Re: [PATCH] Loop Overflow Bug Fix

[KHMan <keinhong@...>]

On 11/20/2017 10:29 PM, Coda Highland wrote:
> On Sun, Nov 19, 2017 at 12:13 AM, KHMan wrote:
> > Keeping 2s complement arithmetic in looping and its flaw(s) isn't
> > perfect but it's a known quantity. Patching the looping code would make it
> > "2s complement but not quite 2s complement". Either way, I hope we can have
> > some updates to the section on numerical loops in the reference manual.
>
> It's NOT 2s complement, though. It's 2s complement if you're working
> with integer loops, but it's signed magnitude if you're working with
> floating point, and Lua doesn't tell you explicitly which you're
> working with unless you ask. So it's already "signed magnitude, except
> when it's 2s complement" and Paige's patch brings it just a little bit
> closer to "always signed magnitude".

Sorry to myself for not being specific enough. I was discussing 
the integer portion, I thought that was obvious. I assumed the 
specified loop pseudo-code intentionally matches C-style loop 
behaviour, and so integer loops are more or less 2s complement 
thingies. I'm happy to follow specifications, that is, the 
pseudo-code.

All those terminology ends up just being terminology when you get 
to corner cases. Try these:

local j = 1
for i = 2^53-10,math.huge do
  print(string.format("%4d %20d", j, i))
  j = j + 1; if j == 20 then return end
end

local j = 1
for i = 2^53-10,2^53 do
  print(string.format("%4d %20d", j, i))
  j = j + 1; if j == 20 then return end
end

local j = 1
for i = 2^53-10,2^53+1 do
  print(string.format("%4d %20d", j, i))
  j = j + 1; if j == 20 then return end
end

local j = 1
for i = 2^53-10,2^53+2 do
  print(string.format("%4d %20d", j, i))
  j = j + 1; if j == 20 then return end
end

Corner cases are fun, as long as one does not use them in serious 
code.

--
Cheers,
Kein-Hong Man (esq.)
Selangor, Malaysia