[Vm-dev] Re: how the h**l do I generate a signed shift?

[Nicolas Cellier]

2014-07-01 0:56 GMT+02:00 Nicolas Cellier <
nicolas.cellier.aka.nice at gmail.com>:

>
>
>
> 2014-07-01 0:41 GMT+02:00 Eliot Miranda <eliot.miranda at gmail.com>:
>
>>
>> Hi All,
>>
>>    sorry for that noise...
>>
>>
>> On Mon, Jun 30, 2014 at 12:24 PM, Eliot Miranda <eliot.miranda at gmail.com>
>> wrote:
>>
>>> Hi All,
>>>
>>>     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.  To whit:
>>>
>>> LargeIntegersPlugin>>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"
>>>  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)"
>>>  z := z // 256].
>>> limit := largeLen - 1.
>>> smallLen to: limit do:
>>>  [:i |
>>> z := z + (pByteLarge at: i) .
>>> pByteRes at: i put: z - (z // 256 * 256).
>>>  "sign-tolerant form of (z bitAnd: 255)"
>>> z := z // 256].
>>>
>>> The "z := z // 256"'s at the end of the loops were being generated as
>>>         z = ((sqInt) z) >> 8;
>>>  which is essential for the signed arithmetic implicit in "z := z +
>>> (pByteLarge at: i) - (pByteSmall at: i)" to work.
>>>
>>> So what's the right thing to do?
>>>
>>> In C -1 // 256 = 0, but in Smalltalk -1 // 256 = -1 (// rounds towards -
>>> infinity), whereas  (-1 quo: 256) = 0 (quo: rounds towards 0).
>>>
>>> I could modify the code generator to generate Smalltalk semantics for
>>> //, but its not pretty (one has to check signedness, check if there's a
>>> remainder, etc).
>>>
>>> What I'd like is to have a signed bitShift:.  Wait you say, bitShift: is
>>> signed.  Ah, but the code generator generates unsigned shifts for all
>>> bitShift:'s !!!!.
>>>
>>> So some ideas:
>>>
>>> 1. change bitShift: to obey the type of the receiver (Slang allows one
>>> to type variables, defaulting to a singed long). This is my preference, but
>>> it risks breaking a good handful of negative bitShift: uses in plugins
>>> (which is where I'm worried about regressions).
>>>
>>> 2. change bitShift: to obey explicit casts, generating a signed shift
>>> for
>>>    foo asInteger bitShift: expr
>>>    (self cCoerceSimple: #foo to: #sqInt) bitShift: expr
>>> Seriously?!?! this stinks.
>>>
>>> 3. write
>>> z := self cCode: [z >>= 8] inSmalltalk: [z // 256]
>>>
>>> Seriously?!?! this stinks too.
>>>
>>> Anything else that makes any sense?
>>>
>>
>> Doh:
>>
>> Intger methdos for *VMMaker
>> signedBitShift: anInteger
>> "For historical reasons Slang generates an unsigned shift from all of the
>> shift operators >>, << & bitShift:.
>>  These are too deeply entrenched to try and redefine the semantics.  So
>> instead we provide a signed bitShift:
>>  that signals to Slang that its argument should be cast to signed, not to
>> unsigned, when being shifted."
>>  ^self bitShift: anInteger
>>
>> apologies
>> --
>> best,
>> Eliot
>>
>>
> Though remember that signed right shift is theoretically implementation
> defined (though I'm not aware of a compiler/processor providing 0
> expansion), and that signed right shift behavior is undefined in case of
> overflow...
>
signed left shift is undefined in case of overflow
I should better read again before sending...

>
> http://stackoverflow.com/questions/4009885/arithmetic-bit-shift-on-a-signed-integer
> C is a very strange language... Portable?
> I just hate its (lack of) signed integer arithmetic model.
>
>
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