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[mir-opt] Optimize calls to CopyNonOverlapping #83785

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[mir-opt] Optimize calls to CopyNonOverlapping #83785

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44 changes: 34 additions & 10 deletions compiler/rustc_mir/src/transform/lower_intrinsics.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -11,6 +11,7 @@ pub struct LowerIntrinsics;

impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
let param_env = tcx.param_env(body.source.def_id()).with_reveal_all_normalized(tcx);
let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
for block in basic_blocks {
let terminator = block.terminator.as_mut().unwrap();
Expand Down Expand Up @@ -43,22 +44,36 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
sym::copy_nonoverlapping => {
let target = destination.unwrap().1;
let mut args = args.drain(..);
block.statements.push(Statement {
source_info: terminator.source_info,
kind: StatementKind::CopyNonOverlapping(
box rustc_middle::mir::CopyNonOverlapping {
src: args.next().unwrap(),
dst: args.next().unwrap(),
count: args.next().unwrap(),
},
),
});
let src = args.next().unwrap();
let dst = args.next().unwrap();
let count = args.next().unwrap();

assert_eq!(
args.next(),
None,
"Extra argument for copy_non_overlapping intrinsic"
);
drop(args);

let stmt = if let (Some(1), Some(src), Some(dst)) = (
eval_operand_to_usize(tcx, param_env, &count),
src.place(),
dst.place(),
) {
StatementKind::Assign(box (
tcx.mk_place_deref(dst),
Rvalue::Use(Operand::Copy(tcx.mk_place_deref(src))),
))
Comment on lines +63 to +66
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@bjorn3 bjorn3 Oct 20, 2021 •
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There was an interesting conversation on zulip around the semantics of copy_nonoverlapping. If it does an "untyped copy", lowering it to a regular assignment is not a sound optimization, so this probably needs to wait on a decision if copy_nonoverlapping does a typed or untyped copy. https://rust-lang.zulipchat.com/#narrow/stream/136281-t-lang.2Fwg-unsafe-code-guidelines/topic/copy_nonoverlapping.20and.20provenance/near/258410065

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Ah, thanks for digging this up, I knew this proposal was living somewhere... (and I think I was advocating for it, not realizing what equating copy_nonoverlapping and assignment would entail).

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If copy_nonoverlapping does an untyped copy does that mean that the implementation of ptr::read needs to change so that it does a typed read?

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@RalfJung RalfJung Oct 20, 2021 •
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It is correct to replace a typed by an untyped copy, just not the other way around.

So, read purporting to be typed but actually doing an untyped operation under the hood is fine. (Typedness is force on it anyway because the result is returned by-value at type T.)

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Sure, but that may pessimize the read. See #73258

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copy_nonoverlapping does an untyped copy as per #97712, so this lowering is off the table now.

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Should we close it then?

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It could be made an assignment at type MaybeUninit<T>.

} else {
StatementKind::CopyNonOverlapping(
box rustc_middle::mir::CopyNonOverlapping { src, dst, count },
)
};

block
.statements
.push(Statement { source_info: terminator.source_info, kind: stmt });

terminator.kind = TerminatorKind::Goto { target };
}
sym::wrapping_add | sym::wrapping_sub | sym::wrapping_mul => {
Expand Down Expand Up @@ -126,6 +141,15 @@ impl<'tcx> MirPass<'tcx> for LowerIntrinsics {
}
}

fn eval_operand_to_usize(
tcx: TyCtxt<'tcx>,
param_env: rustc_middle::ty::ParamEnv<'tcx>,
operand: &Operand<'tcx>,
) -> Option<u64> {
let constant = operand.constant()?;
Some(constant.literal.try_eval_usize(tcx, param_env)?)
}

fn resolve_rust_intrinsic(
tcx: TyCtxt<'tcx>,
func_ty: Ty<'tcx>,
Expand Down
111 changes: 111 additions & 0 deletions src/test/mir-opt/lower_intrinsics.copy_nonoverlapping.LowerIntrinsics.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,111 @@
- // MIR for `copy_nonoverlapping` before LowerIntrinsics
+ // MIR for `copy_nonoverlapping` after LowerIntrinsics

fn copy_nonoverlapping() -> () {
let mut _0: (); // return place in scope 0 at $DIR/lower_intrinsics.rs:76:30: 76:30
let _1: i32; // in scope 0 at $DIR/lower_intrinsics.rs:77:9: 77:12
let _4: (); // in scope 0 at $DIR/lower_intrinsics.rs:82:9: 82:88
let mut _5: *const i32; // in scope 0 at $DIR/lower_intrinsics.rs:82:46: 82:62
let mut _6: *const i32; // in scope 0 at $DIR/lower_intrinsics.rs:82:46: 82:62
let _7: &i32; // in scope 0 at $DIR/lower_intrinsics.rs:82:46: 82:50
let mut _8: *mut i32; // in scope 0 at $DIR/lower_intrinsics.rs:82:64: 82:84
let mut _9: *mut i32; // in scope 0 at $DIR/lower_intrinsics.rs:82:64: 82:84
let mut _10: &mut i32; // in scope 0 at $DIR/lower_intrinsics.rs:82:64: 82:74
let _11: (); // in scope 0 at $DIR/lower_intrinsics.rs:83:9: 83:88
let mut _12: *const i32; // in scope 0 at $DIR/lower_intrinsics.rs:83:46: 83:62
let mut _13: *const i32; // in scope 0 at $DIR/lower_intrinsics.rs:83:46: 83:62
let _14: &i32; // in scope 0 at $DIR/lower_intrinsics.rs:83:46: 83:50
let mut _15: *mut i32; // in scope 0 at $DIR/lower_intrinsics.rs:83:64: 83:84
let mut _16: *mut i32; // in scope 0 at $DIR/lower_intrinsics.rs:83:64: 83:84
let mut _17: &mut i32; // in scope 0 at $DIR/lower_intrinsics.rs:83:64: 83:74
scope 1 {
debug src => _1; // in scope 1 at $DIR/lower_intrinsics.rs:77:9: 77:12
let mut _2: i32; // in scope 1 at $DIR/lower_intrinsics.rs:78:9: 78:18
scope 2 {
debug dst_1 => _2; // in scope 2 at $DIR/lower_intrinsics.rs:78:9: 78:18
let mut _3: i32; // in scope 2 at $DIR/lower_intrinsics.rs:79:9: 79:18
scope 3 {
debug dst_2 => _3; // in scope 3 at $DIR/lower_intrinsics.rs:79:9: 79:18
scope 4 {
}
}
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/lower_intrinsics.rs:77:9: 77:12
_1 = const 42_i32; // scope 0 at $DIR/lower_intrinsics.rs:77:15: 77:17
StorageLive(_2); // scope 1 at $DIR/lower_intrinsics.rs:78:9: 78:18
_2 = const 1_i32; // scope 1 at $DIR/lower_intrinsics.rs:78:21: 78:22
StorageLive(_3); // scope 2 at $DIR/lower_intrinsics.rs:79:9: 79:18
_3 = const 2_i32; // scope 2 at $DIR/lower_intrinsics.rs:79:21: 79:22
StorageLive(_4); // scope 4 at $DIR/lower_intrinsics.rs:82:9: 82:88
StorageLive(_5); // scope 4 at $DIR/lower_intrinsics.rs:82:46: 82:62
StorageLive(_6); // scope 4 at $DIR/lower_intrinsics.rs:82:46: 82:62
StorageLive(_7); // scope 4 at $DIR/lower_intrinsics.rs:82:46: 82:50
_7 = &_1; // scope 4 at $DIR/lower_intrinsics.rs:82:46: 82:50
_6 = &raw const (*_7); // scope 4 at $DIR/lower_intrinsics.rs:82:46: 82:50
_5 = _6; // scope 4 at $DIR/lower_intrinsics.rs:82:46: 82:62
StorageLive(_8); // scope 4 at $DIR/lower_intrinsics.rs:82:64: 82:84
StorageLive(_9); // scope 4 at $DIR/lower_intrinsics.rs:82:64: 82:84
StorageLive(_10); // scope 4 at $DIR/lower_intrinsics.rs:82:64: 82:74
_10 = &mut _2; // scope 4 at $DIR/lower_intrinsics.rs:82:64: 82:74
_9 = &raw mut (*_10); // scope 4 at $DIR/lower_intrinsics.rs:82:64: 82:74
_8 = _9; // scope 4 at $DIR/lower_intrinsics.rs:82:64: 82:84
- _4 = std::intrinsics::copy_nonoverlapping::<i32>(move _5, move _8, const 1_usize) -> bb1; // scope 4 at $DIR/lower_intrinsics.rs:82:9: 82:88
- // mir::Constant
- // + span: $DIR/lower_intrinsics.rs:82:9: 82:45
- // + literal: Const { ty: unsafe extern "rust-intrinsic" fn(*const i32, *mut i32, usize) {std::intrinsics::copy_nonoverlapping::<i32>}, val: Value(Scalar(<ZST>)) }
+ (*_8) = (*_5); // scope 4 at $DIR/lower_intrinsics.rs:82:9: 82:88
+ goto -> bb1; // scope 4 at $DIR/lower_intrinsics.rs:82:9: 82:88
}

bb1: {
StorageDead(_8); // scope 4 at $DIR/lower_intrinsics.rs:82:87: 82:88
StorageDead(_5); // scope 4 at $DIR/lower_intrinsics.rs:82:87: 82:88
StorageDead(_10); // scope 4 at $DIR/lower_intrinsics.rs:82:88: 82:89
StorageDead(_9); // scope 4 at $DIR/lower_intrinsics.rs:82:88: 82:89
StorageDead(_7); // scope 4 at $DIR/lower_intrinsics.rs:82:88: 82:89
StorageDead(_6); // scope 4 at $DIR/lower_intrinsics.rs:82:88: 82:89
StorageDead(_4); // scope 4 at $DIR/lower_intrinsics.rs:82:88: 82:89
StorageLive(_11); // scope 4 at $DIR/lower_intrinsics.rs:83:9: 83:88
StorageLive(_12); // scope 4 at $DIR/lower_intrinsics.rs:83:46: 83:62
StorageLive(_13); // scope 4 at $DIR/lower_intrinsics.rs:83:46: 83:62
StorageLive(_14); // scope 4 at $DIR/lower_intrinsics.rs:83:46: 83:50
_14 = &_1; // scope 4 at $DIR/lower_intrinsics.rs:83:46: 83:50
_13 = &raw const (*_14); // scope 4 at $DIR/lower_intrinsics.rs:83:46: 83:50
_12 = _13; // scope 4 at $DIR/lower_intrinsics.rs:83:46: 83:62
StorageLive(_15); // scope 4 at $DIR/lower_intrinsics.rs:83:64: 83:84
StorageLive(_16); // scope 4 at $DIR/lower_intrinsics.rs:83:64: 83:84
StorageLive(_17); // scope 4 at $DIR/lower_intrinsics.rs:83:64: 83:74
_17 = &mut _3; // scope 4 at $DIR/lower_intrinsics.rs:83:64: 83:74
_16 = &raw mut (*_17); // scope 4 at $DIR/lower_intrinsics.rs:83:64: 83:74
_15 = _16; // scope 4 at $DIR/lower_intrinsics.rs:83:64: 83:84
- _11 = std::intrinsics::copy_nonoverlapping::<i32>(move _12, move _15, const 1_usize) -> bb2; // scope 4 at $DIR/lower_intrinsics.rs:83:9: 83:88
- // mir::Constant
- // + span: $DIR/lower_intrinsics.rs:83:9: 83:45
- // + literal: Const { ty: unsafe extern "rust-intrinsic" fn(*const i32, *mut i32, usize) {std::intrinsics::copy_nonoverlapping::<i32>}, val: Value(Scalar(<ZST>)) }
+ (*_15) = (*_12); // scope 4 at $DIR/lower_intrinsics.rs:83:9: 83:88
+ goto -> bb2; // scope 4 at $DIR/lower_intrinsics.rs:83:9: 83:88
}

bb2: {
StorageDead(_15); // scope 4 at $DIR/lower_intrinsics.rs:83:87: 83:88
StorageDead(_12); // scope 4 at $DIR/lower_intrinsics.rs:83:87: 83:88
StorageDead(_17); // scope 4 at $DIR/lower_intrinsics.rs:83:88: 83:89
StorageDead(_16); // scope 4 at $DIR/lower_intrinsics.rs:83:88: 83:89
StorageDead(_14); // scope 4 at $DIR/lower_intrinsics.rs:83:88: 83:89
StorageDead(_13); // scope 4 at $DIR/lower_intrinsics.rs:83:88: 83:89
StorageDead(_11); // scope 4 at $DIR/lower_intrinsics.rs:83:88: 83:89
_0 = const (); // scope 4 at $DIR/lower_intrinsics.rs:81:5: 84:6
StorageDead(_3); // scope 2 at $DIR/lower_intrinsics.rs:85:1: 85:2
StorageDead(_2); // scope 1 at $DIR/lower_intrinsics.rs:85:1: 85:2
StorageDead(_1); // scope 0 at $DIR/lower_intrinsics.rs:85:1: 85:2
return; // scope 0 at $DIR/lower_intrinsics.rs:85:2: 85:2
}

bb3 (cleanup): {
resume; // scope 0 at $DIR/lower_intrinsics.rs:76:1: 85:2
}
}

12 changes: 12 additions & 0 deletions src/test/mir-opt/lower_intrinsics.rs
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Expand Up @@ -71,3 +71,15 @@ pub fn discriminant<T>(t: T) {
core::intrinsics::discriminant_value(&());
core::intrinsics::discriminant_value(&E::B);
}

// EMIT_MIR lower_intrinsics.copy_nonoverlapping.LowerIntrinsics.diff
pub fn copy_nonoverlapping() {
let src = 42;
let mut dst_1 = 1;
let mut dst_2 = 2;

unsafe {
std::intrinsics::copy_nonoverlapping(&src as *const _, &mut dst_1 as *mut _, 1);
std::intrinsics::copy_nonoverlapping(&src as *const _, &mut dst_2 as *mut _, 1);
}
}