base: Add a macro to expand parameter pack expressions in order.

This wraps up the strange compiler goop necessary to evaluate
expressions based on parameter pack expansions in order.

Change-Id: I16fbd53d22492a8c20524e3ef8bb8ff5e5d59b14
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42033
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/base/compiler.hh

@@ -112,6 +112,16 @@
 // we can't do that with direct substitution.
 #  define M5_LIKELY(cond) __builtin_expect(!!(cond), 1)
 #  define M5_UNLIKELY(cond) __builtin_expect(!!(cond), 0)
+
+// Evaluate an expanded parameter pack in order. Multiple arguments can be
+// passed in which be evaluated in order relative to each other as a group.
+// The argument(s) must include a parameter pack to expand. This works because
+// the elements of a brace inclosed initializer list are evaluated in order,
+// as are the arguments to the comma operator, which evaluates to the last
+// value. This is compiler specific because it uses variadic macros.
+#define M5_FOR_EACH_IN_PACK(...) \
+do { M5_VAR_USED int i[] = { 0, ((void)(__VA_ARGS__), 0)... }; } while (false)
+
 #else
 #  error "Don't know what to do for your compiler."
 #endif