[LLVMdev] help with pointer-to-array conversion

[Naftali Schwartz]

I now understand that IndVarSimplify.cpp is capable of reproducing array 
references when the pointer initialization from the array address is found 
inside the immediately enclosing loop, such that in the following code:

int A[20000], B[100], Z;
int main()
{
         int i, j, *a, *b;
         for ( a = &A[0], i = 0; i != 200; i++ )
                 for ( b = &B[0], j = 0; j != 100; j++ )
                         Z += *a++ * *b++;
}

the pointer b can be transformed into an offset from B[], but a cannot be 
transformed into an offset from A[] because the initialization is two 
levels up.

I started poking around in IndVarSimplify.cpp and I have a patch which 
should (at least partially) enable the transformation for a as well, but I 
find that although the code looks OK in debug printouts, the gccas output 
is unchanged.  So two questions:  1) Am I on the right track with this 
transformation (patch enclosed) and 2) why does the output not reflect my 
changes?

Index: lib/Transforms/Scalar/IndVarSimplify.cpp
===================================================================
RCS file: /var/cvs/llvm/llvm/lib/Transforms/Scalar/IndVarSimplify.cpp,v
retrieving revision 1.78
diff -r1.78 IndVarSimplify.cpp
310c310
<         runOnLoop(*I);
---
>         runOnLoop(LI, *I);
322,323c322,323
<     void runOnLoop(Loop *L);
<     void EliminatePointerRecurrence(PHINode *PN, BasicBlock *Preheader,
---
>     void runOnLoop(LoopInfo *LI, Loop *L);
>     void EliminatePointerRecurrence(LoopInfo *LI, Loop *L, PHINode *PN, 
BasicBlock *Preheader,
361c361
< void IndVarSimplify::EliminatePointerRecurrence(PHINode *PN,
---
> void IndVarSimplify::EliminatePointerRecurrence(LoopInfo *LI, Loop *L, 
PHINode *PN,
366a367
> Value *saved_trip_count = L->getTripCount(); // save before it's 
clobbered
392c393,408
<
---
> std::cerr << "\tGEPI: " << *GEPI;
>       if (PHINode *phi = dyn_cast<PHINode>(GEPI->getOperand(0)))
>       {
>               if (ConstantExpr *CE = 
dyn_cast<ConstantExpr>(phi->getIncomingValue(0)))
>               if (CE->getOpcode() == Instruction::GetElementPtr) // We 
want this to become explicit...
>               {
> std::cerr << "Block Before: " << *GEPI->getParent();
>                       Instruction *mul = 
BinaryOperator::createMul(saved_trip_count,
>                               LI->getLoopFor( phi->getIncomingBlock(1) )
>                               ->getCanonicalInductionVariable(), "mul.", 
GEPI);
>                       Instruction *add = BinaryOperator::createAdd(mul, 
NewAdd, "add.", GEPI);
>                       GEPI->setOperand( 0, phi->getIncomingValue(0) );
>                       GEPI->setOperand( 1, add );
> std::cerr << "Block After: " << *GEPI->getParent();
>               }
>       }
605c621,622
< void IndVarSimplify::runOnLoop(Loop *L) {
---
> void IndVarSimplify::runOnLoop(LoopInfo *LI, Loop *L) {
>
617c634
<       EliminatePointerRecurrence(PN, Preheader, DeadInsts);
---
>       EliminatePointerRecurrence(LI, L, PN, Preheader, DeadInsts);
626c643
<     runOnLoop(*I);
---
>     runOnLoop(LI,*I);
652,653c669,670
<         if (SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SCEV))
<           if (AR->getNumOperands() == 2 && 
isa<SCEVConstant>(AR->getOperand(1)))
---
>         if (1)//SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(SCEV))
>           if (1)//AR->getNumOperands() == 2 && 
isa<SCEVConstant>(AR->getOperand(1)))

Naftali