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Fix PR1938 by forcing the code that uses an undefined value to branch one

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way or the other.  Rewriting the code itself prevents subsequent analysis
passes from making contradictory conclusions about the code that could 
cause an infeasible path to be made feasible.

llvm-svn: 46427
This commit is contained in:
Chris Lattner 2008-01-28 00:32:30 +00:00
parent 724cda19dc
commit 7250586ec9
2 changed files with 55 additions and 4 deletions
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23
lib/Transforms/Scalar/SCCP.cpp
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@ -1313,15 +1313,30 @@ bool SCCPSolver::ResolvedUndefsIn(Function &F) {
continue;
}
// If the edge to the first successor isn't thought to be feasible yet, mark
// it so now.
if (KnownFeasibleEdges.count(Edge(BB, TI->getSuccessor(0))))
// 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(0));
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());
} else {
SwitchInst *SI = cast<SwitchInst>(TI);
SI->setCondition(SI->getCaseValue(1));
}
return true;
}

36
test/Transforms/SCCP/2008-01-27-UndefCorrelate.ll Normal file
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@ -0,0 +1,36 @@
; RUN: llvm-as < %s | opt -sccp | llvm-dis | grep undef | count 1
; PR1938
define i32 @main() {
entry:
br label %bb
bb:
%indvar = phi i32 [ 0, %entry ], [ %k, %bb.backedge ]
%k = add i32 %indvar, 1
br i1 undef, label %cond_true, label %cond_false
cond_true:
%tmp97 = icmp slt i32 %k, 10
br i1 %tmp97, label %bb.backedge, label %bb12
bb.backedge:
br label %bb
cond_false:
%tmp9 = icmp slt i32 %k, 10
br i1 %tmp9, label %bb.backedge, label %bb12
bb12:
%tmp14 = icmp eq i32 %k, 10
br i1 %tmp14, label %cond_next18, label %cond_true17
cond_true17:
tail call void @abort( )
unreachable
cond_next18:
ret i32 0
}
declare void @abort()