Tell LLVM about read-only captures

compiler/rustc_codegen_llvm/src/abi.rs

@@ -24,6 +24,7 @@ use crate::attributes::{self, llfn_attrs_from_instance};
 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;
@@ -41,12 +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]> {
@@ -82,6 +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));
             }
         }
@@ -500,7 +506,16 @@ impl<'ll, 'tcx> FnAbiLlvmExt<'ll, 'tcx> for FnAbi<'tcx, Ty<'tcx>> {
                     }
                 }
                 PassMode::Indirect { attrs, meta_attrs: None, on_stack: false } => {
-                    apply(attrs);
+                    let i = apply(attrs);
+                    if cx.sess().opts.optimize != config::OptLevel::No
+                        && llvm_util::get_version() >= (21, 0, 0)
+                    {
+                        attributes::apply_to_llfn(
+                            llfn,
+                            llvm::AttributePlace::Argument(i),
+                            &[llvm::AttributeKind::DeadOnReturn.create_attr(cx.llcx)],
+                        );
+                    }
                 }
                 PassMode::Indirect { attrs, meta_attrs: Some(meta_attrs), on_stack } => {
                     assert!(!on_stack);

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -249,6 +249,8 @@ pub(crate) enum AttributeKind {
     FnRetThunkExtern = 41,
     Writable = 42,
     DeadOnUnwind = 43,
+    DeadOnReturn = 44,
+    CapturesReadOnly = 45,
 }
 
 /// LLVMIntPredicate

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -277,6 +277,8 @@ enum class LLVMRustAttributeKind {
   FnRetThunkExtern = 41,
   Writable = 42,
   DeadOnUnwind = 43,
+  DeadOnReturn = 44,
+  CapturesReadOnly = 45,
 };
 
 static Attribute::AttrKind fromRust(LLVMRustAttributeKind Kind) {
@@ -369,6 +371,14 @@ static Attribute::AttrKind fromRust(LLVMRustAttributeKind Kind) {
     return Attribute::Writable;
   case LLVMRustAttributeKind::DeadOnUnwind:
     return Attribute::DeadOnUnwind;
+  case LLVMRustAttributeKind::DeadOnReturn:
+#if LLVM_VERSION_GE(21, 0)
+    return Attribute::DeadOnReturn;
+#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");
 }
@@ -423,6 +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)));
 }

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 }

compiler/rustc_ty_utils/src/abi.rs

@@ -355,6 +355,7 @@ fn adjust_for_rust_scalar<'tcx>(
 
             if matches!(kind, PointerKind::SharedRef { frozen: true }) && !is_return {
                 attrs.set(ArgAttribute::ReadOnly);
+                attrs.set(ArgAttribute::CapturesReadOnly);
             }
         }
     }

tests/codegen-llvm/addr-of-mutate.rs

@@ -5,7 +5,7 @@
 // Test for the absence of `readonly` on the argument when it is mutated via `&raw const`.
 // See <https://github.com/rust-lang/rust/issues/111502>.
 
-// CHECK: i8 @foo(ptr noalias{{( nocapture)?}} noundef align 1{{( captures\(none\))?}} dereferenceable(128) %x)
+// CHECK: i8 @foo(ptr{{( dead_on_return)?}} noalias{{( nocapture)?}} noundef align 1{{( captures\(none\))?}} dereferenceable(128) %x)
 #[no_mangle]
 pub fn foo(x: [u8; 128]) -> u8 {
     let ptr = core::ptr::addr_of!(x).cast_mut();
@@ -15,7 +15,7 @@ pub fn foo(x: [u8; 128]) -> u8 {
     x[0]
 }
 
-// CHECK: i1 @second(ptr noalias{{( nocapture)?}} noundef align {{[0-9]+}}{{( captures\(none\))?}} dereferenceable({{[0-9]+}}) %a_ptr_and_b)
+// CHECK: i1 @second(ptr{{( dead_on_return)?}} noalias{{( nocapture)?}} noundef align {{[0-9]+}}{{( captures\(none\))?}} dereferenceable({{[0-9]+}}) %a_ptr_and_b)
 #[no_mangle]
 pub unsafe fn second(a_ptr_and_b: (*mut (i32, bool), (i64, bool))) -> bool {
     let b_bool_ptr = core::ptr::addr_of!(a_ptr_and_b.1.1).cast_mut();
@@ -24,7 +24,7 @@ pub unsafe fn second(a_ptr_and_b: (*mut (i32, bool), (i64, bool))) -> bool {
 }
 
 // If going through a deref (and there are no other mutating accesses), then `readonly` is fine.
-// CHECK: i1 @third(ptr noalias{{( nocapture)?}} noundef readonly align {{[0-9]+}}{{( captures\(none\))?}} dereferenceable({{[0-9]+}}) %a_ptr_and_b)
+// CHECK: i1 @third(ptr{{( dead_on_return)?}} noalias{{( nocapture)?}} noundef readonly align {{[0-9]+}}{{( captures\(none\))?}} dereferenceable({{[0-9]+}}) %a_ptr_and_b)
 #[no_mangle]
 pub unsafe fn third(a_ptr_and_b: (*mut (i32, bool), (i64, bool))) -> bool {
     let b_bool_ptr = core::ptr::addr_of!((*a_ptr_and_b.0).1).cast_mut();

tests/codegen-llvm/dead_on_return.rs

@@ -0,0 +1,31 @@
+//@ compile-flags: -C opt-level=3
+//@ min-llvm-version: 21
+
+#![crate_type = "lib"]
+#![allow(unused_assignments, unused_variables)]
+
+// Check that the old string is deallocated, but a new one is not initialized.
+#[unsafe(no_mangle)]
+pub fn test_str_new(mut s: String) {
+    // CHECK-LABEL: @test_str_new
+    // CHECK: __rust_dealloc
+    // CHECK-NOT: store
+    s = String::new();
+}
+
+#[unsafe(no_mangle)]
+pub fn test_str_take(mut x: String) -> String {
+    // CHECK-LABEL: @test_str_take
+    // CHECK-NEXT: {{.*}}:
+    // CHECK-NEXT: call void @llvm.memcpy
+    // CHECK-NEXT: ret
+    core::mem::take(&mut x)
+}
+
+#[unsafe(no_mangle)]
+pub fn test_array_store(mut x: [u32; 100]) {
+    // CHECK-LABEL: @test_array_store
+    // CHECK-NEXT: {{.*}}:
+    // CHECK-NEXT: ret
+    x[0] = 1;
+}

tests/codegen-llvm/function-arguments.rs

@@ -80,7 +80,7 @@ pub fn option_nonzero_int(x: Option<NonZero<u64>>) -> Option<NonZero<u64>> {
     x
 }
 
-// CHECK: @readonly_borrow(ptr noalias noundef readonly align 4 dereferenceable(4) %_1)
+// CHECK: @readonly_borrow(ptr noalias noundef readonly align 4{{( captures\(address, read_provenance\))}} dereferenceable(4) %_1)
 // FIXME #25759 This should also have `nocapture`
 #[no_mangle]
 pub fn readonly_borrow(_: &i32) {}
@@ -91,12 +91,12 @@ pub fn readonly_borrow_ret() -> &'static i32 {
     loop {}
 }
 
-// CHECK: @static_borrow(ptr noalias noundef readonly align 4 dereferenceable(4) %_1)
+// CHECK: @static_borrow(ptr noalias noundef readonly align 4{{( captures\(address, read_provenance\))}} dereferenceable(4) %_1)
 // static borrow may be captured
 #[no_mangle]
 pub fn static_borrow(_: &'static i32) {}
 
-// CHECK: @named_borrow(ptr noalias noundef readonly align 4 dereferenceable(4) %_1)
+// CHECK: @named_borrow(ptr noalias noundef readonly align 4{{( captures\(address, read_provenance\))}} dereferenceable(4) %_1)
 // borrow with named lifetime may be captured
 #[no_mangle]
 pub fn named_borrow<'r>(_: &'r i32) {}
@@ -129,21 +129,21 @@ pub fn mutable_borrow_ret() -> &'static mut i32 {
 // <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>.
 pub fn mutable_notunpin_borrow(_: &mut NotUnpin) {}
 
-// CHECK: @notunpin_borrow(ptr noalias noundef readonly align 4 dereferenceable(4) %_1)
+// CHECK: @notunpin_borrow(ptr noalias noundef readonly align 4{{( captures\(address, read_provenance\))}} dereferenceable(4) %_1)
 // But `&NotUnpin` behaves perfectly normal.
 #[no_mangle]
 pub fn notunpin_borrow(_: &NotUnpin) {}
 
-// CHECK: @indirect_struct(ptr noalias{{( nocapture)?}} noundef readonly align 4{{( captures\(none\))?}} dereferenceable(32) %_1)
+// CHECK: @indirect_struct(ptr{{( dead_on_return)?}} noalias{{( nocapture)?}} noundef readonly align 4{{( captures\(none\))?}} dereferenceable(32) %_1)
 #[no_mangle]
 pub fn indirect_struct(_: S) {}
 
-// CHECK: @borrowed_struct(ptr noalias noundef readonly align 4 dereferenceable(32) %_1)
+// CHECK: @borrowed_struct(ptr noalias noundef readonly align 4{{( captures\(address, read_provenance\))}} dereferenceable(32) %_1)
 // FIXME #25759 This should also have `nocapture`
 #[no_mangle]
 pub fn borrowed_struct(_: &S) {}
 
-// CHECK: @option_borrow(ptr noalias noundef readonly align 4 dereferenceable_or_null(4) %_x)
+// CHECK: @option_borrow(ptr noalias noundef readonly align 4{{( captures\(address, read_provenance\))}} dereferenceable_or_null(4) %_x)
 #[no_mangle]
 pub fn option_borrow(_x: Option<&i32>) {}
 
@@ -185,7 +185,7 @@ pub fn _box(x: Box<i32>) -> Box<i32> {
 // With a custom allocator, it should *not* have `noalias`. (See
 // <https://github.com/rust-lang/miri/issues/3341> for why.) The second argument is the allocator,
 // which is a reference here that still carries `noalias` as usual.
-// CHECK: @_box_custom(ptr noundef nonnull align 4 %x.0, ptr noalias noundef nonnull readonly align 1 %x.1)
+// CHECK: @_box_custom(ptr noundef nonnull align 4 %x.0, ptr noalias noundef nonnull readonly align 1{{( captures\(address, read_provenance\))}} %x.1)
 #[no_mangle]
 pub fn _box_custom(x: Box<i32, &std::alloc::Global>) {
     drop(x)
@@ -208,7 +208,7 @@ pub fn struct_return() -> S {
 #[no_mangle]
 pub fn helper(_: usize) {}
 
-// CHECK: @slice(ptr noalias noundef nonnull readonly align 1 %_1.0, [[USIZE]] noundef %_1.1)
+// CHECK: @slice(ptr noalias noundef nonnull readonly align 1{{( captures\(address, read_provenance\))}} %_1.0, [[USIZE]] noundef %_1.1)
 // FIXME #25759 This should also have `nocapture`
 #[no_mangle]
 pub fn slice(_: &[u8]) {}
@@ -227,7 +227,7 @@ pub fn unsafe_slice(_: &[UnsafeInner]) {}
 #[no_mangle]
 pub fn raw_slice(_: *const [u8]) {}
 
-// CHECK: @str(ptr noalias noundef nonnull readonly align 1 %_1.0, [[USIZE]] noundef %_1.1)
+// CHECK: @str(ptr noalias noundef nonnull readonly align 1{{( captures\(address, read_provenance\))}} %_1.0, [[USIZE]] noundef %_1.1)
 // FIXME #25759 This should also have `nocapture`
 #[no_mangle]
 pub fn str(_: &[u8]) {}
@@ -259,7 +259,7 @@ pub fn trait_option(x: Option<Box<dyn Drop + Unpin>>) -> Option<Box<dyn Drop + U
     x
 }
 
-// CHECK: { ptr, [[USIZE]] } @return_slice(ptr noalias noundef nonnull readonly align 2 %x.0, [[USIZE]] noundef %x.1)
+// CHECK: { ptr, [[USIZE]] } @return_slice(ptr noalias noundef nonnull readonly align 2{{( captures\(address, read_provenance\))}} %x.0, [[USIZE]] noundef %x.1)
 #[no_mangle]
 pub fn return_slice(x: &[u16]) -> &[u16] {
     x

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

@@ -256,7 +256,7 @@ pub struct IntDoubleInt {
     c: i32,
 }
 
-// CHECK: define void @f_int_double_int_s_arg(ptr noalias{{( nocapture)?}} noundef align 8{{( captures\(none\))?}} dereferenceable(24) %a)
+// CHECK: define void @f_int_double_int_s_arg(ptr{{( dead_on_return)?}} noalias{{( nocapture)?}} noundef align 8{{( captures\(none\))?}} dereferenceable(24) %a)
 #[no_mangle]
 pub extern "C" fn f_int_double_int_s_arg(a: IntDoubleInt) {}
 

tests/codegen-llvm/range-attribute.rs

@@ -67,7 +67,7 @@ pub fn enum2_value(x: Enum2) -> Enum2 {
     x
 }
 
-// CHECK: noundef [[USIZE]] @takes_slice(ptr noalias noundef nonnull readonly align 4 %x.0, [[USIZE]] noundef %x.1)
+// CHECK: noundef [[USIZE]] @takes_slice(ptr {{.*}} %x.0, [[USIZE]] noundef %x.1)
 #[no_mangle]
 pub fn takes_slice(x: &[i32]) -> usize {
     x.len()

tests/ui/abi/debug.generic.stderr

@@ -939,7 +939,7 @@ error: fn_abi_of(assoc_test) = FnAbi {
                    },
                    mode: Direct(
                        ArgAttributes {
-                           regular: NoAlias | NonNull | ReadOnly | NoUndef,
+                           regular: NoAlias | NonNull | ReadOnly | NoUndef | CapturesReadOnly,
                            arg_ext: None,
                            pointee_size: Size(2 bytes),
                            pointee_align: Some(

tests/ui/abi/debug.riscv64.stderr

@@ -939,7 +939,7 @@ error: fn_abi_of(assoc_test) = FnAbi {
                    },
                    mode: Direct(
                        ArgAttributes {
-                           regular: NoAlias | NonNull | ReadOnly | NoUndef,
+                           regular: NoAlias | NonNull | ReadOnly | NoUndef | CapturesReadOnly,
                            arg_ext: None,
                            pointee_size: Size(2 bytes),
                            pointee_align: Some(

tests/ui/codegen/equal-pointers-unequal/as-cast/segfault.rs

@@ -1,7 +1,7 @@
-//@ known-bug: #107975
 //@ compile-flags: -Copt-level=2
-//@ run-pass
+//@ run-fail
 //@ ignore-backends: gcc
+//@ min-llvm-version: 21
 
 // https://github.com/rust-lang/rust/issues/107975#issuecomment-1431758601
 

tests/ui/codegen/equal-pointers-unequal/exposed-provenance/segfault.rs

@@ -1,7 +1,7 @@
-//@ known-bug: #107975
 //@ compile-flags: -Copt-level=2
-//@ run-pass
+//@ run-fail
 //@ ignore-backends: gcc
+//@ min-llvm-version: 21
 
 // https://github.com/rust-lang/rust/issues/107975#issuecomment-1431758601
 

tests/ui/codegen/equal-pointers-unequal/strict-provenance/segfault.rs

@@ -1,7 +1,7 @@
-//@ known-bug: #107975
 //@ compile-flags: -Copt-level=2
-//@ run-pass
+//@ run-fail
 //@ ignore-backends: gcc
+//@ min-llvm-version: 21
 
 // https://github.com/rust-lang/rust/issues/107975#issuecomment-1431758601