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diff --git a/libphobos/libdruntime/__builtins.di b/libphobos/libdruntime/__builtins.di
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+++ b/libphobos/libdruntime/__builtins.di
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+/* This D file is implicitly imported by all ImportC source files.
+ * It provides definitions for C compiler builtin functions and declarations.
+ * The purpose is to make it unnecessary to hardwire them into the compiler.
+ * As the leading double underscore suggests, this is for internal use only.
+ *
+ * Copyright: Copyright Digital Mars 2022
+ * License:   $(HTTP www.boost.org/LICENSE_1_0.txt, Boost License 1.0).
+ * Authors:   Walter Bright
+ * Source: $(DRUNTIMESRC __builtins.d)
+ */
+
+
+module __builtins;
+
+/* gcc relies on internal __builtin_xxxx functions and templates to
+ * accomplish <stdarg.h>. D does the same thing with templates in core.stdc.stdarg.
+ * Here, we redirect the gcc builtin declarations to the equivalent
+ * ones in core.stdc.stdarg, thereby avoiding having to hardware them
+ * into the D compiler.
+ */
+
+import core.stdc.stdarg;
+
+alias va_list = core.stdc.stdarg.va_list;
+
+version (Posix)
+{
+    version (X86_64)
+        alias __va_list_tag = core.stdc.stdarg.__va_list_tag;
+}
+
+alias __builtin_va_start = core.stdc.stdarg.va_start;
+
+alias __builtin_va_end = core.stdc.stdarg.va_end;
+
+alias __builtin_va_copy = core.stdc.stdarg.va_copy;
+
+/* dmd's ImportC rewrites __builtin_va_arg into an instantiation of va_arg
+ */
+alias va_arg = core.stdc.stdarg.va_arg;