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, 10 insertions, 0 deletions

diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index 565bdc3af03..20167a4b45e 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/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");
 }