Add a new ABI to support cmse_nonsecure_call

This commit adds a new ABI to be selected via `extern
"C-cmse-nonsecure-call"` on function pointers in order for the compiler to
apply the corresponding cmse_nonsecure_call callsite attribute.
For Armv8-M targets supporting TrustZone-M, this will perform a
non-secure function call by saving, clearing and calling a non-secure
function pointer using the BLXNS instruction.

See the page on the unstable book for details.

Signed-off-by: Hugues de Valon <hugues.devalon@arm.com>

compiler/rustc_codegen_llvm/src/abi.rs

@@ -389,7 +389,7 @@ impl<'tcx> FnAbiLlvmExt<'tcx> for FnAbi<'tcx, Ty<'tcx>> {
 
     fn llvm_cconv(&self) -> llvm::CallConv {
         match self.conv {
-            Conv::C | Conv::Rust => llvm::CCallConv,
+            Conv::C | Conv::Rust | Conv::CCmseNonSecureCall => llvm::CCallConv,
             Conv::AmdGpuKernel => llvm::AmdGpuKernel,
             Conv::AvrInterrupt => llvm::AvrInterrupt,
             Conv::AvrNonBlockingInterrupt => llvm::AvrNonBlockingInterrupt,
@@ -546,6 +546,18 @@ impl<'tcx> FnAbiLlvmExt<'tcx> for FnAbi<'tcx, Ty<'tcx>> {
         if cconv != llvm::CCallConv {
             llvm::SetInstructionCallConv(callsite, cconv);
         }
+
+        if self.conv == Conv::CCmseNonSecureCall {
+            // This will probably get ignored on all targets but those supporting the TrustZone-M
+            // extension (thumbv8m targets).
+            unsafe {
+                llvm::AddCallSiteAttrString(
+                    callsite,
+                    llvm::AttributePlace::Function,
+                    rustc_data_structures::const_cstr!("cmse_nonsecure_call"),
+                );
+            }
+        }
     }
 }
 

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1100,6 +1100,7 @@ extern "C" {
     // Operations on call sites
     pub fn LLVMSetInstructionCallConv(Instr: &Value, CC: c_uint);
     pub fn LLVMRustAddCallSiteAttribute(Instr: &Value, index: c_uint, attr: Attribute);
+    pub fn LLVMRustAddCallSiteAttrString(Instr: &Value, index: c_uint, Name: *const c_char);
     pub fn LLVMRustAddAlignmentCallSiteAttr(Instr: &Value, index: c_uint, bytes: u32);
     pub fn LLVMRustAddDereferenceableCallSiteAttr(Instr: &Value, index: c_uint, bytes: u64);
     pub fn LLVMRustAddDereferenceableOrNullCallSiteAttr(Instr: &Value, index: c_uint, bytes: u64);

compiler/rustc_codegen_llvm/src/llvm/mod.rs

@@ -43,6 +43,10 @@ pub fn AddFunctionAttrString(llfn: &'a Value, idx: AttributePlace, attr: &CStr)
     }
 }
 
+pub fn AddCallSiteAttrString(callsite: &Value, idx: AttributePlace, attr: &CStr) {
+    unsafe { LLVMRustAddCallSiteAttrString(callsite, idx.as_uint(), attr.as_ptr()) }
+}
+
 #[derive(Copy, Clone)]
 pub enum AttributePlace {
     ReturnValue,