implement va_arg for arm in rustc itself

compiler/rustc_codegen_llvm/src/va_arg.rs

@@ -908,6 +908,21 @@ pub(super) fn emit_va_arg<'ll, 'tcx>(
             )
         }
         "aarch64" => emit_aapcs_va_arg(bx, addr, target_ty),
+        "arm" => {
+            // Types wider than 16 bytes are not currently supported. Clang has special logic for
+            // such types, but `VaArgSafe` is not implemented for any type that is this large.
+            assert!(bx.cx.size_of(target_ty).bytes() <= 16);
+
+            emit_ptr_va_arg(
+                bx,
+                addr,
+                target_ty,
+                PassMode::Direct,
+                SlotSize::Bytes4,
+                AllowHigherAlign::Yes,
+                ForceRightAdjust::No,
+            )
+        }
         "s390x" => emit_s390x_va_arg(bx, addr, target_ty),
         "powerpc" => emit_powerpc_va_arg(bx, addr, target_ty),
         "powerpc64" | "powerpc64le" => emit_ptr_va_arg(

tests/assembly-llvm/c-variadic-arm.rs

@@ -0,0 +1,26 @@
+//@ assembly-output: emit-asm
+//@ compile-flags: -Copt-level=3
+//@ only-arm
+//@ ignore-thumb
+//@ ignore-android
+#![no_std]
+#![crate_type = "lib"]
+#![feature(c_variadic)]
+
+// Check that the assembly that rustc generates matches what clang emits.
+
+#[unsafe(no_mangle)]
+unsafe extern "C" fn variadic(a: f64, mut args: ...) -> f64 {
+    // CHECK-LABEL: variadic
+    // CHECK: sub sp, sp
+
+    // CHECK: vldr
+    // CHECK: vadd.f64
+    // CHECK: vldr
+    // CHECK: vadd.f64
+    let b = args.arg::<f64>();
+    let c = args.arg::<f64>();
+    a + b + c
+
+    // CHECK: add sp, sp
+}