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Switch lowering: handle zero-weight branch probabilities 

After r236617, branch probabilities are no longer guaranteed to be >= 1. This
patch makes the swich lowering code handle that correctly, without bumping the
branch weights by 1 which might cause overflow and skews the probabilities.

Covered by @zero_weight_tree in test/CodeGen/X86/switch.ll.

llvm-svn: 236739
This commit is contained in:
Hans Wennborg 2015-05-07 15:47:15 +00:00
parent 04d9e653ed
commit 44faaa7aa4
1 changed files with 7 additions and 16 deletions
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23
llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
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@ -7927,17 +7927,15 @@ void SelectionDAGBuilder::splitWorkItem(SwitchWorkList &WorkList,
// Move LastLeft and FirstRight towards each other from opposite directions to
// find a partitioning of the clusters which balances the weight on both
// sides.
// sides. If LeftWeight and RightWeight are equal, alternate which side is
// taken to ensure 0-weight nodes are distributed evenly.
unsigned I = 0;
while (LastLeft + 1 < FirstRight) {
// Zero-weight nodes would cause skewed trees since they don't affect
// LeftWeight or RightWeight.
assert(LastLeft->Weight != 0);
assert(FirstRight->Weight != 0);
if (LeftWeight < RightWeight)
if (LeftWeight < RightWeight || (LeftWeight == RightWeight && (I & 1)))
LeftWeight += (++LastLeft)->Weight;
else
RightWeight += (--FirstRight)->Weight;
I++;
}
assert(LastLeft + 1 == FirstRight);
assert(LastLeft >= W.FirstCluster);
@ -8007,15 +8005,8 @@ void SelectionDAGBuilder::visitSwitch(const SwitchInst &SI) {
for (auto I : SI.cases()) {
MachineBasicBlock *Succ = FuncInfo.MBBMap[I.getCaseSuccessor()];
const ConstantInt *CaseVal = I.getCaseValue();
uint32_t Weight = 1;
if (BPI) {
// TODO - BPI used to guarantee non-zero weights, but this produces
// information loss (see PR 22718). Since we can't handle zero weights
// here, use the same flooring mechanism previously used by BPI.
Weight = std::max(
1u, BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex()));
assert(Weight <= UINT32_MAX / SI.getNumSuccessors());
}
uint32_t Weight =
BPI ? BPI->getEdgeWeight(SI.getParent(), I.getSuccessorIndex()) : 0;
Clusters.push_back(CaseCluster::range(CaseVal, CaseVal, Succ, Weight));
}