On 13 February 2012 20:58, Eliot Miranda <eliot.miranda@gmail.com> 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: Â "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.