2014-06-30 22:05 GMT+02:00 Nicolas Cellier <nicolas.cellier.aka.nice@gmail.com>:
2014-06-30 21:24 GMT+02:00 Eliot Miranda <eliot.miranda@gmail.com>:
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: pByteSmalllen: smallLenwith: pByteLargelen: largeLeninto: 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 asz = ((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 forfoo asInteger bitShift: expr(self cCoerceSimple: #foo to: #sqInt) bitShift: exprSeriously?!?! this stinks.3. writez := self cCode: [z >>= 8] inSmalltalk: [z // 256]
Seriously?!?! this stinks too.Anything else that makes any sense?--
best,EliotHi Eliot,look how I did it in the 32bits LargInt variant:
cDigitSub: pWordSmall
len: smallLen
with: pWordLarge
len: largeLen
into: pWordRes
| z limit |
<var: #pWordSmall type: 'unsigned int * '>
<var: #pWordLarge type: 'unsigned int * '>
<var: #pWordRes type: 'unsigned int * '>
<var: #z type: 'unsigned long long '>
z := 0.
limit := smallLen - 1.
0 to: limit do:
[:i |
z := z + (pWordLarge at: i) - (pWordSmall at: i).
pWordRes at: i put: (z bitAnd: 16rFFFFFFFF).
z := 0 - (z >> 63)].
limit := largeLen - 1.
smallLen to: limit do:
[:i |
z := z + (pWordLarge at: i) .
pWordRes at: i put: (z bitAnd: 16rFFFFFFFF).
z := 0 - (z >> 63)].
^0In unsigned arithmetic, all these ops are perfectly well defined, and I don't think they suck.So you can translate it back to unsigned char * and unsigned short (z >> 16)
Hmm, due to int promotion, it might be safer to declare unsigned int z, and perform z := 0 - (z >> 32).