Fix LLVM IR generated for C-variadic arguments

It is possible to create malformed LLVM IR given variadic arguments that
are aggregate types. This occurs due to improper tracking of the current
argument in the functions list of arguments.

Author: dlrobertson

src/librustc_codegen_ssa/mir/block.rs

@@ -20,7 +20,7 @@ use syntax_pos::Pos;
 
 use super::{FunctionCx, LocalRef};
 use super::place::PlaceRef;
-use super::operand::{OperandValue, OperandRef};
+use super::operand::OperandRef;
 use super::operand::OperandValue::{Pair, Ref, Immediate};
 
 /// Used by `FunctionCx::codegen_terminator` for emitting common patterns
@@ -695,18 +695,7 @@ impl<'a, 'tcx: 'a, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             // an "spoofed" `VaList`. This argument is ignored, but we need to
             // populate it with a dummy operand so that the users real arguments
             // are not overwritten.
-            let i = if sig.c_variadic && last_arg_idx.map(|x| x == i).unwrap_or(false) {
-                let layout = match self.cx.tcx().lang_items().va_list() {
-                    Some(did) => bx.cx().layout_of(bx.tcx().type_of(did)),
-                    None => bug!("`va_list` language item required for C-variadics"),
-                };
-                let op = OperandRef {
-                    val: OperandValue::Immediate(
-                        bx.cx().const_undef(bx.cx().immediate_backend_type(layout)
-                    )),
-                    layout: layout,
-                };
-                self.codegen_argument(&mut bx, op, &mut llargs, &fn_ty.args[i]);
+            let i = if sig.c_variadic && last_arg_idx.map(|x| i >= x).unwrap_or(false) {
                 if i + 1 < fn_ty.args.len() {
                     i + 1
                 } else {

src/test/codegen/issue-58881.rs

@@ -0,0 +1,21 @@
+// compile-flags: -C no-prepopulate-passes
+//
+// only-x86_64
+// ignore-windows
+
+#![crate_type = "lib"]
+
+extern "C" {
+    fn variadic_fn(_: i32, ...);
+}
+
+#[repr(C)]
+struct Foo(u8);
+#[repr(C)]
+struct Bar(u64, u64, u64);
+
+// Ensure that emit arguments of the correct type.
+pub unsafe fn test_call_variadic() {
+    // CHECK: call void (i32, ...) @variadic_fn(i32 0, i8 {{.*}}, %Bar* {{.*}})
+    variadic_fn(0, Foo(0), Bar(0, 0, 0))
+}