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GVN: Preserve derefs at terminators that cannot write to memory

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This commit is contained in:
dianqk 2025-10-14 21:30:53 +08:00
parent a673575b24
commit afff0502a6
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5 changed files with 57 additions and 46 deletions
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22
compiler/rustc_middle/src/mir/terminator.rs
View file

@ -696,6 +696,28 @@ impl<'tcx> TerminatorKind<'tcx> {
_ => None,
}
}
/// Returns true if the terminator can write to memory.
pub fn can_write_to_memory(&self) -> bool {
match self {
TerminatorKind::Goto { .. }
| TerminatorKind::SwitchInt { .. }
| TerminatorKind::UnwindResume
| TerminatorKind::UnwindTerminate(_)
| TerminatorKind::Return
| TerminatorKind::Assert { .. }
| TerminatorKind::CoroutineDrop
| TerminatorKind::FalseEdge { .. }
| TerminatorKind::FalseUnwind { .. }
| TerminatorKind::Unreachable => false,
TerminatorKind::Call { .. }
| TerminatorKind::Drop { .. }
| TerminatorKind::TailCall { .. }
// Yield writes to the resume_arg place.
| TerminatorKind::Yield { .. }
| TerminatorKind::InlineAsm { .. } => true,
}
}
}
#[derive(Copy, Clone, Debug)]

9
compiler/rustc_mir_transform/src/gvn.rs
View file

@ -1926,13 +1926,8 @@ impl<'tcx> MutVisitor<'tcx> for VnState<'_, '_, 'tcx> {
self.assign(local, opaque);
}
}
// Function calls and ASM may invalidate (nested) derefs. We must handle them carefully.
// Currently, only preserving derefs for trivial terminators like SwitchInt and Goto.
let safe_to_preserve_derefs = matches!(
terminator.kind,
TerminatorKind::SwitchInt { .. } | TerminatorKind::Goto { .. }
);
if !safe_to_preserve_derefs {
// Terminators that can write to memory may invalidate (nested) derefs.
if terminator.kind.can_write_to_memory() {
self.invalidate_derefs();
}
self.super_terminator(terminator, location);

5
tests/mir-opt/pre-codegen/two_unwrap_unchecked.rs
View file

@ -1,4 +1,3 @@
// skip-filecheck
//@ compile-flags: -O
#![crate_type = "lib"]
@ -6,6 +5,10 @@
// EMIT_MIR two_unwrap_unchecked.two_unwrap_unchecked.GVN.diff
// EMIT_MIR two_unwrap_unchecked.two_unwrap_unchecked.PreCodegen.after.mir
pub fn two_unwrap_unchecked(v: &Option<i32>) -> i32 {
// CHECK-LABEL: fn two_unwrap_unchecked(
// CHECK: [[DEREF_V:_.*]] = copy (*_1);
// CHECK: [[V1V2:_.*]] = copy (([[DEREF_V]] as Some).0: i32);
// CHECK: _0 = Add(copy [[V1V2]], copy [[V1V2]]);
let v1 = unsafe { v.unwrap_unchecked() };
let v2 = unsafe { v.unwrap_unchecked() };
v1 + v2

39
tests/mir-opt/pre-codegen/two_unwrap_unchecked.two_unwrap_unchecked.GVN.diff
View file

@ -41,12 +41,15 @@
bb0: {
- StorageLive(_2);
- StorageLive(_3);
+ nop;
+ nop;
StorageLive(_3);
_3 = copy (*_1);
StorageLive(_8);
- StorageLive(_8);
+ nop;
_8 = discriminant(_3);
switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb1];
- switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb1];
+ switchInt(copy _8) -> [0: bb2, 1: bb3, otherwise: bb1];
}
bb1: {
@ -58,16 +61,21 @@
}
bb3: {
_2 = move ((_3 as Some).0: i32);
StorageDead(_8);
StorageDead(_3);
- StorageLive(_4);
- _2 = move ((_3 as Some).0: i32);
- StorageDead(_8);
- StorageDead(_3);
+ _2 = copy ((_3 as Some).0: i32);
+ nop;
+ nop;
StorageLive(_4);
StorageLive(_5);
_5 = copy (*_1);
- _5 = copy (*_1);
+ _5 = copy _3;
StorageLive(_9);
_9 = discriminant(_5);
switchInt(move _9) -> [0: bb4, 1: bb5, otherwise: bb1];
- _9 = discriminant(_5);
- switchInt(move _9) -> [0: bb4, 1: bb5, otherwise: bb1];
+ _9 = copy _8;
+ switchInt(copy _8) -> [0: bb4, 1: bb5, otherwise: bb1];
}
bb4: {
@ -75,20 +83,21 @@
}
bb5: {
_4 = move ((_5 as Some).0: i32);
- _4 = move ((_5 as Some).0: i32);
+ _4 = copy _2;
StorageDead(_9);
StorageDead(_5);
StorageLive(_6);
_6 = copy _2;
StorageLive(_7);
_7 = copy _4;
- _7 = copy _4;
- _0 = Add(move _6, move _7);
+ _0 = Add(copy _2, copy _4);
+ _7 = copy _2;
+ _0 = Add(copy _2, copy _2);
StorageDead(_7);
StorageDead(_6);
- StorageDead(_4);
StorageDead(_4);
- StorageDead(_2);
+ nop;
+ nop;
return;
}

28
tests/mir-opt/pre-codegen/two_unwrap_unchecked.two_unwrap_unchecked.PreCodegen.after.mir
View file

@ -5,15 +5,12 @@ fn two_unwrap_unchecked(_1: &Option<i32>) -> i32 {
let mut _0: i32;
let mut _2: std::option::Option<i32>;
let _4: i32;
let mut _5: std::option::Option<i32>;
scope 1 {
debug v1 => _4;
let _7: i32;
scope 2 {
debug v2 => _7;
debug v2 => _4;
}
scope 8 (inlined #[track_caller] Option::<i32>::unwrap_unchecked) {
let mut _6: isize;
scope 9 {
}
scope 10 (inlined #[track_caller] unreachable_unchecked) {
@ -37,33 +34,18 @@ fn two_unwrap_unchecked(_1: &Option<i32>) -> i32 {
}
bb0: {
StorageLive(_2);
_2 = copy (*_1);
StorageLive(_3);
_3 = discriminant(_2);
switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb3];
switchInt(copy _3) -> [0: bb2, 1: bb1, otherwise: bb2];
}
bb1: {
_4 = move ((_2 as Some).0: i32);
StorageDead(_3);
StorageDead(_2);
StorageLive(_5);
_5 = copy (*_1);
StorageLive(_6);
_6 = discriminant(_5);
switchInt(move _6) -> [0: bb3, 1: bb2, otherwise: bb3];
}
bb2: {
_7 = move ((_5 as Some).0: i32);
StorageDead(_6);
StorageDead(_5);
_0 = Add(copy _4, copy _7);
_4 = copy ((_2 as Some).0: i32);
_0 = Add(copy _4, copy _4);
return;
}
bb3: {
bb2: {
unreachable;
}
}