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.

1 files changed, 13 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index aec11e9905f..123aef0f310 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -2,6 +2,7 @@ use crate::attributes;
 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 @@ impl<'ll, 'tcx> FnAbiLlvmExt<'ll, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
                     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);