Implement proper C ABI lowering for RISC-V

Author: msizanoen1

Diff for: src/librustc/ty/layout.rs

@@ -2650,6 +2650,7 @@ where
                 .map(|(i, ty)| arg_of(ty, Some(i)))
                 .collect(),
             c_variadic: sig.c_variadic,
+            fixed_count: inputs.len(),
             conv,
         };
         fn_abi.adjust_for_abi(cx, sig.abi);

Diff for: src/librustc_target/abi/call/mod.rs

@@ -120,6 +120,7 @@ impl Reg {
     reg_ctor!(i16, Integer, 16);
     reg_ctor!(i32, Integer, 32);
     reg_ctor!(i64, Integer, 64);
+    reg_ctor!(i128, Integer, 128);
 
     reg_ctor!(f32, Float, 32);
     reg_ctor!(f64, Float, 64);
@@ -493,6 +494,12 @@ pub struct FnAbi<'a, Ty> {
 
     pub c_variadic: bool,
 
+    /// The count of non-variadic arguments.
+    ///
+    /// Should only be different from args.len() when c_variadic is true.
+    /// This can be used to know wether an argument is variadic or not.
+    pub fixed_count: usize,
+
     pub conv: Conv,
 }
 
@@ -534,8 +541,7 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "nvptx" => nvptx::compute_abi_info(self),
             "nvptx64" => nvptx64::compute_abi_info(self),
             "hexagon" => hexagon::compute_abi_info(self),
-            "riscv32" => riscv::compute_abi_info(self, 32),
-            "riscv64" => riscv::compute_abi_info(self, 64),
+            "riscv32" | "riscv64" => riscv::compute_abi_info(cx, self),
             "wasm32" if cx.target_spec().target_os != "emscripten" => {
                 wasm32_bindgen_compat::compute_abi_info(self)
             }