Tell LLVM about read-only captures

`&Freeze` parameters are not only `readonly` within the function,
but any captures of the pointer can also only be used for reads.
This can now be encoded using the `captures(address, read_provenance)`
attribute.

5 files changed, 17 insertions, 1 deletions

diff --git a/compiler/rustc_codegen_llvm/src/abi.rs b/compiler/rustc_codegen_llvm/src/abi.rs
index 043123fcab2..399f8b6e762 100644
--- a/compiler/rustc_codegen_llvm/src/abi.rs
+++ b/compiler/rustc_codegen_llvm/src/abi.rs
@@ -42,12 +42,13 @@ trait ArgAttributesExt {
 const ABI_AFFECTING_ATTRIBUTES: [(ArgAttribute, llvm::AttributeKind); 1] =
     [(ArgAttribute::InReg, llvm::AttributeKind::InReg)];
 
-const OPTIMIZATION_ATTRIBUTES: [(ArgAttribute, llvm::AttributeKind); 5] = [
+const OPTIMIZATION_ATTRIBUTES: [(ArgAttribute, llvm::AttributeKind); 6] = [
     (ArgAttribute::NoAlias, llvm::AttributeKind::NoAlias),
     (ArgAttribute::NoCapture, llvm::AttributeKind::NoCapture),
     (ArgAttribute::NonNull, llvm::AttributeKind::NonNull),
     (ArgAttribute::ReadOnly, llvm::AttributeKind::ReadOnly),
     (ArgAttribute::NoUndef, llvm::AttributeKind::NoUndef),
+    (ArgAttribute::CapturesReadOnly, llvm::AttributeKind::CapturesReadOnly),
 ];
 
 fn get_attrs<'ll>(this: &ArgAttributes, cx: &CodegenCx<'ll, '_>) -> SmallVec<[&'ll Attribute; 8]> {
@@ -83,6 +84,10 @@ fn get_attrs<'ll>(this: &ArgAttributes, cx: &CodegenCx<'ll, '_>) -> SmallVec<[&'
         }
         for (attr, llattr) in OPTIMIZATION_ATTRIBUTES {
             if regular.contains(attr) {
+                // captures(address, read_provenance) is only available since LLVM 21.
+                if attr == ArgAttribute::CapturesReadOnly && llvm_util::get_version() < (21, 0, 0) {
+                    continue;
+                }
                 attrs.push(llattr.create_attr(cx.llcx));
             }
         }
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 55d26a97e2d..2461f70a86e 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -251,6 +251,7 @@ pub(crate) enum AttributeKind {
     Writable = 42,
     DeadOnUnwind = 43,
     DeadOnReturn = 44,
+    CapturesReadOnly = 45,
 }
 
 /// LLVMIntPredicate
diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index e4fe1fc2e42..e699e4b9c13 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
@@ -278,6 +278,7 @@ enum class LLVMRustAttributeKind {
   Writable = 42,
   DeadOnUnwind = 43,
   DeadOnReturn = 44,
+  CapturesReadOnly = 45,
 };
 
 static Attribute::AttrKind fromRust(LLVMRustAttributeKind Kind) {
@@ -376,6 +377,8 @@ static Attribute::AttrKind fromRust(LLVMRustAttributeKind Kind) {
 #else
     report_fatal_error("DeadOnReturn attribute requires LLVM 21 or later");
 #endif
+  case LLVMRustAttributeKind::CapturesReadOnly:
+    report_fatal_error("Should be handled separately");
   }
   report_fatal_error("bad LLVMRustAttributeKind");
 }
@@ -430,6 +433,11 @@ LLVMRustCreateAttrNoValue(LLVMContextRef C, LLVMRustAttributeKind RustAttr) {
   if (RustAttr == LLVMRustAttributeKind::NoCapture) {
     return wrap(Attribute::getWithCaptureInfo(*unwrap(C), CaptureInfo::none()));
   }
+  if (RustAttr == LLVMRustAttributeKind::CapturesReadOnly) {
+    return wrap(Attribute::getWithCaptureInfo(
+        *unwrap(C), CaptureInfo(CaptureComponents::Address |
+                                CaptureComponents::ReadProvenance)));
+  }
 #endif
   return wrap(Attribute::get(*unwrap(C), fromRust(RustAttr)));
 }
diff --git a/compiler/rustc_target/src/callconv/mod.rs b/compiler/rustc_target/src/callconv/mod.rs
index 63e56744aec..5f2a6f7ba38 100644
--- a/compiler/rustc_target/src/callconv/mod.rs
+++ b/compiler/rustc_target/src/callconv/mod.rs
@@ -119,6 +119,7 @@ mod attr_impl {
             const ReadOnly  = 1 << 4;
             const InReg     = 1 << 5;
             const NoUndef = 1 << 6;
+            const CapturesReadOnly = 1 << 7;
         }
     }
     rustc_data_structures::external_bitflags_debug! { ArgAttribute }
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs
index 89d1dd8cf23..e4ed084b073 100644
--- a/compiler/rustc_ty_utils/src/abi.rs
+++ b/compiler/rustc_ty_utils/src/abi.rs
@@ -356,6 +356,7 @@ fn arg_attrs_for_rust_scalar<'tcx>(
 
             if matches!(kind, PointerKind::SharedRef { frozen: true }) && !is_return {
                 attrs.set(ArgAttribute::ReadOnly);
+                attrs.set(ArgAttribute::CapturesReadOnly);
             }
         }
     }