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](https://github.com/rust-lang/rust/pull/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: https://github.com/rust-lang/rust/issues/75835

1 files changed, 6 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_llvm/src/llvm/mod.rs b/compiler/rustc_codegen_llvm/src/llvm/mod.rs
index c09e3659f80..ed9b99188bb 100644
--- a/compiler/rustc_codegen_llvm/src/llvm/mod.rs
+++ b/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,