80 bit floating point emulation

[H. S. Teoh]

On Fri, Jul 08, 2022 at 10:34:34PM +0000, IGotD- via Digitalmars-d wrote:
> On Friday, 8 July 2022 at 22:04:38 UTC, Walter Bright wrote:
> > In order for DMD to become a proper cross compiler, it needs to be
> > able to emulate 80 bit floating point. The full x87 doesn't need to
> > be emulated, just add, sub, mul, div, cmp, neg, tst.
[...]
> Is this something that is important? Now DMD has native support for
> the x87 80-bit floats but is this something worth porting and
> emulating with other architectures. It's something that I think is
> going to be hardly used so wouldn't be better to letting the compiler
> emit an error instead telling the user that 80-bit floats isn't
> supported?

The keyword here is "cross compiler".  Suppose you're compiling on a
platform that doesn't have 80-bit floats, but you're targeting x86.
You'd want your CTFE float values to match the behavior of 80-bit
floats, 'cos otherwise there would be a mismatch between what the
compiler computes at compile-time vs. what the program would have
produced when run on the target x86 platform.


> Keep in mind that the 80-bit float is like an one off in floating
> point history. Some other architectures supports 128-bit floats which
> is more common. I would much rather that we emulate 128-bit floats
> instead than the 80-bit float.
> 
> Also, Intel themselves have more or less deprecated their own x87 in
> favor of the newer SSE floating point instructions. That leads to the
> question, shouldn't DMD do the same? Depreciate 80-bit floats?

This is also a valid point. :-P  I've also been shying away from 80-bit
floats recently because they are slow (80-bit hardware hasn't been
improved since decades ago, 'cos nobody is invested in continuing to
develop it anymore), and also non-standard (only useful for intermediate
computations, not useful for transmission because the target system may
not support it -- also, results are peculiar to x86 CPUs and differ from
results obtained on other CPUs).


T

-- 
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