Auto merge of #121298 - nikic:writable, r=cuviper

Set writable and dead_on_unwind attributes for sret arguments

Set the `writable` and `dead_on_unwind` attributes for `sret` arguments. This allows call slot optimization to remove more memcpy's.

See https://llvm.org/docs/LangRef.html#parameter-attributes for the specification of these attributes. In short, the statement we're making here is that:

 * The return slot is writable.
 * The return slot will not be read if the function unwinds.

Fixes https://github.com/rust-lang/rust/issues/90595.

compiler/rustc_codegen_llvm/src/abi.rs

@@ -2,6 +2,7 @@
 use crate::builder::Builder;
 use crate::context::CodegenCx;
 use crate::llvm::{self, Attribute, AttributePlace};
+use crate::llvm_util;
 use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
@@ -425,6 +426,18 @@ fn apply_attrs_llfn(&self, cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value) {
                     cx.type_array(cx.type_i8(), self.ret.layout.size.bytes()),
                 );
                 attributes::apply_to_llfn(llfn, llvm::AttributePlace::Argument(i), &[sret]);
+                if cx.sess().opts.optimize != config::OptLevel::No
+                    && llvm_util::get_version() >= (18, 0, 0)
+                {
+                    attributes::apply_to_llfn(
+                        llfn,
+                        llvm::AttributePlace::Argument(i),
+                        &[
+                            llvm::AttributeKind::Writable.create_attr(cx.llcx),
+                            llvm::AttributeKind::DeadOnUnwind.create_attr(cx.llcx),
+                        ],
+                    );
+                }
             }
             PassMode::Cast { cast, pad_i32: _ } => {
                 cast.attrs.apply_attrs_to_llfn(llvm::AttributePlace::ReturnValue, cx, llfn);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -200,6 +200,8 @@ pub enum AttributeKind {
     AllocAlign = 39,
     SanitizeSafeStack = 40,
     FnRetThunkExtern = 41,
+    Writable = 42,
+    DeadOnUnwind = 43,
 }
 
 /// LLVMIntPredicate

compiler/rustc_llvm/llvm-wrapper/LLVMWrapper.h

@@ -91,6 +91,8 @@ enum LLVMRustAttribute {
   AllocAlign = 39,
   SanitizeSafeStack = 40,
   FnRetThunkExtern = 41,
+  Writable = 42,
+  DeadOnUnwind = 43,
 };
 
 typedef struct OpaqueRustString *RustStringRef;

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -312,6 +312,16 @@ static Attribute::AttrKind fromRust(LLVMRustAttribute Kind) {
     return Attribute::SafeStack;
   case FnRetThunkExtern:
     return Attribute::FnRetThunkExtern;
+#if LLVM_VERSION_GE(18, 0)
+  case Writable:
+    return Attribute::Writable;
+  case DeadOnUnwind:
+    return Attribute::DeadOnUnwind;
+#else
+  case Writable:
+  case DeadOnUnwind:
+    report_fatal_error("Not supported on this LLVM version");
+#endif
   }
   report_fatal_error("bad AttributeKind");
 }

tests/codegen/function-arguments.rs

@@ -197,7 +197,7 @@ pub fn notunpin_box(x: Box<NotUnpin>) -> Box<NotUnpin> {
   x
 }
 
-// CHECK: @struct_return(ptr noalias nocapture noundef sret([32 x i8]) align 4 dereferenceable(32){{( %_0)?}})
+// CHECK: @struct_return(ptr{{( dead_on_unwind)?}} noalias nocapture noundef{{( writable)?}} sret([32 x i8]) align 4 dereferenceable(32){{( %_0)?}})
 #[no_mangle]
 pub fn struct_return() -> S {
   S {

tests/codegen/loongarch-abi/loongarch64-lp64d-abi.rs

@@ -260,7 +260,7 @@ pub struct IntDoubleInt {
 #[no_mangle]
 pub extern "C" fn f_int_double_int_s_arg(a: IntDoubleInt) {}
 
-// CHECK: define void @f_ret_int_double_int_s(ptr noalias nocapture noundef sret([24 x i8]) align 8 dereferenceable(24) %_0)
+// CHECK: define void @f_ret_int_double_int_s(ptr{{( dead_on_unwind)?}} noalias nocapture noundef{{( writable)?}} sret([24 x i8]) align 8 dereferenceable(24) %_0)
 #[no_mangle]
 pub extern "C" fn f_ret_int_double_int_s() -> IntDoubleInt {
     IntDoubleInt { a: 1, b: 2., c: 3 }

tests/codegen/maybeuninit-rvo.rs

@@ -1,4 +1,6 @@
 //@ compile-flags: -O
+//@ needs-unwind
+//@ min-llvm-version: 18
 #![feature(c_unwind)]
 #![crate_type = "lib"]
 
@@ -23,8 +25,8 @@ pub fn new_from_uninit() -> Foo {
 }
 
 pub fn new_from_uninit_unwind() -> Foo {
-    // CHECK-LABEL: new_from_uninit
+    // CHECK-LABEL: new_from_uninit_unwind
-    // CHECK: call void @llvm.memcpy.
+    // CHECK-NOT: call void @llvm.memcpy.
     let mut x = std::mem::MaybeUninit::uninit();
     unsafe {
         init_unwind(x.as_mut_ptr());