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GVN: transmute adts to their fields if a field projection is immediately transmuted anyway #153085

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GVN: transmute adts to their fields if a field projection is immediately transmuted anyway #153085

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45a1603
Turn field accesses into a transmute if followed by a transmute anyway
oli-obk Feb 24, 2026
c972717
Add test for option transmutes
oli-obk Feb 25, 2026
c44c650
Also skip downcasts if the size of the enum matches the transmute tar…
oli-obk Feb 25, 2026
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37 changes: 37 additions & 0 deletions compiler/rustc_mir_transform/src/gvn.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1494,6 +1494,7 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
}
}

#[instrument(level = "trace", skip(self), ret)]
fn simplify_cast(
&mut self,
initial_kind: &mut CastKind,
Expand Down Expand Up @@ -1549,6 +1550,42 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
}
}

// Field-Access-then-Transmute can just transmute the original value,
// so long as the bytes of a value from only from a single field.
if let Transmute = kind
&& let Value::Projection(field_value, ProjectionElem::Field(field_idx, ())) =
self.get(value)
{
if let Value::Projection(
downcast_value,
ProjectionElem::Downcast(_, _variant_idx),
) = self.get(field_value)
{
let downcast_ty = self.ty(downcast_value);
if let Ok(downcast_layout) = self.ecx.layout_of(downcast_ty)
&& let Ok(projected_layout) = self.ecx.layout_of(from)
&& downcast_layout.size == projected_layout.size
{
from = downcast_ty;
value = downcast_value;
was_updated_this_iteration = true;
if projected_layout.ty == to {
return Some(value);
}
}
} else if let Some((f_idx, field_ty)) =
self.value_is_all_in_one_field(self.ty(field_value), FIRST_VARIANT)
{
assert_eq!(field_idx, f_idx, "{from} -> {field_ty}");
from = self.ty(field_value);
value = field_value;
was_updated_this_iteration = true;
if field_ty == to {
return Some(value);
}
}
}

// Aggregate-then-Transmute can just transmute the original field value,
// so long as the bytes of a value from only from a single field.
if let Transmute = kind
Expand Down
42 changes: 42 additions & 0 deletions tests/mir-opt/const_prop/transmute.option_field.GVN.32bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,42 @@
- // MIR for `option_field` before GVN
+ // MIR for `option_field` after GVN

fn option_field(_1: Option<NonNull<()>>) -> *const () {
debug x => _1;
let mut _0: *const ();
let mut _2: isize;
let mut _4: std::ptr::NonNull<()>;
scope 1 {
debug x => _3;
let _3: std::ptr::NonNull<()>;
}

bb0: {
_2 = discriminant(_1);
switchInt(move _2) -> [1: bb1, otherwise: bb2];
}

bb1: {
- StorageLive(_3);
+ nop;
_3 = copy ((_1 as Some).0: std::ptr::NonNull<()>);
StorageLive(_4);
_4 = copy _3;
- _0 = move _4 as *const () (Transmute);
+ _0 = copy _1 as *const () (Transmute);
StorageDead(_4);
- StorageDead(_3);
+ nop;
goto -> bb3;
}

bb2: {
_0 = const 0_usize as *const () (PointerWithExposedProvenance);
goto -> bb3;
}

bb3: {
return;
}
}

42 changes: 42 additions & 0 deletions tests/mir-opt/const_prop/transmute.option_field.GVN.64bit.diff
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Original file line number Diff line number Diff line change
@@ -0,0 +1,42 @@
- // MIR for `option_field` before GVN
+ // MIR for `option_field` after GVN

fn option_field(_1: Option<NonNull<()>>) -> *const () {
debug x => _1;
let mut _0: *const ();
let mut _2: isize;
let mut _4: std::ptr::NonNull<()>;
scope 1 {
debug x => _3;
let _3: std::ptr::NonNull<()>;
}

bb0: {
_2 = discriminant(_1);
switchInt(move _2) -> [1: bb1, otherwise: bb2];
}

bb1: {
- StorageLive(_3);
+ nop;
_3 = copy ((_1 as Some).0: std::ptr::NonNull<()>);
StorageLive(_4);
_4 = copy _3;
- _0 = move _4 as *const () (Transmute);
+ _0 = copy _1 as *const () (Transmute);
StorageDead(_4);
- StorageDead(_3);
+ nop;
goto -> bb3;
}

bb2: {
_0 = const 0_usize as *const () (PointerWithExposedProvenance);
goto -> bb3;
}

bb3: {
return;
}
}

8 changes: 8 additions & 0 deletions tests/mir-opt/const_prop/transmute.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -84,4 +84,12 @@ pub unsafe fn unreachable_box() -> ! {
match *x {}
}

// EMIT_MIR transmute.option_field.GVN.diff
pub unsafe fn option_field(x: Option<std::ptr::NonNull<()>>) -> *const () {
// CHECK-LABEL: fn option_field(
// CHECK: _3 = copy ((_1 as Some).0: std::ptr::NonNull<()>)
// CHECK: _0 = copy _1 as *const () (Transmute)
if let Some(x) = x { unsafe { transmute(x) } } else { 0 as *const () }
}

enum Never {}
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/transmute.unreachable_box.GVN.32bit.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@
- _1 = const 1_usize as std::boxed::Box<Never> (Transmute);
- _2 = copy ((_1.0: std::ptr::Unique<Never>).0: std::ptr::NonNull<Never>) as *const Never (Transmute);
+ _1 = const Box::<Never>(std::ptr::Unique::<Never> {{ pointer: NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: PhantomData::<Never> }}, std::alloc::Global);
+ _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }} as *const Never (Transmute);
+ _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as *const Never (Transmute);
unreachable;
}
}
Expand Down
2 changes: 1 addition & 1 deletion tests/mir-opt/const_prop/transmute.unreachable_box.GVN.64bit.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -14,7 +14,7 @@
- _1 = const 1_usize as std::boxed::Box<Never> (Transmute);
- _2 = copy ((_1.0: std::ptr::Unique<Never>).0: std::ptr::NonNull<Never>) as *const Never (Transmute);
+ _1 = const Box::<Never>(std::ptr::Unique::<Never> {{ pointer: NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: PhantomData::<Never> }}, std::alloc::Global);
+ _2 = const std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }} as *const Never (Transmute);
+ _2 = const std::boxed::Box::<Never>(std::ptr::Unique::<Never> {{ pointer: std::ptr::NonNull::<Never> {{ pointer: {0x1 as *const Never} }}, _marker: std::marker::PhantomData::<Never> }}, std::alloc::Global) as *const Never (Transmute);
unreachable;
}
}
Expand Down
3 changes: 2 additions & 1 deletion tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -74,7 +74,8 @@
- StorageLive(_5);
+ nop;
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
- _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+ _11 = copy _10 as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
+ nop;
Expand Down
3 changes: 2 additions & 1 deletion tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.32bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -54,7 +54,8 @@
- StorageLive(_5);
+ nop;
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
- _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+ _11 = copy _10 as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
+ nop;
Expand Down
3 changes: 2 additions & 1 deletion tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -74,7 +74,8 @@
- StorageLive(_5);
+ nop;
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
- _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+ _11 = copy _10 as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
+ nop;
Expand Down
3 changes: 2 additions & 1 deletion tests/mir-opt/dont_reset_cast_kind_without_updating_operand.test.GVN.64bit.panic-unwind.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -54,7 +54,8 @@
- StorageLive(_5);
+ nop;
_10 = copy (*_1);
_11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
- _11 = copy ((_10.0: std::ptr::Unique<()>).0: std::ptr::NonNull<()>) as *const () (Transmute);
+ _11 = copy _10 as *const () (Transmute);
_5 = &raw const (*_11);
- StorageLive(_6);
+ nop;
Expand Down
12 changes: 8 additions & 4 deletions ...s/mir-opt/pre-codegen/issue_117368_print_invalid_constant.main.GVN.32bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,8 @@
StorageLive(_3);
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
- StorageLive(_6);
+ nop;
StorageLive(_7);
- _7 = copy _1;
- _6 = std::alloc::Global::alloc_impl_runtime(move _7, const false) -> [return: bb4, unwind unreachable];
Expand All @@ -93,11 +94,14 @@
}

bb6: {
_5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
- _5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
+ _5 = copy ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
StorageDead(_14);
StorageDead(_10);
StorageDead(_6);
_4 = copy _5 as *mut [u8] (Transmute);
- StorageDead(_6);
- _4 = copy _5 as *mut [u8] (Transmute);
+ nop;
+ _4 = copy _6 as *mut [u8] (Transmute);
StorageDead(_5);
_3 = copy _4 as *mut u8 (PtrToPtr);
StorageDead(_4);
Expand Down
12 changes: 8 additions & 4 deletions ...s/mir-opt/pre-codegen/issue_117368_print_invalid_constant.main.GVN.64bit.panic-abort.diff
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Original file line number Diff line number Diff line change
Expand Up @@ -69,7 +69,8 @@
StorageLive(_3);
StorageLive(_4);
StorageLive(_5);
StorageLive(_6);
- StorageLive(_6);
+ nop;
StorageLive(_7);
- _7 = copy _1;
- _6 = std::alloc::Global::alloc_impl_runtime(move _7, const false) -> [return: bb4, unwind unreachable];
Expand All @@ -93,11 +94,14 @@
}

bb6: {
_5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
- _5 = move ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
+ _5 = copy ((_6 as Ok).0: std::ptr::NonNull<[u8]>);
StorageDead(_14);
StorageDead(_10);
StorageDead(_6);
_4 = copy _5 as *mut [u8] (Transmute);
- StorageDead(_6);
- _4 = copy _5 as *mut [u8] (Transmute);
+ nop;
+ _4 = copy _6 as *mut [u8] (Transmute);
StorageDead(_5);
_3 = copy _4 as *mut u8 (PtrToPtr);
StorageDead(_4);
Expand Down
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