[LoopInstSimplify] Re-implement the core logic of loop-instsimplify to
be both simpler and substantially more efficient.
Rather than use a hand-rolled iteration technique that isn't quite the
same as RPO, use the pre-built RPO loop body traversal utility.
Once visiting the loop body in RPO, we can assert that we visit defs
before uses reliably. When this is the case, the only need to iterate is
when simplifying a def that is used by a PHI node along a back-edge.
With this patch, the first pass over the loop body is just a complete
simplification of every instruction across the loop body. When we
encounter a use of a simplified instruction that stems from a PHI node
in the loop body that has already been visited (due to some cyclic CFG,
potentially the loop itself, or a nested loop, or unstructured control
flow), we recall that specific PHI node for the second iteration.
Nothing else needs to be preserved from iteration to iteration.
On the second and later iterations, only instructions known to have
simplified inputs are considered, each time starting from a set of PHIs
that had simplified inputs along the backedges.
Dead instructions are collected along the way, but deleted in a batch at
the end of each iteration making the iterations themselves substantially
simpler. This uses a new batch API for recursively deleting dead
instructions.
This alsa changes the routine to visit subloops. Because simplification
is fundamentally transitive, we may need to visit the entire loop body,
including subloops, to handle knock-on simplification.
I've added a basic test file that helps demonstrate that all of these
changes work. It includes both straight-forward loops with
simplifications as well as interesting PHI-structures, CFG-structures,
and a nested loop case.
Differential Revision: https://reviews.llvm.org/D47407
llvm-svn: 333461
diff --git a/llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp b/llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp
index 661c4fe..da46be9 100644
--- a/llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopInstSimplify.cpp
@@ -20,6 +20,7 @@
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/Utils/Local.h"
@@ -45,118 +46,116 @@
STATISTIC(NumSimplified, "Number of redundant instructions simplified");
-static bool SimplifyLoopInst(Loop *L, DominatorTree *DT, LoopInfo *LI,
- AssumptionCache *AC,
- const TargetLibraryInfo *TLI) {
- SmallVector<BasicBlock *, 8> ExitBlocks;
- L->getUniqueExitBlocks(ExitBlocks);
- array_pod_sort(ExitBlocks.begin(), ExitBlocks.end());
+static bool simplifyLoopInst(Loop &L, DominatorTree &DT, LoopInfo &LI,
+ AssumptionCache &AC,
+ const TargetLibraryInfo &TLI) {
+ const DataLayout &DL = L.getHeader()->getModule()->getDataLayout();
+ SimplifyQuery SQ(DL, &TLI, &DT, &AC);
+ // On the first pass over the loop body we try to simplify every instruction.
+ // On subsequent passes, we can restrict this to only simplifying instructions
+ // where the inputs have been updated. We end up needing two sets: one
+ // containing the instructions we are simplifying in *this* pass, and one for
+ // the instructions we will want to simplify in the *next* pass. We use
+ // pointers so we can swap between two stably allocated sets.
SmallPtrSet<const Instruction *, 8> S1, S2, *ToSimplify = &S1, *Next = &S2;
- // The bit we are stealing from the pointer represents whether this basic
- // block is the header of a subloop, in which case we only process its phis.
- using WorklistItem = PointerIntPair<BasicBlock *, 1>;
- SmallVector<WorklistItem, 16> VisitStack;
- SmallPtrSet<BasicBlock *, 32> Visited;
+ // Track the PHI nodes that have already been visited during each iteration so
+ // that we can identify when it is necessary to iterate.
+ SmallPtrSet<PHINode *, 4> VisitedPHIs;
+
+ // While simplifying we may discover dead code or cause code to become dead.
+ // Keep track of all such instructions and we will delete them at the end.
+ SmallVector<Instruction *, 8> DeadInsts;
+
+ // First we want to create an RPO traversal of the loop body. By processing in
+ // RPO we can ensure that definitions are processed prior to uses (for non PHI
+ // uses) in all cases. This ensures we maximize the simplifications in each
+ // iteration over the loop and minimizes the possible causes for continuing to
+ // iterate.
+ LoopBlocksRPO RPOT(&L);
+ RPOT.perform(&LI);
bool Changed = false;
- bool LocalChanged;
- do {
- LocalChanged = false;
+ for (;;) {
+ for (BasicBlock *BB : RPOT) {
+ for (Instruction &I : *BB) {
+ if (auto *PI = dyn_cast<PHINode>(&I))
+ VisitedPHIs.insert(PI);
- VisitStack.clear();
- Visited.clear();
-
- VisitStack.push_back(WorklistItem(L->getHeader(), false));
-
- while (!VisitStack.empty()) {
- WorklistItem Item = VisitStack.pop_back_val();
- BasicBlock *BB = Item.getPointer();
- bool IsSubloopHeader = Item.getInt();
- const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
-
- // Simplify instructions in the current basic block.
- for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
- Instruction *I = &*BI++;
-
- // The first time through the loop ToSimplify is empty and we try to
- // simplify all instructions. On later iterations ToSimplify is not
- // empty and we only bother simplifying instructions that are in it.
- if (!ToSimplify->empty() && !ToSimplify->count(I))
- continue;
-
- // Don't bother simplifying unused instructions.
- if (!I->use_empty()) {
- Value *V = SimplifyInstruction(I, {DL, TLI, DT, AC});
- if (V && LI->replacementPreservesLCSSAForm(I, V)) {
- // Mark all uses for resimplification next time round the loop.
- for (User *U : I->users())
- Next->insert(cast<Instruction>(U));
-
- I->replaceAllUsesWith(V);
- LocalChanged = true;
- ++NumSimplified;
- }
- }
- if (RecursivelyDeleteTriviallyDeadInstructions(I, TLI)) {
- // RecursivelyDeleteTriviallyDeadInstruction can remove more than one
- // instruction, so simply incrementing the iterator does not work.
- // When instructions get deleted re-iterate instead.
- BI = BB->begin();
- BE = BB->end();
- LocalChanged = true;
- }
-
- if (IsSubloopHeader && !isa<PHINode>(I))
- break;
- }
-
- // Add all successors to the worklist, except for loop exit blocks and the
- // bodies of subloops. We visit the headers of loops so that we can
- // process
- // their phis, but we contract the rest of the subloop body and only
- // follow
- // edges leading back to the original loop.
- for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
- ++SI) {
- BasicBlock *SuccBB = *SI;
- if (!Visited.insert(SuccBB).second)
- continue;
-
- const Loop *SuccLoop = LI->getLoopFor(SuccBB);
- if (SuccLoop && SuccLoop->getHeader() == SuccBB &&
- L->contains(SuccLoop)) {
- VisitStack.push_back(WorklistItem(SuccBB, true));
-
- SmallVector<BasicBlock *, 8> SubLoopExitBlocks;
- SuccLoop->getExitBlocks(SubLoopExitBlocks);
-
- for (unsigned i = 0; i < SubLoopExitBlocks.size(); ++i) {
- BasicBlock *ExitBB = SubLoopExitBlocks[i];
- if (LI->getLoopFor(ExitBB) == L && Visited.insert(ExitBB).second)
- VisitStack.push_back(WorklistItem(ExitBB, false));
- }
-
+ if (I.use_empty()) {
+ if (isInstructionTriviallyDead(&I, &TLI))
+ DeadInsts.push_back(&I);
continue;
}
- bool IsExitBlock =
- std::binary_search(ExitBlocks.begin(), ExitBlocks.end(), SuccBB);
- if (IsExitBlock)
+ // We special case the first iteration which we can detect due to the
+ // empty `ToSimplify` set.
+ bool IsFirstIteration = ToSimplify->empty();
+
+ if (!IsFirstIteration && !ToSimplify->count(&I))
continue;
- VisitStack.push_back(WorklistItem(SuccBB, false));
+ Value *V = SimplifyInstruction(&I, SQ.getWithInstruction(&I));
+ if (!V || !LI.replacementPreservesLCSSAForm(&I, V))
+ continue;
+
+ for (Value::use_iterator UI = I.use_begin(), UE = I.use_end();
+ UI != UE;) {
+ Use &U = *UI++;
+ auto *UserI = cast<Instruction>(U.getUser());
+ U.set(V);
+
+ // If the instruction is used by a PHI node we have already processed
+ // we'll need to iterate on the loop body to converge, so add it to
+ // the next set.
+ if (auto *UserPI = dyn_cast<PHINode>(UserI))
+ if (VisitedPHIs.count(UserPI)) {
+ Next->insert(UserPI);
+ continue;
+ }
+
+ // If we are only simplifying targeted instructions and the user is an
+ // instruction in the loop body, add it to our set of targeted
+ // instructions. Because we process defs before uses (outside of PHIs)
+ // we won't have visited it yet.
+ //
+ // We also skip any uses outside of the loop being simplified. Those
+ // should always be PHI nodes due to LCSSA form, and we don't want to
+ // try to simplify those away.
+ assert((L.contains(UserI) || isa<PHINode>(UserI)) &&
+ "Uses outside the loop should be PHI nodes due to LCSSA!");
+ if (!IsFirstIteration && L.contains(UserI))
+ ToSimplify->insert(UserI);
+ }
+
+ assert(I.use_empty() && "Should always have replaced all uses!");
+ if (isInstructionTriviallyDead(&I, &TLI))
+ DeadInsts.push_back(&I);
+ ++NumSimplified;
+ Changed = true;
}
}
- // Place the list of instructions to simplify on the next loop iteration
- // into ToSimplify.
- std::swap(ToSimplify, Next);
- Next->clear();
+ // Delete any dead instructions found thus far now that we've finished an
+ // iteration over all instructions in all the loop blocks.
+ if (!DeadInsts.empty()) {
+ Changed = true;
+ RecursivelyDeleteTriviallyDeadInstructions(DeadInsts, &TLI);
+ }
- Changed |= LocalChanged;
- } while (LocalChanged);
+ // If we never found a PHI that needs to be simplified in the next
+ // iteration, we're done.
+ if (Next->empty())
+ break;
+
+ // Otherwise, put the next set in place for the next iteration and reset it
+ // and the visited PHIs for that iteration.
+ std::swap(Next, ToSimplify);
+ Next->clear();
+ VisitedPHIs.clear();
+ DeadInsts.clear();
+ }
return Changed;
}
@@ -174,21 +173,20 @@
bool runOnLoop(Loop *L, LPPassManager &LPM) override {
if (skipLoop(L))
return false;
- DominatorTreeWrapperPass *DTWP =
- getAnalysisIfAvailable<DominatorTreeWrapperPass>();
- DominatorTree *DT = DTWP ? &DTWP->getDomTree() : nullptr;
- LoopInfo *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
- AssumptionCache *AC =
- &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
+ DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
+ LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
+ AssumptionCache &AC =
+ getAnalysis<AssumptionCacheTracker>().getAssumptionCache(
*L->getHeader()->getParent());
- const TargetLibraryInfo *TLI =
- &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
+ const TargetLibraryInfo &TLI =
+ getAnalysis<TargetLibraryInfoWrapperPass>().getTLI();
- return SimplifyLoopInst(L, DT, LI, AC, TLI);
+ return simplifyLoopInst(*L, DT, LI, AC, TLI);
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<AssumptionCacheTracker>();
+ AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<TargetLibraryInfoWrapperPass>();
AU.setPreservesCFG();
getLoopAnalysisUsage(AU);
@@ -200,7 +198,7 @@
PreservedAnalyses LoopInstSimplifyPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &) {
- if (!SimplifyLoopInst(&L, &AR.DT, &AR.LI, &AR.AC, &AR.TLI))
+ if (!simplifyLoopInst(L, AR.DT, AR.LI, AR.AC, AR.TLI))
return PreservedAnalyses::all();
auto PA = getLoopPassPreservedAnalyses();
diff --git a/llvm/lib/Transforms/Utils/Local.cpp b/llvm/lib/Transforms/Utils/Local.cpp
index 0be05d8..38a9d34 100644
--- a/llvm/lib/Transforms/Utils/Local.cpp
+++ b/llvm/lib/Transforms/Utils/Local.cpp
@@ -434,18 +434,31 @@
SmallVector<Instruction*, 16> DeadInsts;
DeadInsts.push_back(I);
+ RecursivelyDeleteTriviallyDeadInstructions(DeadInsts, TLI);
- do {
- I = DeadInsts.pop_back_val();
- salvageDebugInfo(*I);
+ return true;
+}
+
+void llvm::RecursivelyDeleteTriviallyDeadInstructions(
+ SmallVectorImpl<Instruction *> &DeadInsts, const TargetLibraryInfo *TLI) {
+ // Process the dead instruction list until empty.
+ while (!DeadInsts.empty()) {
+ Instruction &I = *DeadInsts.pop_back_val();
+ assert(I.use_empty() && "Instructions with uses are not dead.");
+ assert(isInstructionTriviallyDead(&I, TLI) &&
+ "Live instruction found in dead worklist!");
+
+ // Don't lose the debug info while deleting the instructions.
+ salvageDebugInfo(I);
// Null out all of the instruction's operands to see if any operand becomes
// dead as we go.
- for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {
- Value *OpV = I->getOperand(i);
- I->setOperand(i, nullptr);
+ for (Use &OpU : I.operands()) {
+ Value *OpV = OpU.get();
+ OpU.set(nullptr);
- if (!OpV->use_empty()) continue;
+ if (!OpV->use_empty())
+ continue;
// If the operand is an instruction that became dead as we nulled out the
// operand, and if it is 'trivially' dead, delete it in a future loop
@@ -455,10 +468,8 @@
DeadInsts.push_back(OpI);
}
- I->eraseFromParent();
- } while (!DeadInsts.empty());
-
- return true;
+ I.eraseFromParent();
+ }
}
/// areAllUsesEqual - Check whether the uses of a value are all the same.