Add PTX calling conventions to generated LLVM IR

Fixes a TODO. Shouldn't affect anything but why not.

Author: LegNeato

crates/rustc_codegen_nvvm/src/context.rs

@@ -317,8 +317,7 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
         }
     }
 
-    /// Declare a function. All functions use the default ABI, NVVM ignores any calling convention markers.
-    /// All functions calls are generated according to the PTX calling convention.
+    /// Declare a function with appropriate PTX calling conventions.
     /// <https://docs.nvidia.com/cuda/nvvm-ir-spec/index.html#calling-conventions>
     pub fn declare_fn(
         &self,
@@ -332,8 +331,12 @@ impl<'ll, 'tcx> CodegenCx<'ll, 'tcx> {
 
         trace!("Declaring function `{}` with ty `{:?}`", name, ty);
 
-        // TODO(RDambrosio016): we should probably still generate accurate calling conv for functions
-        // just to make it easier to debug IR and/or make it more compatible with compiling using llvm
+        // Set PTX device calling convention for all functions declared here.
+        // Kernel functions will have their calling convention overridden in mono_item.rs
+        unsafe {
+            llvm::LLVMSetFunctionCallConv(llfn, llvm::PtxCallConv::Device as u32);
+        }
+
         llvm::SetUnnamedAddress(llfn, llvm::UnnamedAddr::Global);
         if let Some(abi) = fn_abi {
             abi.apply_attrs_llfn(self, llfn);

crates/rustc_codegen_nvvm/src/llvm.rs

@@ -206,6 +206,20 @@ pub(crate) enum Visibility {
     Protected = 2,
 }
 
+/// PTX/NVPTX calling conventions from LLVM
+/// See: https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/IR/CallingConv.h
+///
+/// While NVVM doesn't strictly require these calling conventions to be set
+/// (it generates PTX according to its own rules), we set them anyway to
+/// make the generated LLVM IR more accurate and easier to debug.
+#[repr(u32)]
+pub(crate) enum PtxCallConv {
+    /// PTX kernel calling convention
+    Kernel = 71,
+    /// PTX device calling convention
+    Device = 72,
+}
+
 /// LLVMUnnamedAddr
 #[repr(C)]
 pub(crate) enum UnnamedAddr {

crates/rustc_codegen_nvvm/src/mono_item.rs

@@ -98,6 +98,9 @@ impl<'tcx> PreDefineCodegenMethods<'tcx> for CodegenCx<'_, 'tcx> {
             // to nvvm.annotations per the nvvm ir docs.
             if nvvm_attrs.kernel {
                 trace!("Marking function `{:?}` as a kernel", symbol_name);
+                llvm::LLVMSetFunctionCallConv(lldecl, llvm::PtxCallConv::Kernel as u32);
+
+                // Add kernel metadata for NVVM
                 let kernel = llvm::LLVMMDStringInContext(self.llcx, "kernel".as_ptr().cast(), 6);
                 let mdvals = &[lldecl, kernel, self.const_i32(1)];
                 let node =