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Second part of pr16069

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The problem this time seems to be a thinko. We were assuming that in the CFG

A
| \
|  B
| /
C

speculating the basic block B would cause only the phi value for the B->C edge
to be speculated. That is not true, the phi's are semantically in the edges, so
if the A->B->C path is taken, any code needed for A->C is not executed and we
have to consider it too when deciding to speculate B.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183226 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Rafael Espindola
2013-06-04 14:11:59 +00:00
parent deb2e9c42f
commit babae05237
2 changed files with 24 additions and 5 deletions
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13
lib/Transforms/Utils/SimplifyCFG.cpp
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@@ -1537,18 +1537,23 @@ static bool SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB) {
Value *OrigV = PN->getIncomingValueForBlock(BB);
Value *ThenV = PN->getIncomingValueForBlock(ThenBB);
// FIXME: Try to remove some of the duplication with HoistThenElseCodeToIf.
// Skip PHIs which are trivial.
if (ThenV == OrigV)
continue;
HaveRewritablePHIs = true;
ConstantExpr *CE = dyn_cast<ConstantExpr>(ThenV);
if (!CE)
ConstantExpr *OrigCE = dyn_cast<ConstantExpr>(OrigV);
ConstantExpr *ThenCE = dyn_cast<ConstantExpr>(ThenV);
if (!OrigCE && !ThenCE)
continue; // Known safe and cheap.
if (!isSafeToSpeculativelyExecute(CE))
if ((ThenCE && !isSafeToSpeculativelyExecute(ThenCE)) ||
(OrigCE && !isSafeToSpeculativelyExecute(OrigCE)))
return false;
if (ComputeSpeculationCost(CE) > PHINodeFoldingThreshold)
unsigned OrigCost = OrigCE ? ComputeSpeculationCost(OrigCE) : 0;
unsigned ThenCost = ThenCE ? ComputeSpeculationCost(ThenCE) : 0;
if (OrigCost + ThenCost > 2 * PHINodeFoldingThreshold)
return false;
// Account for the cost of an unfolded ConstantExpr which could end up