+# on TODO list. It should be noted that for safe deployment in
+# 32-bit *mutli-threaded* context asyncronous signals should be
+# blocked upon entry to SHA512 block routine. This is because
+# 32-bit signaling procedure invalidates upper halves of GPRs.
+# Context switch procedure preserves them, but not signaling:-(
+
+# Second version is true multi-thread safe. Trouble with the original
+# version was that it was using thread local storage pointer register.
+# Well, it scrupulously preserved it, but the problem would arise the
+# moment asynchronous signal was delivered and signal handler would
+# dereference the TLS pointer. While it's never the case in openssl
+# application or test suite, we have to respect this scenario and not
+# use TLS pointer register. Alternative would be to require caller to
+# block signals prior calling this routine. For the record, in 32-bit
+# context R2 serves as TLS pointer, while in 64-bit context - R13.