Re: Floating-point constant folding in Emacs byte compiler

[Paul Eggert]

Stefan Monnier wrote:
> I'm pretty sure those runtime differences are not on-purpose

Although that's most likely true for the examples I gave, it's a problematic 
assumption for code that intentionally inspects runtime behavior.

I mention this because my recently-installed byte-opt patch fixed a problem of 
this sort with lsh. byte-opt was evaluating (lsh -1 -1) at compile-time, whereas 
the intent of the (lsh -1 -1) was to calculate most-positive-fixnum at run-time. 
It's not hard to imagine similar sorts of code to inspect properties of the 
machine's floating-point behavior, code that would yield the wrong value if 
compiled on a machine with different floating-point properties.

Nowadays Emacs hosts almost invariably use IEEE floating point, and I suppose we 
could hardwire that assumption into Emacs. Even so, IEEE floating-point is not 
bit-for-bit identical on all platforms, so if we're constant-folding 
floating-point expressions surely we should at least document that we're doing 
so, so that programmers know that they can't rely on constant expressions 
yielding a different value than what run-time evaluation would deliver.