Auto merge of rust-lang#123886 - scottmcm:more-rvalue-operands, r=<try>

Avoid `alloca`s in codegen for simple pairs and simple transparent structs

Even something simple like constructing a
```rust
#[repr(transparent)] struct Foo(u32);
```
forces an `alloca` to be generated in nightly right now.

Certainly LLVM can optimize that away, but it would be nice if it didn't have to.

Quick example:
```rust
#[repr(transparent)]
pub struct Transparent32(u32);

#[no_mangle]
pub fn make_transparent(x: u32) -> Transparent32 {
    let a = Transparent32(x);
    a
}
```
on nightly we produce <https://rust.godbolt.org/z/zcvoM79ae>
```llvm
define noundef i32 `@make_transparent(i32` noundef %x) unnamed_addr #0 {
  %a = alloca i32, align 4
  store i32 %x, ptr %a, align 4
  %0 = load i32, ptr %a, align 4, !noundef !3
  ret i32 %0
}
```
but after this PR we produce
```llvm
define noundef i32 `@make_transparent(i32` noundef %x) unnamed_addr #0 {
start:
  ret i32 %x
}
```
(even before the optimizer runs).

Author: bors

compiler/rustc_codegen_cranelift/src/vtable.rs

@@ -53,7 +53,7 @@ pub(crate) fn get_ptr_and_method_ref<'tcx>(
                     .layout()
                     .non_1zst_field(fx)
                     .expect("not exactly one non-1-ZST field in a `DispatchFromDyn` type");
-                arg = arg.value_field(fx, FieldIdx::new(idx));
+                arg = arg.value_field(fx, idx);
             }
         }
 

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -1019,7 +1019,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             let (idx, _) = op.layout.non_1zst_field(bx).expect(
                                 "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
                             );
-                            op = op.extract_field(bx, idx);
+                            op = op.extract_field(bx, idx.as_usize());
                         }
 
                         // now that we have `*dyn Trait` or `&dyn Trait`, split it up into its
@@ -1051,7 +1051,7 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                             let (idx, _) = op.layout.non_1zst_field(bx).expect(
                                 "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
                             );
-                            op = op.extract_field(bx, idx);
+                            op = op.extract_field(bx, idx.as_usize());
                         }
 
                         // Make sure that we've actually unwrapped the rcvr down

compiler/rustc_codegen_ssa/src/mir/rvalue.rs

@@ -15,7 +15,7 @@ use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf, TyAndLayout};
 use rustc_middle::ty::{self, adjustment::PointerCoercion, Instance, Ty, TyCtxt};
 use rustc_session::config::OptLevel;
 use rustc_span::{Span, DUMMY_SP};
-use rustc_target::abi::{self, FIRST_VARIANT};
+use rustc_target::abi::{self, FieldIdx, FIRST_VARIANT};
 
 impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
     #[instrument(level = "trace", skip(self, bx))]
@@ -720,12 +720,47 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 OperandRef { val: OperandValue::Immediate(static_), layout }
             }
             mir::Rvalue::Use(ref operand) => self.codegen_operand(bx, operand),
-            mir::Rvalue::Repeat(..) | mir::Rvalue::Aggregate(..) => {
+            mir::Rvalue::Repeat(..) => {
                 // According to `rvalue_creates_operand`, only ZST
-                // aggregate rvalues are allowed to be operands.
+                // repat rvalues are allowed to be operands.
                 let ty = rvalue.ty(self.mir, self.cx.tcx());
                 OperandRef::zero_sized(self.cx.layout_of(self.monomorphize(ty)))
             }
+            mir::Rvalue::Aggregate(ref kind, ref fields) => {
+                let ty = rvalue.ty(self.mir, self.cx.tcx());
+                let ty = self.monomorphize(ty);
+                let layout = self.cx.layout_of(self.monomorphize(ty));
+                match **kind {
+                    _ if layout.is_zst() => OperandRef::zero_sized(layout),
+                    mir::AggregateKind::Tuple => {
+                        debug_assert_eq!(
+                            fields.len(),
+                            2,
+                            "We should only get pairs, but got {rvalue:?}"
+                        );
+                        let a = self.codegen_operand(bx, &fields[FieldIdx::ZERO]);
+                        let b = self.codegen_operand(bx, &fields[FieldIdx::from_u32(1)]);
+                        let val = OperandValue::Pair(a.immediate(), b.immediate());
+                        OperandRef { val, layout }
+                    }
+                    mir::AggregateKind::Adt(..) => {
+                        let (field, _) = layout
+                            .non_1zst_field(self.cx)
+                            .expect("only transparent non-ZST structs should get here");
+                        let val = match self.codegen_operand(bx, &fields[field]).val {
+                            OperandValue::Immediate(a) => {
+                                OperandValue::Immediate(bx.from_immediate(a))
+                            }
+                            OperandValue::Pair(a, b) => {
+                                OperandValue::Pair(bx.from_immediate(a), bx.from_immediate(b))
+                            }
+                            other => bug!("Unexpected operand {other:?}"),
+                        };
+                        OperandRef { val, layout }
+                    }
+                    _ => bug!("Unexpected in codegen_rvalue_operand: {rvalue:?}"),
+                }
+            }
             mir::Rvalue::ShallowInitBox(ref operand, content_ty) => {
                 let operand = self.codegen_operand(bx, operand);
                 let val = operand.immediate();
@@ -1032,12 +1067,37 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             mir::Rvalue::ThreadLocalRef(_) |
             mir::Rvalue::Use(..) => // (*)
                 true,
-            mir::Rvalue::Repeat(..) |
-            mir::Rvalue::Aggregate(..) => {
+            mir::Rvalue::Repeat(..) => {
+                let ty = rvalue.ty(self.mir, self.cx.tcx());
+                let ty = self.monomorphize(ty);
+                let layout = self.cx.spanned_layout_of(ty, span);
+                layout.is_zst()
+            }
+            mir::Rvalue::Aggregate(ref kind, ref fields) => {
                 let ty = rvalue.ty(self.mir, self.cx.tcx());
                 let ty = self.monomorphize(ty);
-                // For ZST this can be `OperandValueKind::ZeroSized`.
-                self.cx.spanned_layout_of(ty, span).is_zst()
+                let layout = self.cx.spanned_layout_of(ty, span);
+                match **kind {
+                    // OperandValue::ZeroSized is easy
+                    _ if layout.is_zst() => true,
+                    // 2-Tuple of scalars is an easy scalar pair
+                    mir::AggregateKind::Tuple => {
+                        fields.len() == 2
+                            && self.cx.is_backend_scalar_pair(layout)
+                            && fields.iter().all(|field| {
+                                let field_ty = field.ty(self.mir, self.cx.tcx());
+                                let field_ty = self.monomorphize(field_ty);
+                                let field_layout = self.cx.spanned_layout_of(field_ty, span);
+                                self.cx.is_backend_immediate(field_layout)
+                            })
+                    }
+                    // If a non-union is transparent, we can pass it along
+                    mir::AggregateKind::Adt(_, _, _, _, None) => {
+                        ty.ty_adt_def().is_some_and(|def| def.repr().transparent())
+                            && !self.cx.is_backend_ref(layout)
+                    }
+                    _ => false,
+                }
             }
         }
 

compiler/rustc_const_eval/src/interpret/terminator.rs

@@ -793,7 +793,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                             let (idx, _) = receiver.layout.non_1zst_field(self).expect(
                                 "not exactly one non-1-ZST field in a `DispatchFromDyn` type",
                             );
-                            receiver = self.project_field(&receiver, idx)?;
+                            receiver = self.project_field(&receiver, idx.as_usize())?;
                         }
                     }
                 };

compiler/rustc_target/src/abi/mod.rs

@@ -279,7 +279,7 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
 
     /// Finds the one field that is not a 1-ZST.
     /// Returns `None` if there are multiple non-1-ZST fields or only 1-ZST-fields.
-    pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(usize, Self)>
+    pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(FieldIdx, Self)>
     where
         Ty: TyAbiInterface<'a, C> + Copy,
     {
@@ -293,8 +293,12 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
                 // More than one non-1-ZST field.
                 return None;
             }
-            found = Some((field_idx, field));
+
+            // Arrays are the only things with more than FieldIdx::MAX fields,
+            // but only 1-element arrays can ever return non-None here.
+            found = Some((FieldIdx::from_usize(field_idx), field));
         }
+
         found
     }
 }

tests/codegen/transparent-aggregates.rs

@@ -0,0 +1,69 @@
+//@ compile-flags: -O -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+
+#[repr(transparent)]
+pub struct Transparent32(u32);
+
+// CHECK: i32 @make_transparent(i32 noundef %x)
+#[no_mangle]
+pub fn make_transparent(x: u32) -> Transparent32 {
+    // CHECK-NOT: alloca
+    // CHECK: ret i32 %x
+    let a = Transparent32(x);
+    a
+}
+
+#[repr(transparent)]
+pub struct TransparentPair((), (u16, u16), ());
+
+// CHECK: { i16, i16 } @make_transparent_pair(i16 noundef %x.0, i16 noundef %x.1)
+#[no_mangle]
+pub fn make_transparent_pair(x: (u16, u16)) -> TransparentPair {
+    // CHECK-NOT: alloca
+    // CHECK: %[[TEMP0:.+]] = insertvalue { i16, i16 } poison, i16 %x.0, 0
+    // CHECK: %[[TEMP1:.+]] = insertvalue { i16, i16 } %[[TEMP0]], i16 %x.1, 1
+    // CHECK: ret { i16, i16 } %[[TEMP1]]
+    let a = TransparentPair((), x, ());
+    a
+}
+
+// CHECK-LABEL: { i32, i32 } @make_2_tuple(i32 noundef %x)
+#[no_mangle]
+pub fn make_2_tuple(x: u32) -> (u32, u32) {
+    // CHECK-NOT: alloca
+    // CHECK: %[[TEMP0:.+]] = insertvalue { i32, i32 } poison, i32 %x, 0
+    // CHECK: %[[TEMP1:.+]] = insertvalue { i32, i32 } %[[TEMP0]], i32 %x, 1
+    // CHECK: ret { i32, i32 } %[[TEMP1]]
+    let pair = (x, x);
+    pair
+}
+
+// CHECK-LABEL: i8 @make_cell_of_bool(i1 noundef zeroext %b)
+#[no_mangle]
+pub fn make_cell_of_bool(b: bool) -> std::cell::Cell<bool> {
+    // CHECK: %[[BYTE:.+]] = zext i1 %b to i8
+    // CHECK: ret i8 %[[BYTE]]
+    std::cell::Cell::new(b)
+}
+
+// CHECK-LABLE: { i8, i16 } @make_cell_of_bool_and_short(i1 noundef zeroext %b, i16 noundef %s)
+#[no_mangle]
+pub fn make_cell_of_bool_and_short(b: bool, s: u16) -> std::cell::Cell<(bool, u16)> {
+    // CHECK-NOT: alloca
+    // CHECK: %[[BYTE:.+]] = zext i1 %b to i8
+    // CHECK: %[[TEMP0:.+]] = insertvalue { i8, i16 } poison, i8 %[[BYTE]], 0
+    // CHECK: %[[TEMP1:.+]] = insertvalue { i8, i16 } %[[TEMP0]], i16 %s, 1
+    // CHECK: ret { i8, i16 } %[[TEMP1]]
+    std::cell::Cell::new((b, s))
+}
+
+// CHECK-LABEL: { i1, i1 } @make_tuple_of_bools(i1 noundef zeroext %a, i1 noundef zeroext %b)
+#[no_mangle]
+pub fn make_tuple_of_bools(a: bool, b: bool) -> (bool, bool) {
+    // CHECK-NOT: alloca
+    // CHECK: %[[TEMP0:.+]] = insertvalue { i1, i1 } poison, i1 %a, 0
+    // CHECK: %[[TEMP1:.+]] = insertvalue { i1, i1 } %[[TEMP0]], i1 %b, 1
+    // CHECK: ret { i1, i1 } %[[TEMP1]]
+    (a, b)
+}