[Stdlib] Make alignedStorageForEmptyVaLists bigger.

Expand alignedStorageForEmptyVaLists to eight Doubles. This produces more predictable output for code that calls a variadic function without parameters where the function still expects them, such as passing an arbitary string to a string formatting function. It's still unsound, but this makes it more predictable.

rdar://145083971

Author: mikeash

stdlib/public/core/VarArgs.swift

@@ -715,7 +715,14 @@ final internal class __VaListBuilder {
   internal var retainer = [CVarArg]()
 #endif
 
-  internal static var alignedStorageForEmptyVaLists: Double = 0
+  // Some code will call a variadic function without passing variadic parameters
+  // where the function will still attempt to read variadic parameters. For
+  // example, calling printf with an arbitrary string as the format string. This
+  // is inherently unsound, but we'll try to be nice to such code by giving it
+  // 64 bytes of zeroes in that case.
+  internal static var alignedStorageForEmptyVaLists:
+     (Double, Double, Double, Double, Double, Double, Double, Double)
+         = (0, 0, 0, 0, 0, 0, 0, 0)
 }
 
 #endif

test/stdlib/VarArgs.swift

@@ -172,6 +172,20 @@ func test_varArgs6() {
 }
 test_varArgs6()
 
+func test_varArgs7() {
+#if canImport(Darwin) && arch(arm64)
+  // Test a workaround for format specifiers and no arguments. We supply eight
+  // words of zeroed memory to give this predictable behavior.
+  my_printf("No parameters: %ld %ld %ld %ld %ld %ld %ld %ld\n")
+#else
+  // va_list is more complicated on other targets so that behavior is not the
+  // same, skip the test by doing a fake print of the expected output.
+  my_printf("No parameters: 0 0 0 0 0 0 0 0\n")
+#endif
+  // CHECK: No parameters: 0 0 0 0 0 0 0 0
+}
+test_varArgs7()
+
 
 // CHECK: done.
 my_printf("done.")