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Simplify transmutes in MIR InstCombine
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Thanks to the combination of rust-lang#108246 and rust-lang#108442 it could already remove identity transmutes.

With this PR, it can also simplify them to `IntToInt` casts, `Discriminant` reads, or `Field` projections.
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@scottmcm
scottmcm committed Mar 29, 2023
1 parent 478cbb4 commit f20af8d
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54 changes: 50 additions & 4 deletions compiler/rustc_mir_transform/src/instcombine.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -3,10 +3,12 @@
use crate::MirPass;
use rustc_hir::Mutability;
use rustc_middle::mir::{
BinOp, Body, Constant, ConstantKind, LocalDecls, Operand, Place, ProjectionElem, Rvalue,
SourceInfo, Statement, StatementKind, SwitchTargets, Terminator, TerminatorKind, UnOp,
BinOp, Body, CastKind, Constant, ConstantKind, Field, LocalDecls, Operand, Place,
ProjectionElem, Rvalue, SourceInfo, Statement, StatementKind, SwitchTargets, Terminator,
TerminatorKind, UnOp,
};
use rustc_middle::ty::layout::ValidityRequirement;
use rustc_middle::ty::util::IntTypeExt;
use rustc_middle::ty::{self, ParamEnv, SubstsRef, Ty, TyCtxt};
use rustc_span::symbol::Symbol;

Expand Down Expand Up @@ -145,9 +147,53 @@ impl<'tcx> InstCombineContext<'tcx, '_> {
}

fn combine_cast(&self, _source_info: &SourceInfo, rvalue: &mut Rvalue<'tcx>) {
if let Rvalue::Cast(_kind, operand, ty) = rvalue {
if operand.ty(self.local_decls, self.tcx) == *ty {
if let Rvalue::Cast(kind, operand, cast_ty) = rvalue {
let operand_ty = operand.ty(self.local_decls, self.tcx);
if operand_ty == *cast_ty {
*rvalue = Rvalue::Use(operand.clone());
} else if *kind == CastKind::Transmute {
// Transmuting an integer to another integer is just a signedness cast
if let (ty::Int(int), ty::Uint(uint)) | (ty::Uint(uint), ty::Int(int)) = (operand_ty.kind(), cast_ty.kind())
&& int.bit_width() == uint.bit_width()
{
// The width check isn't strictly necessary, as different widths
// are UB and thus we'd be allowed to turn it into a cast anyway.
// But let's keep the UB around for codegen to exploit later.
// (If `CastKind::Transmute` ever becomes *not* UB for mismatched sizes,
// then the width check is necessary for big-endian correctness.)
*kind = CastKind::IntToInt;
return;
}

// Transmuting a fieldless enum to its repr is a discriminant read
if let ty::Adt(adt_def, ..) = operand_ty.kind()
&& adt_def.is_enum()
&& adt_def.is_payloadfree()
&& let Some(place) = operand.place()
&& let Some(repr_int) = adt_def.repr().int
&& repr_int.to_ty(self.tcx) == *cast_ty
{
*rvalue = Rvalue::Discriminant(place);
return;
}

// Transmuting a transparent struct/union to a field's type is a projection
if let ty::Adt(adt_def, substs) = operand_ty.kind()
&& adt_def.repr().transparent()
&& (adt_def.is_struct() || adt_def.is_union())
&& let Some(place) = operand.place()
{
let variant = adt_def.non_enum_variant();
for (i, field) in variant.fields.iter().enumerate() {
let field_ty = field.ty(self.tcx, substs);
if field_ty == *cast_ty {
let place = place.project_deeper(&[ProjectionElem::Field(Field::from_usize(i), *cast_ty)], self.tcx);
let operand = if operand.is_move() { Operand::Move(place) } else { Operand::Copy(place) };
*rvalue = Rvalue::Use(operand);
return;
}
}
}
}
}
}
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158 changes: 158 additions & 0 deletions tests/mir-opt/combine_transmutes.adt_transmutes.InstCombine.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,158 @@
- // MIR for `adt_transmutes` before InstCombine
+ // MIR for `adt_transmutes` after InstCombine

fn adt_transmutes() -> () {
let mut _0: (); // return place in scope 0 at $DIR/combine_transmutes.rs:+0:32: +0:32
let _1: u8; // in scope 0 at $DIR/combine_transmutes.rs:+1:9: +1:11
let mut _2: EnumNoRepr; // in scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
let mut _4: EnumNoRepr; // in scope 0 at $DIR/combine_transmutes.rs:+2:28: +2:41
let mut _6: EnumReprIsize; // in scope 0 at $DIR/combine_transmutes.rs:+3:31: +3:47
let mut _8: EnumReprIsize; // in scope 0 at $DIR/combine_transmutes.rs:+4:31: +4:47
let mut _10: std::cmp::Ordering; // in scope 0 at $DIR/combine_transmutes.rs:+5:28: +5:52
let mut _12: std::cmp::Ordering; // in scope 0 at $DIR/combine_transmutes.rs:+6:28: +6:52
let mut _14: std::option::Option<std::num::NonZeroU8>; // in scope 0 at $DIR/combine_transmutes.rs:+7:28: +7:58
let mut _16: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+8:29: +8:54
let mut _18: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+9:29: +9:54
let mut _20: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+10:29: +10:47
let mut _22: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+11:29: +11:47
let mut _24: std::mem::MaybeUninit<std::string::String>; // in scope 0 at $DIR/combine_transmutes.rs:+12:46: +12:77
scope 1 {
debug _a => _1; // in scope 1 at $DIR/combine_transmutes.rs:+1:9: +1:11
let _3: i8; // in scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
scope 2 {
debug _a => _3; // in scope 2 at $DIR/combine_transmutes.rs:+2:9: +2:11
let _5: usize; // in scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
scope 3 {
debug _a => _5; // in scope 3 at $DIR/combine_transmutes.rs:+3:9: +3:11
let _7: isize; // in scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
scope 4 {
debug _a => _7; // in scope 4 at $DIR/combine_transmutes.rs:+4:9: +4:11
let _9: u8; // in scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
scope 5 {
debug _a => _9; // in scope 5 at $DIR/combine_transmutes.rs:+5:9: +5:11
let _11: i8; // in scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
scope 6 {
debug _a => _11; // in scope 6 at $DIR/combine_transmutes.rs:+6:9: +6:11
let _13: u8; // in scope 6 at $DIR/combine_transmutes.rs:+7:9: +7:11
scope 7 {
debug _a => _13; // in scope 7 at $DIR/combine_transmutes.rs:+7:9: +7:11
let _15: i16; // in scope 7 at $DIR/combine_transmutes.rs:+8:9: +8:11
scope 8 {
debug _a => _15; // in scope 8 at $DIR/combine_transmutes.rs:+8:9: +8:11
let _17: u16; // in scope 8 at $DIR/combine_transmutes.rs:+9:9: +9:11
scope 9 {
debug _a => _17; // in scope 9 at $DIR/combine_transmutes.rs:+9:9: +9:11
let _19: u32; // in scope 9 at $DIR/combine_transmutes.rs:+10:9: +10:11
scope 10 {
debug _a => _19; // in scope 10 at $DIR/combine_transmutes.rs:+10:9: +10:11
let _21: i32; // in scope 10 at $DIR/combine_transmutes.rs:+11:9: +11:11
scope 11 {
debug _a => _21; // in scope 11 at $DIR/combine_transmutes.rs:+11:9: +11:11
let _23: std::mem::ManuallyDrop<std::string::String>; // in scope 11 at $DIR/combine_transmutes.rs:+12:9: +12:11
scope 12 {
debug _a => _23; // in scope 12 at $DIR/combine_transmutes.rs:+12:9: +12:11
}
}
}
}
}
}
}
}
}
}
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/combine_transmutes.rs:+1:9: +1:11
StorageLive(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
_2 = EnumNoRepr::A; // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
_1 = move _2 as u8 (Transmute); // scope 0 at $DIR/combine_transmutes.rs:+1:18: +1:42
StorageDead(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:41: +1:42
StorageLive(_3); // scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
StorageLive(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:28: +2:41
_4 = EnumNoRepr::B; // scope 1 at $DIR/combine_transmutes.rs:+2:28: +2:41
_3 = move _4 as i8 (Transmute); // scope 1 at $DIR/combine_transmutes.rs:+2:18: +2:42
StorageDead(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:41: +2:42
StorageLive(_5); // scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
StorageLive(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:31: +3:47
_6 = EnumReprIsize::A; // scope 2 at $DIR/combine_transmutes.rs:+3:31: +3:47
_5 = move _6 as usize (Transmute); // scope 2 at $DIR/combine_transmutes.rs:+3:21: +3:48
StorageDead(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:47: +3:48
StorageLive(_7); // scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
StorageLive(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:31: +4:47
_8 = EnumReprIsize::B; // scope 3 at $DIR/combine_transmutes.rs:+4:31: +4:47
- _7 = move _8 as isize (Transmute); // scope 3 at $DIR/combine_transmutes.rs:+4:21: +4:48
+ _7 = discriminant(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:21: +4:48
StorageDead(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:47: +4:48
StorageLive(_9); // scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
StorageLive(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:28: +5:52
_10 = Less; // scope 4 at $DIR/combine_transmutes.rs:+5:28: +5:52
_9 = move _10 as u8 (Transmute); // scope 4 at $DIR/combine_transmutes.rs:+5:18: +5:53
StorageDead(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:52: +5:53
StorageLive(_11); // scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
StorageLive(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:28: +6:52
_12 = Less; // scope 5 at $DIR/combine_transmutes.rs:+6:28: +6:52
- _11 = move _12 as i8 (Transmute); // scope 5 at $DIR/combine_transmutes.rs:+6:18: +6:53
+ _11 = discriminant(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:18: +6:53
StorageDead(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:52: +6:53
StorageLive(_13); // scope 6 at $DIR/combine_transmutes.rs:+7:9: +7:11
StorageLive(_14); // scope 6 at $DIR/combine_transmutes.rs:+7:28: +7:58
_14 = Option::<NonZeroU8>::Some(const _); // scope 6 at $DIR/combine_transmutes.rs:+7:28: +7:58
// mir::Constant
// + span: $DIR/combine_transmutes.rs:41:33: 41:57
// + literal: Const { ty: NonZeroU8, val: Unevaluated(NonZeroU8::MAX, [], None) }
_13 = move _14 as u8 (Transmute); // scope 6 at $DIR/combine_transmutes.rs:+7:18: +7:59
StorageDead(_14); // scope 6 at $DIR/combine_transmutes.rs:+7:58: +7:59
StorageLive(_15); // scope 7 at $DIR/combine_transmutes.rs:+8:9: +8:11
StorageLive(_16); // scope 7 at $DIR/combine_transmutes.rs:+8:29: +8:54
_16 = Wrapping::<i16>(const 0_i16); // scope 7 at $DIR/combine_transmutes.rs:+8:29: +8:54
- _15 = move _16 as i16 (Transmute); // scope 7 at $DIR/combine_transmutes.rs:+8:19: +8:55
+ _15 = move (_16.0: i16); // scope 7 at $DIR/combine_transmutes.rs:+8:19: +8:55
StorageDead(_16); // scope 7 at $DIR/combine_transmutes.rs:+8:54: +8:55
StorageLive(_17); // scope 8 at $DIR/combine_transmutes.rs:+9:9: +9:11
StorageLive(_18); // scope 8 at $DIR/combine_transmutes.rs:+9:29: +9:54
_18 = Wrapping::<i16>(const 0_i16); // scope 8 at $DIR/combine_transmutes.rs:+9:29: +9:54
_17 = move _18 as u16 (Transmute); // scope 8 at $DIR/combine_transmutes.rs:+9:19: +9:55
StorageDead(_18); // scope 8 at $DIR/combine_transmutes.rs:+9:54: +9:55
StorageLive(_19); // scope 9 at $DIR/combine_transmutes.rs:+10:9: +10:11
StorageLive(_20); // scope 9 at $DIR/combine_transmutes.rs:+10:29: +10:47
_20 = Union32 { u32: const 0_i32 }; // scope 9 at $DIR/combine_transmutes.rs:+10:29: +10:47
_19 = move _20 as u32 (Transmute); // scope 9 at $DIR/combine_transmutes.rs:+10:19: +10:48
StorageDead(_20); // scope 9 at $DIR/combine_transmutes.rs:+10:47: +10:48
StorageLive(_21); // scope 10 at $DIR/combine_transmutes.rs:+11:9: +11:11
StorageLive(_22); // scope 10 at $DIR/combine_transmutes.rs:+11:29: +11:47
_22 = Union32 { u32: const 0_u32 }; // scope 10 at $DIR/combine_transmutes.rs:+11:29: +11:47
_21 = move _22 as i32 (Transmute); // scope 10 at $DIR/combine_transmutes.rs:+11:19: +11:48
StorageDead(_22); // scope 10 at $DIR/combine_transmutes.rs:+11:47: +11:48
StorageLive(_23); // scope 11 at $DIR/combine_transmutes.rs:+12:9: +12:11
StorageLive(_24); // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
_24 = MaybeUninit::<String>::uninit() -> bb1; // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
// mir::Constant
// + span: $DIR/combine_transmutes.rs:46:46: 46:75
// + user_ty: UserType(23)
// + literal: Const { ty: fn() -> MaybeUninit<String> {MaybeUninit::<String>::uninit}, val: Value(<ZST>) }
}

bb1: {
- _23 = move _24 as std::mem::ManuallyDrop<std::string::String> (Transmute); // scope 11 at $DIR/combine_transmutes.rs:+12:36: +12:78
+ _23 = move (_24.1: std::mem::ManuallyDrop<std::string::String>); // scope 11 at $DIR/combine_transmutes.rs:+12:36: +12:78
StorageDead(_24); // scope 11 at $DIR/combine_transmutes.rs:+12:77: +12:78
_0 = const (); // scope 0 at $DIR/combine_transmutes.rs:+0:32: +13:2
StorageDead(_23); // scope 11 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_21); // scope 10 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_19); // scope 9 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_17); // scope 8 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_15); // scope 7 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_13); // scope 6 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_11); // scope 5 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_9); // scope 4 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_7); // scope 3 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_5); // scope 2 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_3); // scope 1 at $DIR/combine_transmutes.rs:+13:1: +13:2
StorageDead(_1); // scope 0 at $DIR/combine_transmutes.rs:+13:1: +13:2
return; // scope 0 at $DIR/combine_transmutes.rs:+13:2: +13:2
}
}

43 changes: 43 additions & 0 deletions tests/mir-opt/combine_transmutes.identity_transmutes.InstCombine.diff
View file
Original file line number Diff line number Diff line change
@@ -0,0 +1,43 @@
- // MIR for `identity_transmutes` before InstCombine
+ // MIR for `identity_transmutes` after InstCombine

fn identity_transmutes() -> () {
let mut _0: (); // return place in scope 0 at $DIR/combine_transmutes.rs:+0:37: +0:37
let _1: i32; // in scope 0 at $DIR/combine_transmutes.rs:+2:9: +2:11
let mut _3: std::vec::Vec<i32>; // in scope 0 at $DIR/combine_transmutes.rs:+3:46: +3:56
scope 1 {
debug _a => _1; // in scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
let _2: std::vec::Vec<i32>; // in scope 1 at $DIR/combine_transmutes.rs:+3:9: +3:11
scope 2 {
debug _a => _2; // in scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
}
}

bb0: {
StorageLive(_1); // scope 0 at $DIR/combine_transmutes.rs:+2:9: +2:11
- _1 = const 1_i32 as i32 (Transmute); // scope 0 at $DIR/combine_transmutes.rs:+2:14: +2:38
+ _1 = const 1_i32; // scope 0 at $DIR/combine_transmutes.rs:+2:14: +2:38
StorageLive(_2); // scope 1 at $DIR/combine_transmutes.rs:+3:9: +3:11
StorageLive(_3); // scope 1 at $DIR/combine_transmutes.rs:+3:46: +3:56
_3 = Vec::<i32>::new() -> bb1; // scope 1 at $DIR/combine_transmutes.rs:+3:46: +3:56
// mir::Constant
// + span: $DIR/combine_transmutes.rs:15:46: 15:54
// + user_ty: UserType(0)
// + literal: Const { ty: fn() -> Vec<i32> {Vec::<i32>::new}, val: Value(<ZST>) }
}

bb1: {
- _2 = move _3 as std::vec::Vec<i32> (Transmute); // scope 1 at $DIR/combine_transmutes.rs:+3:14: +3:57
+ _2 = move _3; // scope 1 at $DIR/combine_transmutes.rs:+3:14: +3:57
StorageDead(_3); // scope 1 at $DIR/combine_transmutes.rs:+3:56: +3:57
_0 = const (); // scope 0 at $DIR/combine_transmutes.rs:+0:37: +4:2
drop(_2) -> bb2; // scope 1 at $DIR/combine_transmutes.rs:+4:1: +4:2
}

bb2: {
StorageDead(_2); // scope 1 at $DIR/combine_transmutes.rs:+4:1: +4:2
StorageDead(_1); // scope 0 at $DIR/combine_transmutes.rs:+4:1: +4:2
return; // scope 0 at $DIR/combine_transmutes.rs:+4:2: +4:2
}
}

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