perlasm/x86_64-xlate.pl: add commentary.

Reviewed-by: Rich Salz <rsalz@openssl.org>

diff --git a/crypto/perlasm/x86_64-xlate.pl b/crypto/perlasm/x86_64-xlate.pl
index f050b23f00c1ac3b6e876c5650c5de7a20e4536d..391362d46ecbabb35729cd8c3efa0cf425364624 100755 (executable)
--- a/crypto/perlasm/x86_64-xlate.pl
+++ b/crypto/perlasm/x86_64-xlate.pl
@@ -692,7 +692,13 @@ sub rex {
    push @$opcode,($rex|0x40) if ($rex);
 }
 
-# older gas and ml64 don't handle SSE>2 instructions
+# Upon initial x86_64 introduction SSE>2 extensions were not introduced
+# yet. In order not to be bothered by tracing exact assembler versions,
+# but at the same time to provide a bare security minimum of AES-NI, we
+# hard-code some instructions. Extensions past AES-NI on the other hand
+# are traced by examining assembler version in individual perlasm
+# modules...
+
 my %regrm = (  "%eax"=>0, "%ecx"=>1, "%edx"=>2, "%ebx"=>3,
                "%esp"=>4, "%ebp"=>5, "%esi"=>6, "%edi"=>7      );