Author: wmat Date: Wed Feb 13 05:51:34 2008 New Revision: 47061 URL: http://llvm.org/viewvc/llvm-project?rev=47061&view=rev Log: Add comments as per review feedback.
Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=47061&r1=47060&r2=47061&view=diff ============================================================================== --- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original) +++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Wed Feb 13 05:51:34 2008 @@ -2525,18 +2525,26 @@ return UnknownValue; if (AddRec->isAffine()) { - // The number of iterations for "{n,+,1} < m", is m-n. However, we don't - // know that m is >= n on input to the loop. If it is, the condition - // returns true zero times. To handle both cases, we return SMAX(m, n)-n. - // FORNOW: We only support unit strides. SCEVHandle One = SE.getIntegerSCEV(1, RHS->getType()); if (AddRec->getOperand(1) != One) return UnknownValue; + // We know the LHS is of the form {n,+,1} and the RHS is some loop-invariant + // m. So, we count the number of iterations in which {n,+,1} < m is true. + // Note that we cannot simply return max(m-n,0) because it's not safe to + // treat m-n as signed nor unsinged due to overflow possibility. + + // First, we get the value of the LHS in the first iteration: n SCEVHandle Start = AddRec->getOperand(0); - SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start) : (SCEVHandle)RHS; + // Then, we get the value of the LHS in the first iteration in which the + // above condition doesn't hold. This equals to max(m,n). + // FIXME (PR2003): we should have an "umax" operator as well. + SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS,Start) : (SCEVHandle)RHS; + + // Finally, we subtract these two values to get the number of times the + // backedge is executed: max(m,n)-n. return SE.getMinusSCEV(End, Start); } _______________________________________________ llvm-commits mailing list llvm-commits@cs.uiuc.edu http://lists.cs.uiuc.edu/mailman/listinfo/llvm-commits