Keep simple transparent things from needing allocas

Author: scottmcm

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 +720,44 @@ 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_idx, _) = layout
+                            .non_1zst_field(self.cx)
+                            .expect("only transparent non-ZST structs should get here");
+                        let field = self.codegen_operand(bx, &fields[field_idx]);
+                        // While the size is the same, since the struct is transparent,
+                        // calling transmute here handles the `i1`-vs-`i8` issues for `bool`.
+                        let Some(val) = self.codegen_transmute_operand(bx, field, layout) else {
+                            bug!("Couldn't transmute {field:?} to {layout:?}");
+                        };
+                        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 +1064,37 @@ 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);
-                // 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);
+                layout.is_zst()
+            }
+            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.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,
+                }
             }
         }
 

tests/codegen/transparent-aggregates.rs

@@ -3,27 +3,67 @@
 #![crate_type = "lib"]
 
 #[repr(transparent)]
-struct Transparent32(u32);
+pub struct Transparent32(u32);
 
 // CHECK: i32 @make_transparent(i32 noundef %x)
 #[no_mangle]
 pub fn make_transparent(x: u32) -> Transparent32 {
-    // CHECK: %a = alloca i32
-    // CHECK: store i32 %x, ptr %a
-    // CHECK: %[[TEMP:.+]] = load i32, ptr %a
-    // CHECK: ret i32 %[[TEMP]]
+    // 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: %pair = alloca { i32, i32 }
-    // CHECK: store i32
-    // CHECK: store i32
-    // CHECK: load i32
-    // CHECK: load i32
+    // 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)
+}