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Auto merge of rust-lang#111568 - scottmcm:undo-opt, r=WaffleLapkin
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Stop turning transmutes into discriminant reads in mir-opt

Partially reverts rust-lang#109612, as after rust-lang#109993 these aren't actually equivalent any more, and I'm no longer confident this was ever an improvement in the first place.

Having this "simplification" meant that similar-looking code actually did somewhat different things.  For example,
```rust
pub unsafe fn demo1(x: std::cmp::Ordering) -> u8 {
    std::mem::transmute(x)
}
pub unsafe fn demo2(x: std::cmp::Ordering) -> i8 {
    std::mem::transmute(x)
}
```
in nightly today is generating <https://rust.godbolt.org/z/dPK58zW18>
```llvm
define noundef i8 `@_ZN7example5demo117h341ef313673d2ee6E(i8` noundef %x) unnamed_addr #0 {
  %0 = icmp uge i8 %x, -1
  %1 = icmp ule i8 %x, 1
  %2 = or i1 %0, %1
  call void `@llvm.assume(i1` %2)
  ret i8 %x
}

define noundef i8 `@_ZN7example5demo217h5ad29f361a3f5700E(i8` noundef %0) unnamed_addr #0 {
  %x = alloca i8, align 1
  store i8 %0, ptr %x, align 1
  %1 = load i8, ptr %x, align 1, !range !2, !noundef !3
  ret i8 %1
}
```

Which feels too different when the original code is essentially identical.

---

Aside: that example is different *after* optimizations too:
```llvm
define noundef i8 `@_ZN7example5demo117h341ef313673d2ee6E(i8` noundef returned %x) unnamed_addr #0 {
  %0 = add i8 %x, 1
  %1 = icmp ult i8 %0, 3
  tail call void `@llvm.assume(i1` %1)
  ret i8 %x
}

define noundef i8 `@_ZN7example5demo217h5ad29f361a3f5700E(i8` noundef returned %0) unnamed_addr #1 {
  ret i8 %0
}
```
so turning the `Transmute` into a `Discriminant` was arguably just making things worse, so leaving it alone instead -- and thus having less code in rustc -- seems clearly better.
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@bors
bors committed May 17, 2023
2 parents ad23942 + 363c202 commit e9e1bbc
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Showing 3 changed files with 49 additions and 152 deletions.
13 changes: 0 additions & 13 deletions compiler/rustc_mir_transform/src/instsimplify.rs
View file
Original file line number Diff line number Diff line change
Expand Up @@ -5,7 +5,6 @@ use crate::MirPass;
use rustc_hir::Mutability;
use rustc_middle::mir::*;
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;
use rustc_target::abi::FieldIdx;
Expand Down Expand Up @@ -163,18 +162,6 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
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()
Expand Down
166 changes: 49 additions & 117 deletions tests/mir-opt/combine_transmutes.adt_transmutes.InstSimplify.diff
View file
Original file line number Diff line number Diff line change
Expand Up @@ -4,59 +4,29 @@
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
let mut _2: std::option::Option<std::num::NonZeroU8>; // in scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
let mut _4: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+2:29: +2:54
let mut _6: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+3:29: +3:54
let mut _8: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+4:29: +4:47
let mut _10: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+5:29: +5:47
let mut _12: std::mem::MaybeUninit<std::string::String>; // in scope 0 at $DIR/combine_transmutes.rs:+6:46: +6:77
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
let _3: i16; // 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
let _5: u16; // 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
let _7: u32; // 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
let _9: i32; // 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
let _11: std::mem::ManuallyDrop<std::string::String>; // 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
}
}
}
}
}
}
}
}
}
Expand All @@ -66,93 +36,55 @@

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(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
_2 = Option::<NonZeroU8>::Some(const _); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
// mir::Constant
// + span: $DIR/combine_transmutes.rs:35:33: 35:57
// + literal: Const { ty: NonZeroU8, val: Unevaluated(NonZeroU8::MAX, [], None) }
_1 = move _2 as u8 (Transmute); // scope 0 at $DIR/combine_transmutes.rs:+1:18: +1:59
StorageDead(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:58: +1:59
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(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:29: +2:54
_4 = Wrapping::<i16>(const 0_i16); // scope 1 at $DIR/combine_transmutes.rs:+2:29: +2:54
- _3 = move _4 as i16 (Transmute); // scope 1 at $DIR/combine_transmutes.rs:+2:19: +2:55
+ _3 = move (_4.0: i16); // scope 1 at $DIR/combine_transmutes.rs:+2:19: +2:55
StorageDead(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:54: +2:55
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(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:29: +3:54
_6 = Wrapping::<i16>(const 0_i16); // scope 2 at $DIR/combine_transmutes.rs:+3:29: +3:54
_5 = move _6 as u16 (Transmute); // scope 2 at $DIR/combine_transmutes.rs:+3:19: +3:55
StorageDead(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:54: +3:55
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
StorageLive(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:29: +4:47
_8 = Union32 { u32: const 0_i32 }; // scope 3 at $DIR/combine_transmutes.rs:+4:29: +4:47
_7 = move _8 as u32 (Transmute); // scope 3 at $DIR/combine_transmutes.rs:+4:19: +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(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:29: +5:47
_10 = Union32 { u32: const 0_u32 }; // scope 4 at $DIR/combine_transmutes.rs:+5:29: +5:47
_9 = move _10 as i32 (Transmute); // scope 4 at $DIR/combine_transmutes.rs:+5:19: +5:48
StorageDead(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:47: +5:48
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() -> [return: bb1, unwind unreachable]; // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
StorageLive(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:46: +6:77
_12 = MaybeUninit::<String>::uninit() -> [return: bb1, unwind unreachable]; // scope 5 at $DIR/combine_transmutes.rs:+6:46: +6:77
// mir::Constant
// + span: $DIR/combine_transmutes.rs:46:46: 46:75
// + user_ty: UserType(23)
// + span: $DIR/combine_transmutes.rs:40:46: 40:75
// + user_ty: UserType(11)
// + 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
- _11 = move _12 as std::mem::ManuallyDrop<std::string::String> (Transmute); // scope 5 at $DIR/combine_transmutes.rs:+6:36: +6:78
+ _11 = move (_12.1: std::mem::ManuallyDrop<std::string::String>); // scope 5 at $DIR/combine_transmutes.rs:+6:36: +6:78
StorageDead(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:77: +6:78
_0 = const (); // scope 0 at $DIR/combine_transmutes.rs:+0:32: +7:2
StorageDead(_11); // scope 5 at $DIR/combine_transmutes.rs:+7:1: +7:2
StorageDead(_9); // scope 4 at $DIR/combine_transmutes.rs:+7:1: +7:2
StorageDead(_7); // scope 3 at $DIR/combine_transmutes.rs:+7:1: +7:2
StorageDead(_5); // scope 2 at $DIR/combine_transmutes.rs:+7:1: +7:2
StorageDead(_3); // scope 1 at $DIR/combine_transmutes.rs:+7:1: +7:2
StorageDead(_1); // scope 0 at $DIR/combine_transmutes.rs:+7:1: +7:2
return; // scope 0 at $DIR/combine_transmutes.rs:+7:2: +7:2
}
}

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