Re: Lua string library partially reimplemented in Lua

[Mike Pall <mikelu-0809@...>]

Alex Davies wrote:
> Mike Pall wrote:
>> Since Lua doesn't have a standard bit manipulation library (yet),
>> I took the liberty to define its semantics:
>
> Much needed addition. And I like the rules, can see how they can provide 
> for efficient optimizations, but may I ask why it's necessary to make 
> non-integral numbers behave in an implementation defined behaviour on 
> modern processors? (Is it to allow for more algebraic simplifications?)

Well, truncation would make the most sense. But this is not the
default rounding mode, and flipping it all the time can be quite
expensive.

There's another difficulty: the double -> 32 bit int conversions
return 0x80000000 for values out of range and cannot be used. But
the conversion to 64 bit ints is only available for x87 and not
for SSE2 (except in x64 mode).

So I have to use the 2^52 + 2^51 biasing trick for SSE2. But
changing the SSE rounding mode is very expensive. And roundsd
(with selectable rounding modes) is only available starting with
SSE4.1 (45nm Core2). Ah, the joys of the x86 instruction set. :-|

Also thinking ahead about implementations for non-x86 machines,
it's probably best to leave the behaviour of bit operations for
non-integral numbers implementation-defined.

BTW: the JavaScript guys now have a big problem because the
official spec defined some very rigid rules for conversions to
integers. Trying to conform _and_ make this fast proves to be a
challenge.

--Mike