ppc64-mont.pl: commentary update.

# December 2009

# Adapted for 32-bit build this module delivers 25-120%, more for

# longer keys, performance improvement on 1.8GHz PPC970. However!

# This implementation utilizes even 64-bit integer operations and

# trouble is that most PPC operating systems don't preserve upper

# halves of general purpose registers upong signal delivery. They do

# preserve them upon context switch, but not signalling:-( This means

# that asynchronous signals have to be blocked upon entry to this

# subroutine. Signal masking (and complementary unmasking) has quite

# an impact on performance, naturally larger for shorter keys. It's

# so severe that 512-bit key performance can be as low as 1/3 of

# expected one. This is why this routine can be engaged for longer

# key operations only, see crypto/ppccap.c for further details.

# Alternative is to break dependence on upper halves on GPRs...

# MacOS X is an exception from this and doesn't require signal

# masking, and that's where above improvement coefficients were

# collected.

# Adapted for 32-bit build this module delivers 25-120%, yes, more

# than *twice* for longer keys, performance improvement over 32-bit

# ppc-mont.pl on 1.8GHz PPC970. However! This implementation utilizes

# even 64-bit integer operations and the trouble is that most PPC

# operating systems don't preserve upper halves of general purpose

# registers upon 32-bit signal delivery. They do preserve them upon

# context switch, but not signalling:-( This means that asynchronous

# signals have to be blocked upon entry to this subroutine. Signal

# masking (and of course complementary unmasking) has quite an impact

# on performance, naturally larger for shorter keys. It's so severe

# that 512-bit key performance can be as low as 1/3 of expected one.

# This is why this routine can be engaged for longer key operations

# only on these OSes, see crypto/ppccap.c for further details. MacOS X

# is an exception from this and doesn't require signal masking, and

# that's where above improvement coefficients were collected. For

# others alternative would be to break dependence on upper halves of

# GPRs by sticking to 32-bit integer operations...

$flavour = shift;