From 3a4cbe2a41f2c4233df2f061d7bcd45d7b46e1a1 Mon Sep 17 00:00:00 2001 From: Wojciech Matyjewicz Date: Wed, 13 Feb 2008 11:51:34 +0000 Subject: [PATCH] Add comments as per review feedback. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47061 91177308-0d34-0410-b5e6-96231b3b80d8 --- lib/Analysis/ScalarEvolution.cpp | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp index 65cee82a964..4a440df07b2 100644 --- a/lib/Analysis/ScalarEvolution.cpp +++ b/lib/Analysis/ScalarEvolution.cpp @@ -2525,18 +2525,26 @@ HowManyLessThans(SCEV *LHS, SCEV *RHS, const Loop *L, bool isSigned) { 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; - SCEVHandle Start = AddRec->getOperand(0); - SCEVHandle End = isSigned ? SE.getSMaxExpr(RHS, Start) : (SCEVHandle)RHS; + // 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); + + // 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); }