Re: exotic CPU hardware

[Paul Eggert]

On 12/4/20 12:47 PM, Bruno Haible wrote:
> The currently sold Unisys ClearPath Dorado
> machines contain Xeon processors.

They're Xeon processors but they come with Unisys-supplied firmware that emulate 
the older architectures, which means that C programs on these platforms should 
follow the old rules. Five or ten years ago Unisys found that this performed 
better than the chips they were designing themselves. Unisys did this for both 
their Dorado and Libra mainframes (quite different ISAs: the former is 
ones'-complement, the latter signed-magnitude), at about the same time.

Come to think of it, IBM has also done something similar, for its IBM i 
(formerly AS/400, formerly System/38) line, which originally used 48-bit words. 
Starting around 2000 they switched it to use IBM's POWER chips with 64-bit 
words. A difference is that IBM i programs have always been translated as needed 
by the runtime from a machine-independent format, instead of being interpreted 
by an emulator.

From a Gnulib viewpoint, the main problem with IBM i C is that the preferred 
pointers size is 128 bits and there is no intptr_t type; although POSIX and the 
C standard allow this, it'd break a lot of GNU code. In practice, anybody 
running GNUish code on the IBM i would run it under PASE, which can execute AIX 
programs in a mostly binary-compatible way (with 64-bit pointers and with intptr_t).

We might as well document this, if only to state more clearly what platforms 
Gnulib  (and most GNU apps) clearly don't support. I installed the attached, 
which adds a section "Exotic platforms" and documents the IBM i and the two 
Unisys platforms. I didn't bother mentioning platforms like the old x86 where 
int is 16 bits and there are near and far pointers, as the GNU coding standards 
explicitly reject tiny-int platforms.

0001-doc-mention-intptr_t-etc.-and-IBM-i.patch
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