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

Nicolas Cellier nicolas.cellier.aka.nice at gmail.com
Mon Jun 30 22:56:52 UTC 2014


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...
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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