Add noundef metadata for fits-in-target-pointer-size array immediate arguments

compiler/rustc_ty_utils/src/abi.rs

@@ -8,8 +8,8 @@ use rustc_middle::ty::{self, InstanceDef, Ty, TyCtxt};
 use rustc_session::config::OptLevel;
 use rustc_span::def_id::DefId;
 use rustc_target::abi::call::{
-    ArgAbi, ArgAttribute, ArgAttributes, ArgExtension, Conv, FnAbi, PassMode, Reg, RegKind,
-    RiscvInterruptKind,
+    ArgAbi, ArgAttribute, ArgAttributes, ArgExtension, CastTarget, Conv, FnAbi, PassMode, Reg,
+    RegKind, RiscvInterruptKind,
 };
 use rustc_target::abi::*;
 use rustc_target::spec::abi::Abi as SpecAbi;
@@ -784,6 +784,21 @@ fn fn_abi_adjust_for_abi<'tcx>(
                 // an LLVM aggregate type for this leads to bad optimizations,
                 // so we pick an appropriately sized integer type instead.
                 arg.cast_to(Reg { kind: RegKind::Integer, size });
+
+                // Let's see if we can add a `noundef`. This is only legal for arrays, definitely
+                // not for unions. This is also legal for `#[repr(transparent)] struct` or
+                // `#[repr(transparent)] enum` containing array. Note that `#[repr(transparent)]`
+                // can contain other `#[repr(transparent)]` structs or enums, which can eventually
+                // contain an array!
+                if arg.layout.ty.is_array() || is_transparent_array(cx, arg.layout) {
+                    // Fixup arg attribute with `noundef`.
+                    let PassMode::Cast { ref mut cast, .. } = &mut arg.mode else {
+                        bug!("this cannot fail because of the previous cast_to `Reg`");
+                    };
+
+                    let box CastTarget { ref mut attrs, .. } = cast;
+                    attrs.set(ArgAttribute::NoUndef);
+                }
             }
 
             // If we deduced that this parameter was read-only, add that to the attribute list now.
@@ -819,6 +834,20 @@ fn fn_abi_adjust_for_abi<'tcx>(
     Ok(())
 }
 
+fn is_transparent_array<'tcx>(
+    cx: &LayoutCx<'tcx, TyCtxt<'tcx>>,
+    outermost_layout: TyAndLayout<'tcx>,
+) -> bool {
+    let mut adt_layout = outermost_layout;
+    // Recursively walk a layout, seeing through all `#[repr(transparent)]` layers.
+    while adt_layout.is_transparent::<LayoutCx<'tcx, TyCtxt<'tcx>>>()
+        && let Some((_, layout)) = adt_layout.non_1zst_field(cx)
+    {
+        adt_layout = layout;
+    }
+    adt_layout.ty.is_array()
+}
+
 #[tracing::instrument(level = "debug", skip(cx))]
 fn make_thin_self_ptr<'tcx>(
     cx: &(impl HasTyCtxt<'tcx> + HasParamEnv<'tcx>),

tests/codegen/array-immediate-param-noundef.rs

@@ -0,0 +1,125 @@
+// Check that small array immediates that fits in target pointer size in argument position have
+// `noundef` parameter metadata. Note that the `noundef` parameter metadata is only applied if:
+// - `!arg.layout.is_unsized() && size <= Pointer(AddressSpace::DATA).size(cx)`
+// - optimizations are turned on.
+//
+//@ only-64bit (presence of noundef depends on pointer width)
+//@ compile-flags: -C no-prepopulate-passes -O
+#![crate_type = "lib"]
+
+// CHECK: define noundef i64 @short_array_u64x1(i64 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u64x1(v: [u64; 1]) -> [u64; 1] {
+    v
+}
+
+// CHECK: define noundef i32 @short_array_u32x1(i32 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u32x1(v: [u32; 1]) -> [u32; 1] {
+    v
+}
+
+// CHECK: define noundef i64 @short_array_u32x2(i64 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u32x2(v: [u32; 2]) -> [u32; 2] {
+    v
+}
+
+// CHECK: define noundef i16 @short_array_u16x1(i16 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u16x1(v: [u16; 1]) -> [u16; 1] {
+    v
+}
+
+// CHECK: define noundef i32 @short_array_u16x2(i32 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u16x2(v: [u16; 2]) -> [u16; 2] {
+    v
+}
+
+// CHECK: define noundef i48 @short_array_u16x3(i48 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u16x3(v: [u16; 3]) -> [u16; 3] {
+    v
+}
+
+// CHECK: define noundef i64 @short_array_u16x4(i64 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u16x4(v: [u16; 4]) -> [u16; 4] {
+    v
+}
+
+// CHECK: define noundef i8 @short_array_u8x1(i8 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u8x1(v: [u8; 1]) -> [u8; 1] {
+    v
+}
+
+// CHECK: define noundef i16 @short_array_u8x2(i16 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u8x2(v: [u8; 2]) -> [u8; 2] {
+    v
+}
+
+// CHECK: define noundef i24 @short_array_u8x3(i24 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u8x3(v: [u8; 3]) -> [u8; 3] {
+    v
+}
+
+// CHECK: define noundef i64 @short_array_u8x8(i64 noundef %{{.*}})
+#[no_mangle]
+pub fn short_array_u8x8(v: [u8; 8]) -> [u8; 8] {
+    v
+}
+
+#[repr(transparent)]
+pub struct Foo([u8; 4]);
+
+// CHECK: define noundef i32 @repr_transparent_struct_short_array(i32 noundef %{{.*}})
+#[no_mangle]
+pub fn repr_transparent_struct_short_array(v: Foo) -> Foo {
+    v
+}
+
+#[repr(transparent)]
+pub enum Bar {
+    Default([u8; 4]),
+}
+
+// CHECK: define noundef i32 @repr_transparent_enum_short_array(i32 noundef %{{.*}})
+#[no_mangle]
+pub fn repr_transparent_enum_short_array(v: Bar) -> Bar {
+    v
+}
+
+#[repr(transparent)]
+pub struct Owo([u8; 4]);
+
+#[repr(transparent)]
+pub struct Uwu(Owo);
+
+#[repr(transparent)]
+pub struct Oowoo(Uwu);
+
+// CHECK: define noundef i32 @repr_transparent_nested_struct_short_array(i32 noundef %{{.*}})
+#[no_mangle]
+pub fn repr_transparent_nested_struct_short_array(v: Oowoo) -> Oowoo {
+    v
+}
+
+// # Negative examples
+
+// This inner struct is *not* `#[repr(transparent)]`, so we must not emit `noundef` for the outer
+// struct.
+pub struct NotTransparent([u8; 4]);
+
+#[repr(transparent)]
+pub struct Transparent(NotTransparent);
+
+// CHECK-LABEL: not_all_transparent_nested_struct_short_array
+// CHECK-NOT: noundef
+#[no_mangle]
+pub fn not_all_transparent_nested_struct_short_array(v: Transparent) -> Transparent {
+    v
+}