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fix a really nasty bug that Evan was tracking in SCCP. When resolving

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undefs in branches/switches, we have two cases: a branch on a literal
undef or a branch on a symbolic value which is undef.  If we have a
literal undef, the code was correct: forcing it to a constant is the
right thing to do.

If we have a branch on a symbolic value that is undef, we should force
the symbolic value to a constant, which then makes the successor block
live.  Forcing the condition of the branch to being a constant isn't 
safe if later paths become live and the value becomes overdefined.  This
is the case that 'forcedconstant' is designed to handle, so just use it.

This fixes rdar://7765019 but there is no good testcase for this, the
one I have is too insane to be useful in the future.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@100478 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2010-04-05 22:14:48 +00:00
parent 9e185800c0
commit e597f00e1a
1 changed files with 32 additions and 29 deletions
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61
lib/Transforms/Scalar/SCCP.cpp
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@ -1521,45 +1521,48 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
}
}
// Check to see if we have a branch or switch on an undefined value. If so
// we force the branch to go one way or the other to make the successor
// values live. It doesn't really matter which way we force it.
TerminatorInst *TI = BB->getTerminator();
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
if (!BI->isConditional()) continue;
if (!getValueState(BI->getCondition()).isUndefined())
continue;
} else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
// If the input to SCCP is actually branch on undef, fix the undef to
// false.
if (isa<UndefValue>(BI->getCondition())) {
BI->setCondition(ConstantInt::getFalse(BI->getContext()));
markEdgeExecutable(BB, TI->getSuccessor(1));
return true;
}
// Otherwise, it is a branch on a symbolic value which is currently
// considered to be undef. Handle this by forcing the input value to the
// branch to false.
markForcedConstant(BI->getCondition(),
ConstantInt::getFalse(TI->getContext()));
return true;
}
if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
if (SI->getNumSuccessors() < 2) // no cases
continue;
if (!getValueState(SI->getCondition()).isUndefined())
continue;
} else {
continue;
// If the input to SCCP is actually switch on undef, fix the undef to
// the first constant.
if (isa<UndefValue>(SI->getCondition())) {
SI->setCondition(SI->getCaseValue(1));
markEdgeExecutable(BB, TI->getSuccessor(1));
return true;
}
markForcedConstant(SI->getCondition(), SI->getCaseValue(1));
return true;
}
// If the edge to the second successor isn't thought to be feasible yet,
// mark it so now. We pick the second one so that this goes to some
// enumerated value in a switch instead of going to the default destination.
if (KnownFeasibleEdges.count(Edge(BB, TI->getSuccessor(1))))
continue;
// Otherwise, it isn't already thought to be feasible. Mark it as such now
// and return. This will make other blocks reachable, which will allow new
// values to be discovered and existing ones to be moved in the lattice.
markEdgeExecutable(BB, TI->getSuccessor(1));
// This must be a conditional branch of switch on undef. At this point,
// force the old terminator to branch to the first successor. This is
// required because we are now influencing the dataflow of the function with
// the assumption that this edge is taken. If we leave the branch condition
// as undef, then further analysis could think the undef went another way
// leading to an inconsistent set of conclusions.
if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
BI->setCondition(ConstantInt::getFalse(BI->getContext()));
} else {
SwitchInst *SI = cast<SwitchInst>(TI);
SI->setCondition(SI->getCaseValue(1));
}
return true;
}
return false;