On 13 February 2012 20:58, Eliot Miranda <[email protected]> wrote: > Hi All, > > (whitewash alert) I just had occasion to look at the bytecode generated > by the standard compiler for HashedCollection class>>#goodPrimeAtLeast:. > Here's the source, with the issue in bold: > > goodPrimeAtLeast: lowerLimit > "Answer the next good prime >= lowerlimit. > If lowerLimit is larger than the largest known good prime, > just make it odd." > | primes low mid high prime | > primes := self goodPrimes. > low := 1. > high := primes size. > lowerLimit > (primes at: high) ifTrue: [ > ^lowerLimit bitOr: 1 ]. > [ high - low <= 1 ] whileFalse: [ > mid := high + low // 2. > prime := primes at: mid. > prime = lowerLimit ifTrue: [ ^prime ]. > prime < lowerLimit > ifTrue: [ low := mid ] > ifFalse: [ high := mid ] ]. > (primes at: low) >= lowerLimit ifTrue: [ ^primes at: low ]. > ^primes at: high > > The code for this sequence is > 58 <15> pushTemp: 5 > 59 <10> pushTemp: 0 > 60 <B2> send: < > 61 <9B> jumpFalse: 66 > 62 <13> pushTemp: 3 > 63 <81 42> storeIntoTemp: 2 > 65 <92> jumpTo: 69 > 66 <13> pushTemp: 3 > 67 <81 44> storeIntoTemp: 4 > 69 <87> pop > > where-as the following would be better: > > 58 <15> pushTemp: 5 > 59 <10> pushTemp: 0 > 60 <B2> send: < > 61 <9B> jumpFalse: 66 > 62 <13> pushTemp: 3 > 63 <82 42> popIntoTemp: 2 > 65 <92> jumpTo: 69 > 66 <13> pushTemp: 3 > 67 <82 44> popIntoTemp: 4 >
[ (a) ifTrue:[ b] ifFalse: [c] ] value should answer b or c (depending on a) now if you use pop instead of store, it will break. Of course in while loop, the [ loop-body closure ] value is always discarded, so yes we can eliminate extra pop. But you can do even more: 1. loop head ... .... 61 <9B> jumpFalse: 66 62 <13> pushTemp: 3 63 <82 42> popIntoTemp: 2 ** 65 <92> jumpTo: 1 66 <13> pushTemp: 3 67 <82 44> popIntoTemp: 4 69 ... jumpTo: 1 so, we can have 1 less jump when code enters first branch. > The reason is that the code generator favours using a single pop for both > arms of the if: > > MessageNode>>sizeCodeForIf: encoder value: forValue > | thenExpr elseExpr branchSize thenSize elseSize | > thenExpr := arguments at: 1. > elseExpr := arguments at: 2. > (forValue > or: [(thenExpr isJust: NodeNil) > or: [elseExpr isJust: NodeNil]]) not > "(...not ifTrue: avoids using ifFalse: alone during this compile)" > ifTrue: "Two-armed IFs forEffect share a single pop" > [^super sizeCodeForEffect: encoder]. > ... > > MessageNode>>emitCodeForIf: stack encoder: encoder value: forValue > | thenExpr thenSize elseExpr elseSize | > thenSize := sizes at: 1. > elseSize := sizes at: 2. > (forValue not and: [elseSize * thenSize > 0]) ifTrue: > "Two-armed IFs forEffect share a single pop" > [^super emitCodeForEffect: stack encoder: encoder]. > > It would be nice if this only happened if doing so actually reduced the size > of the generated code. > -- > best, > Eliot > > > > -- Best regards, Igor Stasenko.
