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Auto merge of rust-lang#111568 - scottmcm:undo-opt, r=WaffleLapkin
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.
2 parents ad23942 + 363c202 commit e9e1bbc

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‎compiler/rustc_mir_transform/src/instsimplify.rs‎

Lines changed: 0 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -5,7 +5,6 @@ use crate::MirPass;
55
use rustc_hir::Mutability;
66
use rustc_middle::mir::*;
77
use rustc_middle::ty::layout::ValidityRequirement;
8-
use rustc_middle::ty::util::IntTypeExt;
98
use rustc_middle::ty::{self, ParamEnv, SubstsRef, Ty, TyCtxt};
109
use rustc_span::symbol::Symbol;
1110
use rustc_target::abi::FieldIdx;
@@ -163,18 +162,6 @@ impl<'tcx> InstSimplifyContext<'tcx, '_> {
163162
return;
164163
}
165164

166-
// Transmuting a fieldless enum to its repr is a discriminant read
167-
if let ty::Adt(adt_def, ..) = operand_ty.kind()
168-
&& adt_def.is_enum()
169-
&& adt_def.is_payloadfree()
170-
&& let Some(place) = operand.place()
171-
&& let Some(repr_int) = adt_def.repr().int
172-
&& repr_int.to_ty(self.tcx) == *cast_ty
173-
{
174-
*rvalue = Rvalue::Discriminant(place);
175-
return;
176-
}
177-
178165
// Transmuting a transparent struct/union to a field's type is a projection
179166
if let ty::Adt(adt_def, substs) = operand_ty.kind()
180167
&& adt_def.repr().transparent()

‎tests/mir-opt/combine_transmutes.adt_transmutes.InstSimplify.diff‎

Lines changed: 49 additions & 117 deletions
Original file line numberDiff line numberDiff line change
@@ -4,59 +4,29 @@
44
fn adt_transmutes() -> () {
55
let mut _0: (); // return place in scope 0 at $DIR/combine_transmutes.rs:+0:32: +0:32
66
let _1: u8; // in scope 0 at $DIR/combine_transmutes.rs:+1:9: +1:11
7-
let mut _2: EnumNoRepr; // in scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
8-
let mut _4: EnumNoRepr; // in scope 0 at $DIR/combine_transmutes.rs:+2:28: +2:41
9-
let mut _6: EnumReprIsize; // in scope 0 at $DIR/combine_transmutes.rs:+3:31: +3:47
10-
let mut _8: EnumReprIsize; // in scope 0 at $DIR/combine_transmutes.rs:+4:31: +4:47
11-
let mut _10: std::cmp::Ordering; // in scope 0 at $DIR/combine_transmutes.rs:+5:28: +5:52
12-
let mut _12: std::cmp::Ordering; // in scope 0 at $DIR/combine_transmutes.rs:+6:28: +6:52
13-
let mut _14: std::option::Option<std::num::NonZeroU8>; // in scope 0 at $DIR/combine_transmutes.rs:+7:28: +7:58
14-
let mut _16: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+8:29: +8:54
15-
let mut _18: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+9:29: +9:54
16-
let mut _20: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+10:29: +10:47
17-
let mut _22: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+11:29: +11:47
18-
let mut _24: std::mem::MaybeUninit<std::string::String>; // in scope 0 at $DIR/combine_transmutes.rs:+12:46: +12:77
7+
let mut _2: std::option::Option<std::num::NonZeroU8>; // in scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
8+
let mut _4: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+2:29: +2:54
9+
let mut _6: std::num::Wrapping<i16>; // in scope 0 at $DIR/combine_transmutes.rs:+3:29: +3:54
10+
let mut _8: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+4:29: +4:47
11+
let mut _10: Union32; // in scope 0 at $DIR/combine_transmutes.rs:+5:29: +5:47
12+
let mut _12: std::mem::MaybeUninit<std::string::String>; // in scope 0 at $DIR/combine_transmutes.rs:+6:46: +6:77
1913
scope 1 {
2014
debug _a => _1; // in scope 1 at $DIR/combine_transmutes.rs:+1:9: +1:11
21-
let _3: i8; // in scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
15+
let _3: i16; // in scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
2216
scope 2 {
2317
debug _a => _3; // in scope 2 at $DIR/combine_transmutes.rs:+2:9: +2:11
24-
let _5: usize; // in scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
18+
let _5: u16; // in scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
2519
scope 3 {
2620
debug _a => _5; // in scope 3 at $DIR/combine_transmutes.rs:+3:9: +3:11
27-
let _7: isize; // in scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
21+
let _7: u32; // in scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
2822
scope 4 {
2923
debug _a => _7; // in scope 4 at $DIR/combine_transmutes.rs:+4:9: +4:11
30-
let _9: u8; // in scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
24+
let _9: i32; // in scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
3125
scope 5 {
3226
debug _a => _9; // in scope 5 at $DIR/combine_transmutes.rs:+5:9: +5:11
33-
let _11: i8; // in scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
27+
let _11: std::mem::ManuallyDrop<std::string::String>; // in scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
3428
scope 6 {
3529
debug _a => _11; // in scope 6 at $DIR/combine_transmutes.rs:+6:9: +6:11
36-
let _13: u8; // in scope 6 at $DIR/combine_transmutes.rs:+7:9: +7:11
37-
scope 7 {
38-
debug _a => _13; // in scope 7 at $DIR/combine_transmutes.rs:+7:9: +7:11
39-
let _15: i16; // in scope 7 at $DIR/combine_transmutes.rs:+8:9: +8:11
40-
scope 8 {
41-
debug _a => _15; // in scope 8 at $DIR/combine_transmutes.rs:+8:9: +8:11
42-
let _17: u16; // in scope 8 at $DIR/combine_transmutes.rs:+9:9: +9:11
43-
scope 9 {
44-
debug _a => _17; // in scope 9 at $DIR/combine_transmutes.rs:+9:9: +9:11
45-
let _19: u32; // in scope 9 at $DIR/combine_transmutes.rs:+10:9: +10:11
46-
scope 10 {
47-
debug _a => _19; // in scope 10 at $DIR/combine_transmutes.rs:+10:9: +10:11
48-
let _21: i32; // in scope 10 at $DIR/combine_transmutes.rs:+11:9: +11:11
49-
scope 11 {
50-
debug _a => _21; // in scope 11 at $DIR/combine_transmutes.rs:+11:9: +11:11
51-
let _23: std::mem::ManuallyDrop<std::string::String>; // in scope 11 at $DIR/combine_transmutes.rs:+12:9: +12:11
52-
scope 12 {
53-
debug _a => _23; // in scope 12 at $DIR/combine_transmutes.rs:+12:9: +12:11
54-
}
55-
}
56-
}
57-
}
58-
}
59-
}
6030
}
6131
}
6232
}
@@ -66,93 +36,55 @@
6636

6737
bb0: {
6838
StorageLive(_1); // scope 0 at $DIR/combine_transmutes.rs:+1:9: +1:11
69-
StorageLive(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
70-
_2 = EnumNoRepr::A; // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:41
71-
_1 = move _2 as u8 (Transmute); // scope 0 at $DIR/combine_transmutes.rs:+1:18: +1:42
72-
StorageDead(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:41: +1:42
39+
StorageLive(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
40+
_2 = Option::<NonZeroU8>::Some(const _); // scope 0 at $DIR/combine_transmutes.rs:+1:28: +1:58
41+
// mir::Constant
42+
// + span: $DIR/combine_transmutes.rs:35:33: 35:57
43+
// + literal: Const { ty: NonZeroU8, val: Unevaluated(NonZeroU8::MAX, [], None) }
44+
_1 = move _2 as u8 (Transmute); // scope 0 at $DIR/combine_transmutes.rs:+1:18: +1:59
45+
StorageDead(_2); // scope 0 at $DIR/combine_transmutes.rs:+1:58: +1:59
7346
StorageLive(_3); // scope 1 at $DIR/combine_transmutes.rs:+2:9: +2:11
74-
StorageLive(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:28: +2:41
75-
_4 = EnumNoRepr::B; // scope 1 at $DIR/combine_transmutes.rs:+2:28: +2:41
76-
_3 = move _4 as i8 (Transmute); // scope 1 at $DIR/combine_transmutes.rs:+2:18: +2:42
77-
StorageDead(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:41: +2:42
47+
StorageLive(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:29: +2:54
48+
_4 = Wrapping::<i16>(const 0_i16); // scope 1 at $DIR/combine_transmutes.rs:+2:29: +2:54
49+
- _3 = move _4 as i16 (Transmute); // scope 1 at $DIR/combine_transmutes.rs:+2:19: +2:55
50+
+ _3 = move (_4.0: i16); // scope 1 at $DIR/combine_transmutes.rs:+2:19: +2:55
51+
StorageDead(_4); // scope 1 at $DIR/combine_transmutes.rs:+2:54: +2:55
7852
StorageLive(_5); // scope 2 at $DIR/combine_transmutes.rs:+3:9: +3:11
79-
StorageLive(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:31: +3:47
80-
_6 = EnumReprIsize::A; // scope 2 at $DIR/combine_transmutes.rs:+3:31: +3:47
81-
_5 = move _6 as usize (Transmute); // scope 2 at $DIR/combine_transmutes.rs:+3:21: +3:48
82-
StorageDead(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:47: +3:48
53+
StorageLive(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:29: +3:54
54+
_6 = Wrapping::<i16>(const 0_i16); // scope 2 at $DIR/combine_transmutes.rs:+3:29: +3:54
55+
_5 = move _6 as u16 (Transmute); // scope 2 at $DIR/combine_transmutes.rs:+3:19: +3:55
56+
StorageDead(_6); // scope 2 at $DIR/combine_transmutes.rs:+3:54: +3:55
8357
StorageLive(_7); // scope 3 at $DIR/combine_transmutes.rs:+4:9: +4:11
84-
StorageLive(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:31: +4:47
85-
_8 = EnumReprIsize::B; // scope 3 at $DIR/combine_transmutes.rs:+4:31: +4:47
86-
- _7 = move _8 as isize (Transmute); // scope 3 at $DIR/combine_transmutes.rs:+4:21: +4:48
87-
+ _7 = discriminant(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:21: +4:48
58+
StorageLive(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:29: +4:47
59+
_8 = Union32 { u32: const 0_i32 }; // scope 3 at $DIR/combine_transmutes.rs:+4:29: +4:47
60+
_7 = move _8 as u32 (Transmute); // scope 3 at $DIR/combine_transmutes.rs:+4:19: +4:48
8861
StorageDead(_8); // scope 3 at $DIR/combine_transmutes.rs:+4:47: +4:48
8962
StorageLive(_9); // scope 4 at $DIR/combine_transmutes.rs:+5:9: +5:11
90-
StorageLive(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:28: +5:52
91-
_10 = Less; // scope 4 at $DIR/combine_transmutes.rs:+5:28: +5:52
92-
_9 = move _10 as u8 (Transmute); // scope 4 at $DIR/combine_transmutes.rs:+5:18: +5:53
93-
StorageDead(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:52: +5:53
63+
StorageLive(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:29: +5:47
64+
_10 = Union32 { u32: const 0_u32 }; // scope 4 at $DIR/combine_transmutes.rs:+5:29: +5:47
65+
_9 = move _10 as i32 (Transmute); // scope 4 at $DIR/combine_transmutes.rs:+5:19: +5:48
66+
StorageDead(_10); // scope 4 at $DIR/combine_transmutes.rs:+5:47: +5:48
9467
StorageLive(_11); // scope 5 at $DIR/combine_transmutes.rs:+6:9: +6:11
95-
StorageLive(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:28: +6:52
96-
_12 = Less; // scope 5 at $DIR/combine_transmutes.rs:+6:28: +6:52
97-
- _11 = move _12 as i8 (Transmute); // scope 5 at $DIR/combine_transmutes.rs:+6:18: +6:53
98-
+ _11 = discriminant(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:18: +6:53
99-
StorageDead(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:52: +6:53
100-
StorageLive(_13); // scope 6 at $DIR/combine_transmutes.rs:+7:9: +7:11
101-
StorageLive(_14); // scope 6 at $DIR/combine_transmutes.rs:+7:28: +7:58
102-
_14 = Option::<NonZeroU8>::Some(const _); // scope 6 at $DIR/combine_transmutes.rs:+7:28: +7:58
103-
// mir::Constant
104-
// + span: $DIR/combine_transmutes.rs:41:33: 41:57
105-
// + literal: Const { ty: NonZeroU8, val: Unevaluated(NonZeroU8::MAX, [], None) }
106-
_13 = move _14 as u8 (Transmute); // scope 6 at $DIR/combine_transmutes.rs:+7:18: +7:59
107-
StorageDead(_14); // scope 6 at $DIR/combine_transmutes.rs:+7:58: +7:59
108-
StorageLive(_15); // scope 7 at $DIR/combine_transmutes.rs:+8:9: +8:11
109-
StorageLive(_16); // scope 7 at $DIR/combine_transmutes.rs:+8:29: +8:54
110-
_16 = Wrapping::<i16>(const 0_i16); // scope 7 at $DIR/combine_transmutes.rs:+8:29: +8:54
111-
- _15 = move _16 as i16 (Transmute); // scope 7 at $DIR/combine_transmutes.rs:+8:19: +8:55
112-
+ _15 = move (_16.0: i16); // scope 7 at $DIR/combine_transmutes.rs:+8:19: +8:55
113-
StorageDead(_16); // scope 7 at $DIR/combine_transmutes.rs:+8:54: +8:55
114-
StorageLive(_17); // scope 8 at $DIR/combine_transmutes.rs:+9:9: +9:11
115-
StorageLive(_18); // scope 8 at $DIR/combine_transmutes.rs:+9:29: +9:54
116-
_18 = Wrapping::<i16>(const 0_i16); // scope 8 at $DIR/combine_transmutes.rs:+9:29: +9:54
117-
_17 = move _18 as u16 (Transmute); // scope 8 at $DIR/combine_transmutes.rs:+9:19: +9:55
118-
StorageDead(_18); // scope 8 at $DIR/combine_transmutes.rs:+9:54: +9:55
119-
StorageLive(_19); // scope 9 at $DIR/combine_transmutes.rs:+10:9: +10:11
120-
StorageLive(_20); // scope 9 at $DIR/combine_transmutes.rs:+10:29: +10:47
121-
_20 = Union32 { u32: const 0_i32 }; // scope 9 at $DIR/combine_transmutes.rs:+10:29: +10:47
122-
_19 = move _20 as u32 (Transmute); // scope 9 at $DIR/combine_transmutes.rs:+10:19: +10:48
123-
StorageDead(_20); // scope 9 at $DIR/combine_transmutes.rs:+10:47: +10:48
124-
StorageLive(_21); // scope 10 at $DIR/combine_transmutes.rs:+11:9: +11:11
125-
StorageLive(_22); // scope 10 at $DIR/combine_transmutes.rs:+11:29: +11:47
126-
_22 = Union32 { u32: const 0_u32 }; // scope 10 at $DIR/combine_transmutes.rs:+11:29: +11:47
127-
_21 = move _22 as i32 (Transmute); // scope 10 at $DIR/combine_transmutes.rs:+11:19: +11:48
128-
StorageDead(_22); // scope 10 at $DIR/combine_transmutes.rs:+11:47: +11:48
129-
StorageLive(_23); // scope 11 at $DIR/combine_transmutes.rs:+12:9: +12:11
130-
StorageLive(_24); // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
131-
_24 = MaybeUninit::<String>::uninit() -> [return: bb1, unwind unreachable]; // scope 11 at $DIR/combine_transmutes.rs:+12:46: +12:77
68+
StorageLive(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:46: +6:77
69+
_12 = MaybeUninit::<String>::uninit() -> [return: bb1, unwind unreachable]; // scope 5 at $DIR/combine_transmutes.rs:+6:46: +6:77
13270
// mir::Constant
133-
// + span: $DIR/combine_transmutes.rs:46:46: 46:75
134-
// + user_ty: UserType(23)
71+
// + span: $DIR/combine_transmutes.rs:40:46: 40:75
72+
// + user_ty: UserType(11)
13573
// + literal: Const { ty: fn() -> MaybeUninit<String> {MaybeUninit::<String>::uninit}, val: Value(<ZST>) }
13674
}
13775

13876
bb1: {
139-
- _23 = move _24 as std::mem::ManuallyDrop<std::string::String> (Transmute); // scope 11 at $DIR/combine_transmutes.rs:+12:36: +12:78
140-
+ _23 = move (_24.1: std::mem::ManuallyDrop<std::string::String>); // scope 11 at $DIR/combine_transmutes.rs:+12:36: +12:78
141-
StorageDead(_24); // scope 11 at $DIR/combine_transmutes.rs:+12:77: +12:78
142-
_0 = const (); // scope 0 at $DIR/combine_transmutes.rs:+0:32: +13:2
143-
StorageDead(_23); // scope 11 at $DIR/combine_transmutes.rs:+13:1: +13:2
144-
StorageDead(_21); // scope 10 at $DIR/combine_transmutes.rs:+13:1: +13:2
145-
StorageDead(_19); // scope 9 at $DIR/combine_transmutes.rs:+13:1: +13:2
146-
StorageDead(_17); // scope 8 at $DIR/combine_transmutes.rs:+13:1: +13:2
147-
StorageDead(_15); // scope 7 at $DIR/combine_transmutes.rs:+13:1: +13:2
148-
StorageDead(_13); // scope 6 at $DIR/combine_transmutes.rs:+13:1: +13:2
149-
StorageDead(_11); // scope 5 at $DIR/combine_transmutes.rs:+13:1: +13:2
150-
StorageDead(_9); // scope 4 at $DIR/combine_transmutes.rs:+13:1: +13:2
151-
StorageDead(_7); // scope 3 at $DIR/combine_transmutes.rs:+13:1: +13:2
152-
StorageDead(_5); // scope 2 at $DIR/combine_transmutes.rs:+13:1: +13:2
153-
StorageDead(_3); // scope 1 at $DIR/combine_transmutes.rs:+13:1: +13:2
154-
StorageDead(_1); // scope 0 at $DIR/combine_transmutes.rs:+13:1: +13:2
155-
return; // scope 0 at $DIR/combine_transmutes.rs:+13:2: +13:2
77+
- _11 = move _12 as std::mem::ManuallyDrop<std::string::String> (Transmute); // scope 5 at $DIR/combine_transmutes.rs:+6:36: +6:78
78+
+ _11 = move (_12.1: std::mem::ManuallyDrop<std::string::String>); // scope 5 at $DIR/combine_transmutes.rs:+6:36: +6:78
79+
StorageDead(_12); // scope 5 at $DIR/combine_transmutes.rs:+6:77: +6:78
80+
_0 = const (); // scope 0 at $DIR/combine_transmutes.rs:+0:32: +7:2
81+
StorageDead(_11); // scope 5 at $DIR/combine_transmutes.rs:+7:1: +7:2
82+
StorageDead(_9); // scope 4 at $DIR/combine_transmutes.rs:+7:1: +7:2
83+
StorageDead(_7); // scope 3 at $DIR/combine_transmutes.rs:+7:1: +7:2
84+
StorageDead(_5); // scope 2 at $DIR/combine_transmutes.rs:+7:1: +7:2
85+
StorageDead(_3); // scope 1 at $DIR/combine_transmutes.rs:+7:1: +7:2
86+
StorageDead(_1); // scope 0 at $DIR/combine_transmutes.rs:+7:1: +7:2
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return; // scope 0 at $DIR/combine_transmutes.rs:+7:2: +7:2
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}
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}
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