Re: New back end ia16: 16-bit Intel x86

[rridge@csclub.uwaterloo.ca (Ross Ridge)]

Rask Ingemann Lambertsen writes:
>You know, over the years Intel (and for x86-64, AMD) fixed (or worked
>around) many shortcomings of the i8086:
>
>   1) Addressing modes, most notably the lack of sp relative addressing.
>   2) Tight register constraints.
>   3) Registers with no 8-bit move instructions.
>   4) Lack (or prohibitive slowness) of useful instructions.

Except for the first point, your port should take advantage of the
improvements made in those areas.

As I see it, there's two groups of developers who would want to use
your port.  Thoses writing embedded applications targeting the 80186
and those writing low-level 16-bit system code, like boot loaders.
The later group of people are targetting at least i386 processors, and
can often assume more modern processors are being used.  I have 16-bit
C code, that when compiled with my 16 year-old Borland C++ compiler,
produces code that uses 32-bit registers and instructions.  I think
features like this would be a lot more useful today in a 16-bit IA-32
compiler than getting the RTX costs right on a processor like the 80286
that hasn't been made in a couple of decades.

>> Ideally a 16-bit IA-32 port should
>> be merged into the i386 port, just like the x86_64 port.
>
>   Q: How do I know you didn't discuss that with the i386 maintainers?

Like I said, while ideally they should be merged, I can see good reasons
for not doing so.  I'm not really arguing for that, at least not at
this point.

>Now, x87 hardware floating point gives an easy upportunity for sharing
>code between 16-bit and 32-bit modes[1], but with reg-stack.c already split
>out from the i386 back end and 16-bit x87 not having all that many
>instructions to write patterns for, I doubt the saving is worth the amount
>of extra grot that a merge of ia16 and i386 would produce.

Ideally, the 16-bit port should support the entire IA-32 instruction set,
and I don't see that happening if the ports aren't merged.  Maybe it
will be never worth the trouble, but it would make your port a lot more
useful to me.

					Ross Ridge