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Add the missing sub identity "A-(A-B) -> B" to DAGCombine.

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This happens e.g. for code like "X - X%10" where we lower the modulo operation
to a series of multiplies and shifts that are then subtracted from X, leading to
this missed optimization.

llvm-svn: 124532
This commit is contained in:
Benjamin Kramer 2011-01-29 12:34:05 +00:00
parent 20433f6339
commit 4a40190f76
1 changed files with 3 additions and 0 deletions
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3
lib/CodeGen/SelectionDAG/DAGCombiner.cpp
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@ -1542,6 +1542,9 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
// Canonicalize (sub -1, x) -> ~x, i.e. (xor x, -1)
if (N0C && N0C->isAllOnesValue())
return DAG.getNode(ISD::XOR, N->getDebugLoc(), VT, N1, N0);
// fold A-(A-B) -> B
if (N1.getOpcode() == ISD::SUB && N0 == N1.getOperand(0))
return N1.getOperand(1);
// fold (A+B)-A -> B
if (N0.getOpcode() == ISD::ADD && N0.getOperand(0) == N1)
return N0.getOperand(1);