at $DIR/inst_combine_deref.rs:21:16: 21:17
StorageDead(_10); // scope 9 at $DIR/inst_combine_deref.rs:21:16: 21:17
StorageDead(_9); // scope 8 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_8); // scope 7 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_7); // scope 6 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_6); // scope 5 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_5); // scope 4 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_4); // scope 3 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_3); // scope 2 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_2); // scope 1 at $DIR/inst_combine_deref.rs:22:1: 22:2
StorageDead(_1); // scope 0 at $DIR/inst_combine_deref.rs:22:1: 22:2
return; // scope 0 at $DIR/inst_combine_deref.rs:22:2: 22:2
}
}

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