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authorChris Lattner <sabre@nondot.org>2004-02-01 06:32:28 +0000
committerChris Lattner <sabre@nondot.org>2004-02-01 06:32:28 +0000
commitc23396e8de5d28b964b0d3cd74c7e3681853445e (patch)
tree971c10e66d166336403c31d1f61d1a3038e85464 /lib
parentd4baf0f74ca54dcbb61d199f4b184d6012d7179f (diff)
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Disable tail duplication in any "hard" cases, where it might break SSA form.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11052 91177308-0d34-0410-b5e6-96231b3b80d8
Diffstat (limited to 'lib')
-rw-r--r--lib/Transforms/Scalar/TailDuplication.cpp28
1 files changed, 27 insertions, 1 deletions
diff --git a/lib/Transforms/Scalar/TailDuplication.cpp b/lib/Transforms/Scalar/TailDuplication.cpp
index 95226a2..52552d5 100644
--- a/lib/Transforms/Scalar/TailDuplication.cpp
+++ b/lib/Transforms/Scalar/TailDuplication.cpp
@@ -41,6 +41,7 @@ namespace {
bool runOnFunction(Function &F);
private:
inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
+ inline bool canEliminateUnconditionalBranch(TerminatorInst *TI);
inline void eliminateUnconditionalBranch(BranchInst *BI);
inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
BasicBlock *NewBlock);
@@ -63,7 +64,8 @@ Pass *llvm::createTailDuplicationPass() { return new TailDup(); }
bool TailDup::runOnFunction(Function &F) {
bool Changed = false;
for (Function::iterator I = F.begin(), E = F.end(); I != E; )
- if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
+ if (shouldEliminateUnconditionalBranch(I->getTerminator()) &&
+ canEliminateUnconditionalBranch(I->getTerminator())) {
eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
Changed = true;
} else {
@@ -111,6 +113,30 @@ bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
return true;
}
+/// canEliminateUnconditionalBranch - Unfortunately, the general form of tail
+/// duplication can do very bad things to SSA form, by destroying arbitrary
+/// relationships between dominators and dominator frontiers as it processes the
+/// program. The right solution for this is to have an incrementally updating
+/// dominator data structure, which can gracefully react to arbitrary
+/// "addEdge/removeEdge" changes to the CFG. Implementing this is nontrivial,
+/// however, so we just disable the transformation in cases where it is not
+/// currently safe.
+///
+bool TailDup::canEliminateUnconditionalBranch(TerminatorInst *TI) {
+ // Basically, we refuse to make the transformation if any of the values
+ // computed in the 'tail' are used in any other basic blocks.
+ BasicBlock *Tail = TI->getSuccessor(0);
+
+ for (BasicBlock::iterator I = Tail->begin(), E = Tail->end(); I != E; ++I)
+ for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
+ ++UI) {
+ Instruction *User = cast<Instruction>(*UI);
+ if (User->getParent() != Tail || isa<PHINode>(User))
+ return false;
+ }
+ return true;
+}
+
/// eliminateUnconditionalBranch - Clone the instructions from the destination
/// block into the source block, eliminating the specified unconditional branch.