Auto merge of #75810 - hug-dev:cmse-nonsecure-entry, r=jonas-schievink

Add support for cmse_nonsecure_entry attribute

This pull request adds the `cmse_nonsecure_entry` attribute under an unstable feature.

I was not sure if it was fine for me to send directly the pull-request or if I should submit a RFC first. I was told on Zulip that it was fine to do so but please close it if I need first submit a RFC or follow another process instead.

The `cmse_nonsecure_entry` attribute is a LLVM attribute that will be available in LLVM 11. I plan to rebase on the [upgrade PR](#73526) once merged to make this one compile.

This attribute modifies code generation of the function as explained [here](https://developer.arm.com/documentation/ecm0359818/latest/) to make it work with the TrustZone-M hardware feature. This feature is only available on `thumbv8m` targets so I created an error for that if one tries to use this attribute for another target.

I added this attribute in Rust as any other LLVM attribute are added but since this one is target-dependent I am not sure if it was the best thing to do. Please indicate me if you think of other ways, like isolating target-dependent attributes together.

----------------

Tracking issue: #75835

Author: bors

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -294,6 +294,9 @@ pub fn from_fn_attrs(cx: &CodegenCx<'ll, 'tcx>, llfn: &'ll Value, instance: ty::
     if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::ALLOCATOR) {
         Attribute::NoAlias.apply_llfn(llvm::AttributePlace::ReturnValue, llfn);
     }
+    if codegen_fn_attrs.flags.contains(CodegenFnAttrFlags::CMSE_NONSECURE_ENTRY) {
+        llvm::AddFunctionAttrString(llfn, Function, const_cstr!("cmse_nonsecure_entry"));
+    }
     sanitize(cx, codegen_fn_attrs.no_sanitize, llfn);
 
     // Always annotate functions with the target-cpu they are compiled for.

compiler/rustc_codegen_llvm/src/back/write.rs

@@ -344,6 +344,13 @@ unsafe extern "C" fn diagnostic_handler(info: &DiagnosticInfo, user: *mut c_void
             .expect("non-UTF8 diagnostic");
             diag_handler.warn(&msg);
         }
+        llvm::diagnostic::Unsupported(diagnostic_ref) => {
+            let msg = llvm::build_string(|s| {
+                llvm::LLVMRustWriteDiagnosticInfoToString(diagnostic_ref, s)
+            })
+            .expect("non-UTF8 diagnostic");
+            diag_handler.err(&msg);
+        }
         llvm::diagnostic::UnknownDiagnostic(..) => {}
     }
 }

compiler/rustc_codegen_llvm/src/llvm/diagnostic.rs

@@ -118,6 +118,7 @@ pub enum Diagnostic<'ll> {
     InlineAsm(InlineAsmDiagnostic<'ll>),
     PGO(&'ll DiagnosticInfo),
     Linker(&'ll DiagnosticInfo),
+    Unsupported(&'ll DiagnosticInfo),
 
     /// LLVM has other types that we do not wrap here.
     UnknownDiagnostic(&'ll DiagnosticInfo),
@@ -159,6 +160,7 @@ impl Diagnostic<'ll> {
 
             Dk::PGOProfile => PGO(di),
             Dk::Linker => Linker(di),
+            Dk::Unsupported => Unsupported(di),
 
             _ => UnknownDiagnostic(di),
         }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -483,6 +483,7 @@ pub enum DiagnosticKind {
     OptimizationFailure,
     PGOProfile,
     Linker,
+    Unsupported,
 }
 
 /// LLVMRustDiagnosticLevel

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -37,6 +37,12 @@ pub fn AddFunctionAttrStringValue(llfn: &'a Value, idx: AttributePlace, attr: &C
     }
 }
 
+pub fn AddFunctionAttrString(llfn: &'a Value, idx: AttributePlace, attr: &CStr) {
+    unsafe {
+        LLVMRustAddFunctionAttrStringValue(llfn, idx.as_uint(), attr.as_ptr(), std::ptr::null())
+    }
+}
+
 #[derive(Copy, Clone)]
 pub enum AttributePlace {
     ReturnValue,

compiler/rustc_error_codes/src/error_codes.rs

@@ -458,6 +458,8 @@ E0770: include_str!("./error_codes/E0770.md"),
 E0771: include_str!("./error_codes/E0771.md"),
 E0773: include_str!("./error_codes/E0773.md"),
 E0774: include_str!("./error_codes/E0774.md"),
+E0775: include_str!("./error_codes/E0775.md"),
+E0776: include_str!("./error_codes/E0776.md"),
 ;
 //  E0006, // merged with E0005
 //  E0008, // cannot bind by-move into a pattern guard

compiler/rustc_error_codes/src/error_codes/E0775.md

@@ -0,0 +1,17 @@
+`#[cmse_nonsecure_entry]` is only valid for targets with the TrustZone-M
+extension.
+
+Erroneous code example:
+
+```compile_fail,E0775
+#![feature(cmse_nonsecure_entry)]
+
+#[cmse_nonsecure_entry]
+pub extern "C" fn entry_function() {}
+```
+
+To fix this error, compile your code for a Rust target that supports the
+TrustZone-M extension. The current possible targets are:
+* `thumbv8m.main-none-eabi`
+* `thumbv8m.main-none-eabihf`
+* `thumbv8m.base-none-eabi`

compiler/rustc_error_codes/src/error_codes/E0776.md

@@ -0,0 +1,13 @@
+`#[cmse_nonsecure_entry]` functions require a C ABI
+
+Erroneous code example:
+
+```compile_fail,E0776
+#![feature(cmse_nonsecure_entry)]
+
+#[no_mangle]
+#[cmse_nonsecure_entry]
+pub fn entry_function(input: Vec<u32>) {}
+```
+
+To fix this error, declare your entry function with a C ABI, using `extern "C"`.

compiler/rustc_feature/src/active.rs

@@ -590,6 +590,9 @@ declare_features! (
     /// Allows using and casting function pointers in a `const fn`.
     (active, const_fn_fn_ptr_basics, "1.48.0", Some(57563), None),
 
+    /// Allows to use the `#[cmse_nonsecure_entry]` attribute.
+    (active, cmse_nonsecure_entry, "1.48.0", Some(75835), None),
+
     // -------------------------------------------------------------------------
     // feature-group-end: actual feature gates
     // -------------------------------------------------------------------------

compiler/rustc_feature/src/builtin_attrs.rs

@@ -349,6 +349,8 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         experimental!(register_tool),
     ),
 
+    gated!(cmse_nonsecure_entry, AssumedUsed, template!(Word), experimental!(cmse_nonsecure_entry)),
+
     // ==========================================================================
     // Internal attributes: Stability, deprecation, and unsafe:
     // ==========================================================================