Fix MaybeUninit codegen using GVN

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -24,6 +24,12 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     ) {
         match *rvalue {
             mir::Rvalue::Use(ref operand) => {
+                if let mir::Operand::Constant(const_op) = operand {
+                    let val = self.eval_mir_constant(&const_op);
+                    if val.all_bytes_uninit(self.cx.tcx()) {
+                        return;
+                    }
+                }
                 let cg_operand = self.codegen_operand(bx, operand);
                 // Crucially, we do *not* use `OperandValue::Ref` for types with
                 // `BackendRepr::Scalar | BackendRepr::ScalarPair`. This ensures we match the MIR

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -522,6 +522,10 @@ impl<'tcx, Prov: Provenance> OpTy<'tcx, Prov> {
     pub(super) fn op(&self) -> &Operand<Prov> {
         &self.op
     }
+
+    pub fn is_immediate_uninit(&self) -> bool {
+        matches!(self.op, Operand::Immediate(Immediate::Uninit))
+    }
 }
 
 impl<'tcx, Prov: Provenance> Projectable<'tcx, Prov> for OpTy<'tcx, Prov> {

compiler/rustc_middle/src/mir/pretty.rs

@@ -1872,6 +1872,13 @@ fn pretty_print_const_value_tcx<'tcx>(
         return Ok(());
     }
 
+    // Printing [MaybeUninit<u8>::uninit(); N] or any other aggregate where all fields are uninit
+    // becomes very verbose. This special case makes the dump terse and clear.
+    if ct.all_bytes_uninit(tcx) {
+        fmt.write_str("<uninit>")?;
+        return Ok(());
+    }
+
     let u8_type = tcx.types.u8;
     match (ct, ty.kind()) {
         // Byte/string slices, printed as (byte) string literals.

compiler/rustc_mir_transform/src/gvn.rs

@@ -570,9 +570,19 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
             _ if ty.is_zst() => ImmTy::uninit(ty).into(),
 
             Opaque(_) => return None,
-            // Do not bother evaluating repeat expressions. This would uselessly consume memory.
-            Repeat(..) => return None,
 
+            // In general, evaluating repeat expressions just consumes a lot of memory.
+            // But in the special case that the element is just Immediate::Uninit, we can evaluate
+            // it without extra memory! If we don't propagate uninit values like this, LLVM can get
+            // very confused: https://github.com/rust-lang/rust/issues/139355
+            Repeat(value, _count) => {
+                let value = self.eval_to_const(value)?;
+                if value.is_immediate_uninit() {
+                    ImmTy::uninit(ty).into()
+                } else {
+                    return None;
+                }
+            }
             Constant { ref value, disambiguator: _ } => {
                 self.ecx.eval_mir_constant(value, DUMMY_SP, None).discard_err()?
             }
@@ -608,8 +618,12 @@ impl<'body, 'a, 'tcx> VnState<'body, 'a, 'tcx> {
             }
             Union(active_field, field) => {
                 let field = self.eval_to_const(field)?;
-                if matches!(ty.backend_repr, BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..))
-                {
+                if field.layout.layout.is_zst() {
+                    ImmTy::from_immediate(Immediate::Uninit, ty).into()
+                } else if matches!(
+                    ty.backend_repr,
+                    BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..)
+                ) {
                     let dest = self.ecx.allocate(ty, MemoryKind::Stack).discard_err()?;
                     let field_dest = self.ecx.project_field(&dest, active_field).discard_err()?;
                     self.ecx.copy_op(field, &field_dest).discard_err()?;
@@ -1705,7 +1719,11 @@ fn op_to_prop_const<'tcx>(
 
     // Do not synthetize too large constants. Codegen will just memcpy them, which we'd like to
     // avoid.
-    if !matches!(op.layout.backend_repr, BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..)) {
+    // But we *do* want to synthesize any size constant if it is entirely uninit because that
+    // benefits codegen, which has special handling for them.
+    if !op.is_immediate_uninit()
+        && !matches!(op.layout.backend_repr, BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..))
+    {
         return None;
     }
 

tests/codegen-llvm/maybeuninit-array.rs

@@ -0,0 +1,15 @@
+// This is a regression test for https://github.com/rust-lang/rust/issues/139355
+
+//@ compile-flags: -Copt-level=3
+
+#![crate_type = "lib"]
+
+use std::mem::MaybeUninit;
+
+#[no_mangle]
+pub fn create_uninit_array() -> [[MaybeUninit<u8>; 4]; 200] {
+    // CHECK-LABEL: create_uninit_array
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: ret void
+    [[MaybeUninit::<u8>::uninit(); 4]; 200]
+}

tests/codegen-llvm/uninit-consts.rs

@@ -11,21 +11,19 @@ pub struct PartiallyUninit {
     y: MaybeUninit<[u8; 10]>,
 }
 
-// CHECK: [[FULLY_UNINIT:@.*]] = private unnamed_addr constant [10 x i8] undef
-
 // CHECK: [[PARTIALLY_UNINIT:@.*]] = private unnamed_addr constant <{ [4 x i8], [12 x i8] }> <{ [4 x i8] c"{{\\EF\\BE\\AD\\DE|\\DE\\AD\\BE\\EF}}", [12 x i8] undef }>, align 4
 
 // This shouldn't contain undef, since it contains more chunks
 // than the default value of uninit_const_chunk_threshold.
 // CHECK: [[UNINIT_PADDING_HUGE:@.*]] = private unnamed_addr constant [32768 x i8] c"{{.+}}", align 4
 
-// CHECK: [[FULLY_UNINIT_HUGE:@.*]] = private unnamed_addr constant [16384 x i8] undef
-
 // CHECK-LABEL: @fully_uninit
 #[no_mangle]
 pub const fn fully_uninit() -> MaybeUninit<[u8; 10]> {
     const M: MaybeUninit<[u8; 10]> = MaybeUninit::uninit();
-    // CHECK: call void @llvm.memcpy.{{.+}}(ptr align 1 %_0, ptr align 1 {{.*}}[[FULLY_UNINIT]]{{.*}}, i{{(32|64)}} 10, i1 false)
+    // returning uninit doesn't need to do anything to the return place at all
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: ret void
     M
 }
 
@@ -49,6 +47,8 @@ pub const fn uninit_padding_huge() -> [(u32, u8); 4096] {
 #[no_mangle]
 pub const fn fully_uninit_huge() -> MaybeUninit<[u32; 4096]> {
     const F: MaybeUninit<[u32; 4096]> = MaybeUninit::uninit();
-    // CHECK: call void @llvm.memcpy.{{.+}}(ptr align 4 %_0, ptr align 4 {{.*}}[[FULLY_UNINIT_HUGE]]{{.*}}, i{{(32|64)}} 16384, i1 false)
+    // returning uninit doesn't need to do anything to the return place at all
+    // CHECK-NEXT: start:
+    // CHECK-NEXT: ret void
     F
 }

tests/mir-opt/const_prop/maybe_uninit.rs

@@ -0,0 +1,10 @@
+//@ compile-flags: -O
+
+use std::mem::MaybeUninit;
+
+// EMIT_MIR maybe_uninit.u8_array.GVN.diff
+pub fn u8_array() -> [MaybeUninit<u8>; 8] {
+    // CHECK: fn u8_array(
+    // CHECK: _0 = const <uninit>;
+    [MaybeUninit::uninit(); 8]
+}

tests/mir-opt/const_prop/maybe_uninit.u8_array.GVN.diff

@@ -0,0 +1,28 @@
+- // MIR for `u8_array` before GVN
++ // MIR for `u8_array` after GVN
+
+  fn u8_array() -> [MaybeUninit<u8>; 8] {
+      let mut _0: [std::mem::MaybeUninit<u8>; 8];
+      let mut _1: std::mem::MaybeUninit<u8>;
+      scope 1 (inlined MaybeUninit::<u8>::uninit) {
+      }
+
+      bb0: {
+          StorageLive(_1);
+-         _1 = MaybeUninit::<u8> { uninit: const () };
+-         _0 = [move _1; 8];
++         _1 = const <uninit>;
++         _0 = const <uninit>;
+          StorageDead(_1);
+          return;
+      }
++ }
++ 
++ ALLOC0 (size: 8, align: 1) {
++     __ __ __ __ __ __ __ __                         │ ░░░░░░░░
++ }
++ 
++ ALLOC1 (size: 1, align: 1) {
++     __                                              │ ░
+  }
+

tests/mir-opt/const_prop/transmute.rs

@@ -43,8 +43,7 @@ pub unsafe fn invalid_bool() -> bool {
 // EMIT_MIR transmute.undef_union_as_integer.GVN.diff
 pub unsafe fn undef_union_as_integer() -> u32 {
     // CHECK-LABEL: fn undef_union_as_integer(
-    // CHECK: _1 = const Union32
-    // CHECK: _0 = const {{.*}}: u32;
+    // CHECK: _0 = const <uninit>;
     union Union32 {
         value: u32,
         unit: (),

tests/mir-opt/const_prop/transmute.undef_union_as_integer.GVN.32bit.diff

@@ -12,16 +12,20 @@
 -         _2 = ();
 -         _1 = Union32 { value: move _2 };
 +         _2 = const ();
-+         _1 = const Union32 {{ value: Indirect { alloc_id: ALLOC0, offset: Size(0 bytes) }: u32, unit: () }};
++         _1 = const <uninit>;
           StorageDead(_2);
 -         _0 = move _1 as u32 (Transmute);
-+         _0 = const Indirect { alloc_id: ALLOC0, offset: Size(0 bytes) }: u32;
++         _0 = const <uninit>;
           StorageDead(_1);
           return;
       }
 + }
 + 
 + ALLOC0 (size: 4, align: 4) {
++     __ __ __ __                                     │ ░░░░
++ }
++ 
++ ALLOC1 (size: 4, align: 4) {
 +     __ __ __ __                                     │ ░░░░
   }
 

tests/mir-opt/const_prop/transmute.undef_union_as_integer.GVN.64bit.diff

@@ -12,16 +12,20 @@
 -         _2 = ();
 -         _1 = Union32 { value: move _2 };
 +         _2 = const ();
-+         _1 = const Union32 {{ value: Indirect { alloc_id: ALLOC0, offset: Size(0 bytes) }: u32, unit: () }};
++         _1 = const <uninit>;
           StorageDead(_2);
 -         _0 = move _1 as u32 (Transmute);
-+         _0 = const Indirect { alloc_id: ALLOC0, offset: Size(0 bytes) }: u32;
++         _0 = const <uninit>;
           StorageDead(_1);
           return;
       }
 + }
 + 
 + ALLOC0 (size: 4, align: 4) {
++     __ __ __ __                                     │ ░░░░
++ }
++ 
++ ALLOC1 (size: 4, align: 4) {
 +     __ __ __ __                                     │ ░░░░
   }