Exotic architectures the standards committees care about

Question

Take a look at this one

Unisys ClearPath Dorado Servers

offering backward compatibility for people who have not yet migrated all their Univac software.

Key points:

• 36-bit words
• CHAR_BIT == 9
• one's complement
• 72-bit non-IEEE floating point
• separate address space for code and data
• word-addressed
• no dedicated stack pointer

Don't know if they offer a C++ compiler though, but they could.

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And now a link to a recent edition of their C manual has surfaced:

Unisys C Compiler Programming Reference Manual

Section 4.5 has a table of data types with 9, 18, 36, and 72 bits.

None of your assumptions hold for mainframes. For starters, I don't know of a mainframe which uses IEEE 754: IBM uses base 16 floating point, and both of the Unisys mainframes use base 8. The Unisys machines are a bit special in many other respects: Bo has mentioned the 2200 architecture, but the MPS architecture is even stranger: 48 bit tagged words. (Whether the word is a pointer or not depends on a bit in the word.) And the numeric representations are designed so that there is no real distinction between floating point and integral arithmetic: the floating point is base 8; it doesn't require normalization, and unlike every other floating point I've seen, it puts the decimal to the right of the mantissa, rather than the left, and uses signed magnitude for the exponent (in addition to the mantissa). With the results that an integral floating point value has (or can have) exactly the same bit representation as a signed magnitude integer. And there are no floating point arithmetic instructions: if the exponents of the two values are both 0, the instruction does integral arithmetic, otherwise, it does floating point arithmetic. (A continuation of the tagging philosophy in the architecture.) Which means that while int may occupy 48 bits, 8 of them must be 0, or the value won't be treated as an integer.

I found this link listing some systems where CHAR_BIT != 8. They include

some TI DSPs have CHAR_BIT == 16

BlueCore-5 chip (a Bluetooth chip from Cambridge Silicon Radio) which has CHAR_BIT == 16.

And of course there is a question on Stack Overflow: What platforms have something other than 8-bit char

As for non two's-complement systems there is an interesting read on comp.lang.c++.moderated. Summarized: there are platforms having ones' complement or sign and magnitude representation.

Full IEEE 754 compliance is rare in floating-point implementations. And weakening the specification in that regard allows lots of optimizations.

For example the subnorm support differers between x87 and SSE.

Optimizations like fusing a multiplication and addition which were separate in the source code slightly change the results too, but is nice optimization on some architectures.

Or on x86 strict IEEE compliance might require certain flags being set or additional transfers between floating point registers and normal memory to force it to use the specified floating point type instead of its internal 80bit floats.

And some platforms have no hardware floats at all and thus need to emulate them in software. And some of the requirements of IEEE 754 might be expensive to implement in software. In particular the rounding rules might be a problem.

My conclusion is that you don't need exotic architectures in order to get into situations were you don't always want to guarantee strict IEEE compliance. For this reason were few programming languages guarantee strict IEEE compliance.

I'm fairly sure that VAX systems are still in use. They don't support IEEE floating-point; they use their own formats. Alpha supports both VAX and IEEE floating-point formats.

Cray vector machines, like the T90, also have their own floating-point format, though newer Cray systems use IEEE. (The T90 I used was decommissioned some years ago; I don't know whether any are still in active use.)

The T90 also had/has some interesting representations for pointers and integers. A native address can only point to a 64-bit word. The C and C++ compilers had CHAR_BIT==8 (necessary because it ran Unicos, a flavor of Unix, and had to interoperate with other systems), but a native address could only point to a 64-bit word. All byte-level operations were synthesized by the compiler, and a void* or char* stored a byte offset in the high-order 3 bits of the word. And I think some integer types had padding bits.

IBM mainframes are another example.

On the other hand, these particular systems needn't necessarily preclude changes to the language standard. Cray didn't show any particular interest in upgrading its C compiler to C99; presumably the same thing applied to the C++ compiler. It might be reasonable to tighten the requirements for hosted implementations, such as requiring CHAR_BIT==8, IEEE format floating-point if not the full semantics, and 2's-complement without padding bits for signed integers. Old systems could continue to support earlier language standards (C90 didn't die when C99 came out), and the requirements could be looser for freestanding implementations (embedded systems) such as DSPs.

On the other other hand, there might be good reasons for future systems to do things that would be considered exotic today.

CHAR_BITS

According to gcc source code:

CHAR_BIT is 16 bits for 1750a, dsp16xx architectures.
CHAR_BIT is 24 bits for dsp56k architecture.
CHAR_BIT is 32 bits for c4x architecture.

You can easily find more by doing:

find $GCC_SOURCE_TREE -type f | xargs grep "#define CHAR_TYPE_SIZE"

or

find $GCC_SOURCE_TREE -type f | xargs grep "#define BITS_PER_UNIT"

if CHAR_TYPE_SIZE is appropriately defined.

IEEE 754 compliance

If target architecture doesn't support floating point instructions, gcc may generate software fallback witch is not the standard compliant by default. More than, special options (like -funsafe-math-optimizations witch also disables sign preserving for zeros) can be used.

IEEE 754 binary representation was uncommon on GPUs until recently, see GPU Floating-Point Paranoia.

EDIT: a question has been raised in the comments whether GPU floating point is relevant to the usual computer programming, unrelated to graphics. Hell, yes! Most high performance thing industrially computed today is done on GPUs; the list includes AI, data mining, neural networks, physical simulations, weather forecast, and much much more. One of the links in the comments shows why: an order of magnitude floating point advantage of GPUs.

Another thing I'd like to add, which is more relevant to the OP question: what did people do 10-15 years ago when GPU floating point was not IEEE and when there was no API such as today's OpenCL or CUDA to program GPUs? Believe it or not, early GPU computing pioneers managed to program GPUs without an API to do that! I met one of them in my company. Here's what he did: he encoded the data he needed to compute as an image with pixels representing the values he was working on, then used OpenGL to perform the operations he needed (such as "gaussian blur" to represent a convolution with a normal distribution, etc), and decoded the resulting image back into an array of results. And this still was faster than using CPU!

Things like that is what prompted NVidia to finally make their internal data binary compatible with IEEE and to introduce an API oriented on computation rather than image manipulation.