[patch] fix comment on findGCD

[Mingjie Xing]

Thank you. I've commited.

--- llvm/trunk/lib/Analysis/DependenceAnalysis.cpp (original)
+++ llvm/trunk/lib/Analysis/DependenceAnalysis.cpp Mon Jan  6 19:54:16 2014
@@ -1275,8 +1275,8 @@ bool DependenceAnalysis::weakCrossingSIV
 //
 // Program 2.1, page 29.
 // Computes the GCD of AM and BM.
-// Also finds a solution to the equation ax - by = gdc(a, b).
-// Returns true iff the gcd divides Delta.
+// Also finds a solution to the equation ax - by = gcd(a, b).
+// Returns true if dependence disproved; i.e., gcd does not divide Delta.
 static
 bool findGCD(unsigned Bits, APInt AM, APInt BM, APInt Delta,
              APInt &G, APInt &X, APInt &Y) {

See http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20140106/200614.html.

Best Regards
Mingjie

2014/1/4 Preston Briggs <preston.briggs at gmail.com>:
> The patch looks ok to me,
> but it also looks more complex than simply fixing the comment.
> Rather than negating the conditional each time we call findGCD,
> let's leave the code alone and make the comment say something like:
>
> findGCD returns true if dependence has been disproven;
> that is, if gcd does not divide Delta.
>
>
> Preston
>
>
>
> On Thu, Jan 2, 2014 at 10:31 PM, Mingjie Xing <mingjie.xing at gmail.com>
> wrote:
>>
>> Hi,
>>
>> While reading source code again, I think it is not a simple comment
>> typo. The function
>> findGCD always finds the GCD. So the boolean return value depends on the
>> meaning
>> what we want it to be. We say the Diophantine equation has a solution
>> iff GCD divides
>> Delta. And the dependence is disproved if there is no solution.
>>
>> So if we want findGCD return true when GCD divides Delta, souce code
>> should be fixed
>> as the following patch,
>>
>> Index: lib/Analysis/DependenceAnalysis.cpp
>> ===================================================================
>> --- lib/Analysis/DependenceAnalysis.cpp (revision 198188)
>> +++ lib/Analysis/DependenceAnalysis.cpp (working copy)
>> @@ -1275,7 +1275,7 @@
>>  //
>>  // Program 2.1, page 29.
>>  // Computes the GCD of AM and BM.
>> -// Also finds a solution to the equation ax - by = gdc(a, b).
>> +// Also finds a solution to the equation ax - by = gcd(a, b).
>>  // Returns true iff the gcd divides Delta.
>>  static
>>  bool findGCD(unsigned Bits, APInt AM, APInt BM, APInt Delta,
>> @@ -1301,11 +1301,11 @@
>>    // make sure gcd divides Delta
>>    R = Delta.srem(G);
>>    if (R != 0)
>> -    return true; // gcd doesn't divide Delta, no dependence
>> +    return false; // gcd doesn't divide Delta, no dependence
>>    Q = Delta.sdiv(G);
>>    X *= Q;
>>    Y *= Q;
>> -  return false;
>> +  return true;
>>  }
>>
>>
>> @@ -1398,7 +1398,7 @@
>>    APInt AM = ConstSrcCoeff->getValue()->getValue();
>>    APInt BM = ConstDstCoeff->getValue()->getValue();
>>    unsigned Bits = AM.getBitWidth();
>> -  if (findGCD(Bits, AM, BM, ConstDelta->getValue()->getValue(), G, X, Y))
>> {
>> +  if (!findGCD(Bits, AM, BM, ConstDelta->getValue()->getValue(), G, X,
>> Y)) {
>>      // gcd doesn't divide Delta, no dependence
>>      ++ExactSIVindependence;
>>      ++ExactSIVsuccesses;
>> @@ -1802,7 +1802,7 @@
>>    APInt AM = ConstSrcCoeff->getValue()->getValue();
>>    APInt BM = ConstDstCoeff->getValue()->getValue();
>>    unsigned Bits = AM.getBitWidth();
>> -  if (findGCD(Bits, AM, BM, ConstDelta->getValue()->getValue(), G, X, Y))
>> {
>> +  if (!findGCD(Bits, AM, BM, ConstDelta->getValue()->getValue(), G, X,
>> Y)) {
>>      // gcd doesn't divide Delta, no dependence
>>      ++ExactRDIVindependence;
>>      return true;
>>
>> Best Regards,
>> Mingjie
>
>