[llvm] r188236 - Fix an oversight in isPotentiallyReachable where we wouldn't do any CFG-walking
Nick Lewycky
nicholas at mxc.ca
Mon Aug 12 17:03:48 PDT 2013
Author: nicholas
Date: Mon Aug 12 19:03:47 2013
New Revision: 188236
URL: http://llvm.org/viewvc/llvm-project?rev=188236&view=rev
Log:
Fix an oversight in isPotentiallyReachable where we wouldn't do any CFG-walking
to find loops if the From and To instructions were in the same block.
Refactor the code a little now that we need to fill to start the CFG-walking
algorithm with more than one starting basic block sometimes.
Special thanks to Andrew Trick for catching an error in my understanding of
natural loops in code review.
Modified:
llvm/trunk/include/llvm/Analysis/CFG.h
llvm/trunk/lib/Analysis/CFG.cpp
llvm/trunk/unittests/Analysis/CFGTest.cpp
Modified: llvm/trunk/include/llvm/Analysis/CFG.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/CFG.h?rev=188236&r1=188235&r2=188236&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/CFG.h (original)
+++ llvm/trunk/include/llvm/Analysis/CFG.h Mon Aug 12 19:03:47 2013
@@ -49,6 +49,9 @@ unsigned GetSuccessorNumber(BasicBlock *
bool isCriticalEdge(const TerminatorInst *TI, unsigned SuccNum,
bool AllowIdenticalEdges = false);
+/// \brief Determine whether instruction 'To' is reachable from 'From',
+/// returning true if uncertain.
+///
/// Determine whether there is a path from From to To within a single function.
/// Returns false only if we can prove that once 'From' has been executed then
/// 'To' can not be executed. Conservatively returns true.
@@ -64,6 +67,15 @@ bool isCriticalEdge(const TerminatorInst
bool isPotentiallyReachable(const Instruction *From, const Instruction *To,
DominatorTree *DT = 0, LoopInfo *LI = 0);
+/// \brief Determine whether block 'To' is reachable from 'From', returning
+/// true if uncertain.
+///
+/// Determine whether there is a path from From to To within a single function.
+/// Returns false only if we can prove that once 'From' has been reached then
+/// 'To' can not be executed. Conservatively returns true.
+bool isPotentiallyReachable(const BasicBlock *From, const BasicBlock *To,
+ DominatorTree *DT = 0, LoopInfo *LI = 0);
+
} // End llvm namespace
#endif
Modified: llvm/trunk/lib/Analysis/CFG.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/CFG.cpp?rev=188236&r1=188235&r2=188236&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/CFG.cpp (original)
+++ llvm/trunk/lib/Analysis/CFG.cpp Mon Aug 12 19:03:47 2013
@@ -133,53 +133,9 @@ static bool loopContainsBoth(LoopInfo *L
return L1 != NULL && L1 == L2;
}
-static bool isPotentiallyReachableSameBlock(const Instruction *A,
- const Instruction *B,
- LoopInfo *LI) {
- // The same block case is special because it's the only time we're looking
- // within a single block to see which comes first. Once we start looking at
- // multiple blocks, the first instruction of the block is reachable, so we
- // only need to determine reachability between whole blocks.
-
- const BasicBlock *BB = A->getParent();
- // If the block is in a loop then we can reach any instruction in the block
- // from any other instruction in the block by going around the backedge.
- // Check whether we're in a loop (or aren't sure).
-
- // Can't be in a loop if it's the entry block -- the entry block may not
- // have predecessors.
- bool HasLoop = BB != &BB->getParent()->getEntryBlock();
-
- // Can't be in a loop if LoopInfo doesn't know about it.
- if (LI && HasLoop) {
- HasLoop = LI->getLoopFor(BB) != 0;
- }
- if (HasLoop)
- return true;
-
- // Linear scan, start at 'A', see whether we hit 'B' or the end first.
- for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
- if (&*I == B)
- return true;
- }
- return false;
-}
-
-bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
- DominatorTree *DT, LoopInfo *LI) {
- assert(A->getParent()->getParent() == B->getParent()->getParent() &&
- "This analysis is function-local!");
-
- const BasicBlock *StopBB = B->getParent();
-
- if (A->getParent() == B->getParent())
- return isPotentiallyReachableSameBlock(A, B, LI);
-
- if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
- return true;
- if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
- return false;
-
+static bool isPotentiallyReachableInner(SmallVectorImpl<BasicBlock *> &Worklist,
+ BasicBlock *StopBB,
+ DominatorTree *DT, LoopInfo *LI) {
// When the stop block is unreachable, it's dominated from everywhere,
// regardless of whether there's a path between the two blocks.
if (DT && !DT->isReachableFromEntry(StopBB))
@@ -188,11 +144,7 @@ bool llvm::isPotentiallyReachable(const
// Limit the number of blocks we visit. The goal is to avoid run-away compile
// times on large CFGs without hampering sensible code. Arbitrarily chosen.
unsigned Limit = 32;
-
SmallSet<const BasicBlock*, 64> Visited;
- SmallVector<BasicBlock*, 32> Worklist;
- Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
-
do {
BasicBlock *BB = Worklist.pop_back_val();
if (!Visited.insert(BB))
@@ -221,7 +173,72 @@ bool llvm::isPotentiallyReachable(const
}
} while (!Worklist.empty());
- // We have exhaustived all possible paths and are certain that 'To' can not
- // be reached from 'From'.
+ // We have exhausted all possible paths and are certain that 'To' can not be
+ // reached from 'From'.
return false;
}
+
+bool llvm::isPotentiallyReachable(const BasicBlock *A, const BasicBlock *B,
+ DominatorTree *DT, LoopInfo *LI) {
+ assert(A->getParent() == B->getParent() &&
+ "This analysis is function-local!");
+
+ SmallVector<BasicBlock*, 32> Worklist;
+ Worklist.push_back(const_cast<BasicBlock*>(A));
+
+ return isPotentiallyReachableInner(Worklist, const_cast<BasicBlock*>(B),
+ DT, LI);
+}
+
+bool llvm::isPotentiallyReachable(const Instruction *A, const Instruction *B,
+ DominatorTree *DT, LoopInfo *LI) {
+ assert(A->getParent()->getParent() == B->getParent()->getParent() &&
+ "This analysis is function-local!");
+
+ SmallVector<BasicBlock*, 32> Worklist;
+
+ if (A->getParent() == B->getParent()) {
+ // The same block case is special because it's the only time we're looking
+ // within a single block to see which instruction comes first. Once we
+ // start looking at multiple blocks, the first instruction of the block is
+ // reachable, so we only need to determine reachability between whole
+ // blocks.
+ BasicBlock *BB = const_cast<BasicBlock *>(A->getParent());
+
+ // If the block is in a loop then we can reach any instruction in the block
+ // from any other instruction in the block by going around a backedge.
+ if (LI && LI->getLoopFor(BB) != 0)
+ return true;
+
+ // Linear scan, start at 'A', see whether we hit 'B' or the end first.
+ for (BasicBlock::const_iterator I = A, E = BB->end(); I != E; ++I) {
+ if (&*I == B)
+ return true;
+ }
+
+ // Can't be in a loop if it's the entry block -- the entry block may not
+ // have predecessors.
+ if (BB == &BB->getParent()->getEntryBlock())
+ return false;
+
+ // Otherwise, continue doing the normal per-BB CFG walk.
+ for (succ_iterator I = succ_begin(BB), E = succ_end(BB); I != E; ++I)
+ Worklist.push_back(*I);
+
+ if (Worklist.empty()) {
+ // We've proven that there's no path!
+ return false;
+ }
+ } else {
+ Worklist.push_back(const_cast<BasicBlock*>(A->getParent()));
+ }
+
+ if (A->getParent() == &A->getParent()->getParent()->getEntryBlock())
+ return true;
+ if (B->getParent() == &A->getParent()->getParent()->getEntryBlock())
+ return false;
+
+ return isPotentiallyReachableInner(Worklist,
+ const_cast<BasicBlock*>(B->getParent()),
+ DT, LI);
+}
Modified: llvm/trunk/unittests/Analysis/CFGTest.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Analysis/CFGTest.cpp?rev=188236&r1=188235&r2=188236&view=diff
==============================================================================
--- llvm/trunk/unittests/Analysis/CFGTest.cpp (original)
+++ llvm/trunk/unittests/Analysis/CFGTest.cpp Mon Aug 12 19:03:47 2013
@@ -147,6 +147,23 @@ TEST_F(IsPotentiallyReachableTest, SameB
ExpectPath(true);
}
+TEST_F(IsPotentiallyReachableTest, SameBlockNoLoop) {
+ ParseAssembly(
+ "define void @test() {\n"
+ "entry:\n"
+ " br label %middle\n"
+ "middle:\n"
+ " %B = bitcast i8 undef to i8\n"
+ " bitcast i8 undef to i8\n"
+ " bitcast i8 undef to i8\n"
+ " %A = bitcast i8 undef to i8\n"
+ " br label %nextblock\n"
+ "nextblock:\n"
+ " ret void\n"
+ "}\n");
+ ExpectPath(false);
+}
+
TEST_F(IsPotentiallyReachableTest, StraightNoPath) {
ParseAssembly(
"define void @test() {\n"
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