[Vm-dev] What problem are we trying to solve? (was: VM Maker:
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).
> > >> 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
> > 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
> 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
> to use unsigned integer C arithmetic unless there is some specific good
> 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
> - 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
> > to signed, and hence broke unsigned division. That's what used to
> > controlled by the UseRightShiftForDivide class var of
> > Yesterday I found out that that optimization is the only thing
> > keeping the LargeIntegers plugin afloat.
> - At that point we had a problem in the Spur/Cog VMs that led to some
> 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
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
> regard to its use of UseRightShiftForDivide resulted in a problem?
> - Something else?
I'm pretty sure I'm missing something here.
You don't appear to be.
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