Fix alignment of generated UNWIND_INFO structures

I missed a few parts of the Windows documentation:

> The UNWIND_INFO structure must be DWORD aligned in memory.

> For alignment purposes, this array [unwind codes] always has an even
> number of entries, and the final entry is potentially unused. In that
> case, the array is one longer than indicated by the count of unwind
> codes field.

https://learn.microsoft.com/en-us/cpp/build/exception-handling-x64?view=msvc-170

This didn't seem to have any practical effect (unwinding tests worked
as-is), but I noticed this while rewriting some handwritten codes.

Bug: 259
Change-Id: I655f3a7f3a907797e7665a276f4926a31a1e1639
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/68407
Reviewed-by: Adam Langley <agl@google.com>
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>

1 files changed, 10 insertions, 0 deletions

diff --git a/crypto/perlasm/x86_64-xlate.pl b/crypto/perlasm/x86_64-xlate.pl
index 5b7705d..3011c91 100755
--- a/crypto/perlasm/x86_64-xlate.pl
+++ b/crypto/perlasm/x86_64-xlate.pl
@@ -903,6 +903,16 @@ $info{info_label}:
 $info{unwind_codes}
 ____
 
+	# UNWIND_INFOs must be 4-byte aligned. If needed, we must add an extra
+	# unwind code. This does not change the unwind code count. Windows
+	# documentation says "For alignment purposes, this array always has an
+	# even number of entries, and the final entry is potentially unused. In
+	# that case, the array is one longer than indicated by the count of
+	# unwind codes field."
+	if ($info{num_codes} & 1) {
+	    $xdata .= "\t.value\t0\n";
+	}
+
 	%info = ();
 	return $end_label;
     }