Fix segfaults in release build C-variadic fns

`va_start` and `va_end` must be called to initialize/cleanup the
"spoofed" `VaList` in a Rust defined C-variadic function even if
the `VaList` is not used.

src/librustc_codegen_ssa/mir/block.rs

@@ -233,8 +233,13 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         mut bx: Bx,
     ) {
         if self.fn_ty.c_variadic {
-            if let Some(va_list) = self.va_list_ref {
-                bx.va_end(va_list.llval);
+            match self.va_list_ref {
+                Some(va_list) => {
+                    bx.va_end(va_list.llval);
+                }
+                None => {
+                    bug!("C-variadic function must have a `va_list_ref`");
+                }
             }
         }
         let llval = match self.fn_ty.ret.mode {

src/librustc_codegen_ssa/mir/mod.rs

@@ -532,13 +532,7 @@ fn arg_local_refs<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>>(
                 PassMode::Ignore(IgnoreMode::Zst) => {
                     return local(OperandRef::new_zst(bx.cx(), arg.layout));
                 }
-                PassMode::Ignore(IgnoreMode::CVarArgs) => {
-                    let backend_type = bx.cx().immediate_backend_type(arg.layout);
-                    return local(OperandRef {
-                        val: OperandValue::Immediate(bx.cx().const_undef(backend_type)),
-                        layout: arg.layout,
-                    });
-                }
+                PassMode::Ignore(IgnoreMode::CVarArgs) => {}
                 PassMode::Direct(_) => {
                     let llarg = bx.get_param(bx.llfn(), llarg_idx as c_uint);
                     bx.set_value_name(llarg, &name);

src/test/codegen/c-variadic-opt.rs

@@ -0,0 +1,19 @@
+// compile-flags: -C opt-level=3
+
+#![crate_type = "lib"]
+#![feature(c_variadic)]
+#![no_std]
+use core::ffi::VaList;
+
+extern "C" {
+    fn vprintf(fmt: *const i8, ap: VaList) -> i32;
+}
+
+// Ensure that `va_start` and `va_end` are properly injected even
+// when the "spoofed" `VaList` is not used.
+#[no_mangle]
+pub unsafe extern "C" fn c_variadic_no_use(fmt: *const i8, mut ap: ...) -> i32 {
+    // CHECK: call void @llvm.va_start
+    vprintf(fmt, ap)
+    // CHECK: call void @llvm.va_end
+}