From edf729ea34cf3217c91dfb4c33e68e57aad63498 Mon Sep 17 00:00:00 2001
From: Vastargazing <vebohr@gmail.com>
Date: Sun, 2 Aug 2026 09:03:11 +0300
Subject: [PATCH] Emit noundef on small aggregate returns in the x86 Win32 ABI

The x86 Win32 (MSVC) ABI returns small aggregates in an integer register via
`cast_to`, which drops the argument attributes -- so these returns never carried
`noundef`, unlike scalars, which get it from `arg_attrs_for_rust_scalar`.

Switch those casts to `cast_to_maybe_noundef` so a return whose layout is provably
free of uninit bytes gets `noundef`, while padded layouts correctly omit it.
---
 .../rustc_target/src/callconv/x86_win32.rs    |  8 +-
 tests/codegen-llvm/abi-noundef-cast-win32.rs  | 95 +++++++++++++++++++
 2 files changed, 99 insertions(+), 4 deletions(-)
 create mode 100644 tests/codegen-llvm/abi-noundef-cast-win32.rs

diff --git a/compiler/rustc_target/src/callconv/x86_win32.rs b/compiler/rustc_target/src/callconv/x86_win32.rs
index 824e7cc098a46..4cee6579a0b09 100644
--- a/compiler/rustc_target/src/callconv/x86_win32.rs
+++ b/compiler/rustc_target/src/callconv/x86_win32.rs
@@ -27,10 +27,10 @@ pub(crate) fn compute_abi_info<'a, Ty, C>(
             // - fixed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85667
             if t.abi_return_struct_as_int || opts.reg_struct_return {
                 match fn_abi.ret.layout.size.bytes() {
-                    1 => fn_abi.ret.cast_to(Reg::i8()),
-                    2 => fn_abi.ret.cast_to(Reg::i16()),
-                    4 => fn_abi.ret.cast_to(Reg::i32()),
-                    8 => fn_abi.ret.cast_to(Reg::i64()),
+                    1 => fn_abi.ret.cast_to_maybe_noundef(Reg::i8(), cx),
+                    2 => fn_abi.ret.cast_to_maybe_noundef(Reg::i16(), cx),
+                    4 => fn_abi.ret.cast_to_maybe_noundef(Reg::i32(), cx),
+                    8 => fn_abi.ret.cast_to_maybe_noundef(Reg::i64(), cx),
                     _ => fn_abi.ret.make_indirect(),
                 }
             } else {
diff --git a/tests/codegen-llvm/abi-noundef-cast-win32.rs b/tests/codegen-llvm/abi-noundef-cast-win32.rs
new file mode 100644
index 0000000000000..2f928a41d8410
--- /dev/null
+++ b/tests/codegen-llvm/abi-noundef-cast-win32.rs
@@ -0,0 +1,95 @@
+// Checks that small aggregates returned as an integer under the Win32 (MSVC) ABI
+// (`abi_return_struct_as_int`) carry `noundef` when the layout is provably free of
+// uninit bytes, and omit it when the layout may contain uninit bytes (padding or a
+// union).
+//
+// See <https://github.com/rust-lang/rust/issues/123183>.
+
+//@ add-minicore
+//@ compile-flags: --target i686-pc-windows-msvc -Cno-prepopulate-passes -Copt-level=3
+//@ needs-llvm-components: x86
+
+#![feature(no_core, lang_items)]
+#![no_core]
+#![crate_type = "lib"]
+
+extern crate minicore;
+use minicore::*;
+
+// Fully-defined layouts, one per integer register size (i8/i16/i32/i64).
+
+#[repr(C)]
+pub struct OneU8 {
+    x: u8,
+}
+
+#[repr(C)]
+pub struct TwoU8 {
+    x: u8,
+    y: u8,
+}
+
+#[repr(C)]
+pub struct TwoU16 {
+    x: u16,
+    y: u16,
+}
+
+#[repr(C)]
+pub struct TwoU32 {
+    x: u32,
+    y: u32,
+}
+
+// Layouts that may contain uninit bytes.
+
+// Tail padding: `u8` at offset 4 leaves bytes 5..8 uninit.
+#[repr(C)]
+pub struct Padded {
+    x: u32,
+    y: u8,
+}
+
+#[repr(C)]
+pub union U {
+    x: u32,
+    y: u32,
+}
+
+// CHECK: define noundef i8 @ret_one_u8()
+#[no_mangle]
+pub extern "C" fn ret_one_u8() -> OneU8 {
+    OneU8 { x: 1 }
+}
+
+// CHECK: define noundef i16 @ret_two_u8()
+#[no_mangle]
+pub extern "C" fn ret_two_u8() -> TwoU8 {
+    TwoU8 { x: 1, y: 2 }
+}
+
+// CHECK: define noundef i32 @ret_two_u16()
+#[no_mangle]
+pub extern "C" fn ret_two_u16() -> TwoU16 {
+    TwoU16 { x: 1, y: 2 }
+}
+
+// CHECK: define noundef i64 @ret_two_u32()
+#[no_mangle]
+pub extern "C" fn ret_two_u32() -> TwoU32 {
+    TwoU32 { x: 1, y: 2 }
+}
+
+// Tail padding -> no `noundef`.
+// CHECK: define i64 @ret_padded()
+#[no_mangle]
+pub extern "C" fn ret_padded() -> Padded {
+    Padded { x: 1, y: 2 }
+}
+
+// Union -> no `noundef`.
+// CHECK: define i32 @ret_union()
+#[no_mangle]
+pub extern "C" fn ret_union() -> U {
+    U { x: 1 }
+}