2014-07-01 0:41 GMT+02:00 Eliot Miranda <eliot.miranda@gmail.com>:

 

Hi All,

   sorry for that noise...

On Mon, Jun 30, 2014 at 12:24 PM, Eliot Miranda <eliot.miranda@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.