2014-07-01 0:56 GMT+02:00 Nicolas Cellier < nicolas.cellier.aka.nice@gmail.com>:
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 adivision 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:
- 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).
- 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.
- 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-... C is a very strange language... Portable? I just hate its (lack of) signed integer arithmetic model.