From 3565b0699d6830dc31732afa96272bcbd1f83606 Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Fri, 19 Sep 2025 20:39:47 +0200
Subject: [PATCH] emit attribute for readonly non-pure inline assembly

---
 compiler/rustc_codegen_llvm/src/asm.rs        |  4 +-
 compiler/rustc_codegen_llvm/src/llvm/ffi.rs   |  1 +
 .../rustc_llvm/llvm-wrapper/RustWrapper.cpp   |  5 ++
 tests/codegen-llvm/asm/readonly-not-pure.rs   | 48 +++++++++++++++++++
 4 files changed, 56 insertions(+), 2 deletions(-)
 create mode 100644 tests/codegen-llvm/asm/readonly-not-pure.rs

diff --git a/compiler/rustc_codegen_llvm/src/asm.rs b/compiler/rustc_codegen_llvm/src/asm.rs
index b79176e909818..838eb0db0403d 100644
--- a/compiler/rustc_codegen_llvm/src/asm.rs
+++ b/compiler/rustc_codegen_llvm/src/asm.rs
@@ -340,8 +340,8 @@ impl<'ll, 'tcx> AsmBuilderMethods<'tcx> for Builder<'_, 'll, 'tcx> {
             attrs.push(llvm::AttributeKind::WillReturn.create_attr(self.cx.llcx));
         } else if options.contains(InlineAsmOptions::NOMEM) {
             attrs.push(llvm::MemoryEffects::InaccessibleMemOnly.create_attr(self.cx.llcx));
-        } else {
-            // LLVM doesn't have an attribute to represent ReadOnly + SideEffect
+        } else if options.contains(InlineAsmOptions::READONLY) {
+            attrs.push(llvm::MemoryEffects::ReadOnlyNotPure.create_attr(self.cx.llcx));
         }
         attributes::apply_to_callsite(result, llvm::AttributePlace::Function, &{ attrs });
 
diff --git a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
index 9a86e4373d8f4..fd972f371df49 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
+++ b/compiler/rustc_codegen_llvm/src/llvm/ffi.rs
@@ -710,6 +710,7 @@ pub(crate) enum MemoryEffects {
     None,
     ReadOnly,
     InaccessibleMemOnly,
+    ReadOnlyNotPure,
 }
 
 /// LLVMOpcode
diff --git a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
index 64151962321fd..414274f24fb50 100644
--- a/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
+++ b/compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp
@@ -553,6 +553,7 @@ enum class LLVMRustMemoryEffects {
   None,
   ReadOnly,
   InaccessibleMemOnly,
+  ReadOnlyNotPure,
 };
 
 extern "C" LLVMAttributeRef
@@ -568,6 +569,10 @@ LLVMRustCreateMemoryEffectsAttr(LLVMContextRef C,
   case LLVMRustMemoryEffects::InaccessibleMemOnly:
     return wrap(Attribute::getWithMemoryEffects(
         *unwrap(C), MemoryEffects::inaccessibleMemOnly()));
+  case LLVMRustMemoryEffects::ReadOnlyNotPure:
+    return wrap(Attribute::getWithMemoryEffects(
+        *unwrap(C),
+        MemoryEffects::readOnly() | MemoryEffects::inaccessibleMemOnly()));
   default:
     report_fatal_error("bad MemoryEffects.");
   }
diff --git a/tests/codegen-llvm/asm/readonly-not-pure.rs b/tests/codegen-llvm/asm/readonly-not-pure.rs
new file mode 100644
index 0000000000000..a3c0e276c7f5e
--- /dev/null
+++ b/tests/codegen-llvm/asm/readonly-not-pure.rs
@@ -0,0 +1,48 @@
+//@ add-core-stubs
+//@ compile-flags: -Copt-level=3 --target x86_64-unknown-linux-gnu
+//@ needs-llvm-components: x86
+
+#![crate_type = "rlib"]
+#![feature(no_core)]
+#![no_core]
+
+// Test that when an inline assembly block specifies `readonly` but not `pure`, a detailed
+// `MemoryEffects` is provided to LLVM: this assembly block is not allowed to perform writes,
+// but it may have side-effects.
+
+extern crate minicore;
+use minicore::*;
+
+pub static mut VAR: i32 = 0;
+
+// CHECK-LABEL: @no_options
+// CHECK: call i32 asm
+#[no_mangle]
+pub unsafe fn no_options() -> i32 {
+    VAR = 1;
+    let _ignored: i32;
+    asm!("mov {0}, 1", out(reg) _ignored);
+    VAR
+}
+
+// CHECK-LABEL: @readonly_pure
+// CHECK-NOT: call i32 asm
+#[no_mangle]
+pub unsafe fn readonly_pure() -> i32 {
+    VAR = 1;
+    let _ignored: i32;
+    asm!("mov {0}, 1", out(reg) _ignored, options(pure, readonly));
+    VAR
+}
+
+// CHECK-LABEL: @readonly_not_pure
+// CHECK: call i32 asm {{.*}} #[[ATTR:[0-9]+]]
+#[no_mangle]
+pub unsafe fn readonly_not_pure() -> i32 {
+    VAR = 1;
+    let _ignored: i32;
+    asm!("mov {0}, 1", out(reg) _ignored, options(readonly));
+    VAR
+}
+
+// CHECK: attributes #[[ATTR]] = { nounwind memory(read, inaccessiblemem: readwrite) }