[Vm-dev] What problem are we trying to solve? (was: VM Maker: VMMaker.oscog-eem.790.mcz)

Eliot Miranda eliot.miranda at gmail.com
Wed Jul 2 03:44:24 UTC 2014


On Tue, Jul 1, 2014 at 7:51 PM, David T. Lewis <lewis at mail.msen.com> wrote:

>
> On Tue, Jul 01, 2014 at 05:36:54PM -0700, Eliot Miranda wrote:
> >
> > On Tue, Jul 1, 2014 at 12:21 PM, Nicolas Cellier <
> > nicolas.cellier.aka.nice at gmail.com> wrote:
> >
> > >
> > > 2014-07-01 4:22 GMT+02:00 <commits at source.squeak.org>:
> > >>
> > >> Item was changed:
> > >>   ----- Method: LargeIntegersPlugin>>cDigitSub:len:with:len:into: (in
> > >> category 'C core') -----
> > >> + cDigitSub: pByteSmall len: smallLen with: pByteLarge len: largeLen
> > >> into: pByteRes
> > >> +       | z |
> > >> - cDigitSub: pByteSmall
> > >> -               len: smallLen
> > >> -               with: pByteLarge
> > >> -               len: largeLen
> > >> -               into: pByteRes
> > >> -       | z limit |
> > >>         <var: #pByteSmall type: 'unsigned char * '>
> > >>         <var: #pByteLarge type: 'unsigned char * '>
> > >>         <var: #pByteRes type: 'unsigned char * '>
> > >>
> > >> +       z := 0. "Loop invariant is -1<=z<=1"
> > >> +       0 to: smallLen - 1 do:
> > >> -       z := 0.
> > >> -       "Loop invariant is -1<=z<=1"
> > >> -       limit := smallLen - 1.
> > >> -       0 to: limit do:
> > >>                 [:i |
> > >>                 z := z + (pByteLarge at: i) - (pByteSmall at: i).
> > >> +               pByteRes at: i put: z - (z // 256 * 256).
> "sign-tolerant
> > >> form of (z bitAnd: 255)"
> > >>
> > >
> > > Frankly, having z declared unsigned int and just doing  pByteRes at: i
> > > put: (z bitAnd: 16rFF) as I suggested would be way way simpler and will
> > > ALWAYS work.
> > > Why the hell invoke the complications of signed arithmetic when the
> > > content pByteRes is unsigned???
> > >
> >
> > I'm not maintaining the plugin.  But I broke it in fixing the unsigned
> > division anomaly.  I just wanted it to work again as quickly as possibly
> > without expending effort.  I made the minimum changes I could to keep it
> > working.  I'm much happier to have you maintain the plugin.  You have the
> > expertise and experience.
> >
> > Nicolas, my priority is to have Spur working.  I don't want to have to
> > expend lots of energy changing plugins to get Spur working.  My
> submitting
> > this fix is not an endorsement of any kind.  It's merely expediency.
> >
>
> After catching up with the email thread, I am confused as to what problem
> we
> are trying to solve.
>
> As near as I can tell, the situation is:
>
>  - The original LargeIntegersPlugin>>cDigitSub:len:with:len:into: works
> with all
>    combinations of 32/64 bit VMs and images.
>
>  - Nicolas has proposed a better implementation, along with the
> recommendation
>    to use unsigned integer C arithmetic unless there is some specific good
> reason
>    to do otherwise. This seems right in principle, although the
> implementation in
>    VMMaker-nice.348 is not working for 64-bit VMs, so some issues remain
> to be
>    resolved.
>
>  - Eliot's original question began with this:
>
>      > I recently eliminated the optimization in Slang that replaces a
>      > division by a power of two with a shift, because the code cast the
> argument
>      > to signed, and hence broke unsigned division.  That's what used to
> be
>      > controlled by the UseRightShiftForDivide class var of
> CCodeGenerator.
>      >
>      > Yesterday I found out that that optimization is the only thing
> that's
>      > keeping the LargeIntegers plugin afloat.
>
>  - At that point we had a problem in the Spur/Cog VMs that led to some
> patching
>    of the code generation and so forth, along with this email thread.
>
> So now I am confused. Is the problem that:
>
>   - The original implementation was broken?
>

Arguably yes.  It relied on incorrect Slang translation to work,
specifically the assumption that N / D where D is a power of two is
equivalent to (sqInt)N >> S where S is log2 of D.  That's wrong on two
counts:

  If N is -1 then (sqInt)-1 >> S is -1, whereas -1 / D is zero for D > 1.
  If N is unsigned (sqInt)N >> S ~= N >> S if N has the top bit set.


  - There is something different in the Spur/Cog environment that exposed
>     problems in the original implementation?
>

Yes.  I hit examples where the division optimization was generating
incorrect code, e.g. in SpurMemoryManager>>allocateOldSpaceChunkOfBytes:

initialIndex := chunkBytes / self allocationUnit.

even though chunkBytes is unsigned, Slang was generating (sqInt)chunkBytes
>> 3, which generates garbage when chunkBytes >= 2^31.  I decided to rip
out the optimization (it is incorrect in the -1 / D case) rather than hack
these.  That in turn surfaced the bug in the LargeIntegers plugin.

  - The original implementation worked, but the change to CCodeGenerator
> with
>     regard to its use of UseRightShiftForDivide resulted in a problem?
>

Yes.


>   - Something else?
>

No.

I'm pretty sure I'm missing something here.
>

You don't appear to be.
-- 
best,
Eliot
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