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Re: Transitioning to 64bit, is it worth it, and how

Andrew J. Barr wrote:
> Ron Johnson wrote:
> > Andrew J. Barr wrote:
> > > Yes. Unlike true 64-bit architectures like PowerPC, there is a penalty
> > > for executing 32 bit code on amd64/em64t processors.

Most certainly incorrect.  In fact natively compiled AMD64 64-bit
compiled code *may* run faster due to better optimizations available
by having more registers available.  But typically amd64 runs both
32-bit and 64-bit at similar performance levels.

> > Penalty?  Absolutely fscking not!!!!  The A64 processes 32 bit code
> > at a similar speed to "regular" Athlon CPUs.

Most emphatically agree!

> Do you have any sources for this?

Search the web.  I am too lazy to go looking myself at this moment.

> I remember quite distinctly seeing on a mailing list, I think it was
> debian-powerpc in fact, that the only reason that AMD64 operating
> systems have an all-64-bit user land is because there is a penalty
> for executing 32-bit code, one which is not present on other true
> 64-bit architectures.

Ahem, that is incorrect.  For example Red Hat and SuSE GNU/Linux on
amd64 install a "biarch" model with dual support for both 32-bit and
64-bit.  So I would put forward that most amd64 systems support both
32-bit and 64-bit out of the box.  The reason Debian is a pure 64-bit
system is simply that the "biarch" model used by Red Hat and SuSE is
terrible.  It is too ugly for words.  There was huge discussion about
how to support amd64 for Debian.  The obvious way to go is the
"multiarch" model but no one could agree to actually do it.

Meanwhile a pure 64-bit port was pretty simple, direct and without
serious complication.  A pure 64-bit port would be quite useful to a
large segment of the population.  Even if agreement could not be
reached on either biarch or multiarch a pure 64-bit port could be made
and would be useful and could transition to multiarch in the future
when the issues were resolved with it.  But even so there was huge
resistance because it was not a perfect solution and did not support
32-bit applications out of the box.  Arguments ensued.  The amd64 port
was prevented from entering Debian Sarge.  A sad time all things

Debian went with a pure 64-bit model on amd64 out of a pragmatic
attitude that it was good enough and that pursuit of a perfect
multiarch model should not prevent using 64-bit amd64 now.  There was
never any issue of performance difference entering into the
discussion.  A review of the early days of the debian-amd64 mailing
list would contain the history of these decisions.


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