[CaptureTracking] Ignore ephemeral values when determining pointer es…

…capeness

Ephemeral values cannot cause a pointer to escape.

No change in compile time:
https://llvm-compile-time-tracker.com/compare.php?from=4371710085ba1c376a094948b806ddd3b88319de&to=c5ddbcc4866f38026737762ee8d7b9b00395d4f4&stat=instructions

This partially fixes some regressions caused by more calls to `__builtin_assume` (D122397).

Reviewed By: asbirlea

Differential Revision: https://reviews.llvm.org/D123162

llvm/include/llvm/Analysis/CaptureTracking.h

@@ -25,6 +25,7 @@ namespace llvm {
   class DominatorTree;
   class LoopInfo;
   class Function;
+  template <typename T> class SmallPtrSetImpl;
 
   /// getDefaultMaxUsesToExploreForCaptureTracking - Return default value of
   /// the maximal number of uses to explore before giving up. It is used by
@@ -41,8 +42,14 @@ namespace llvm {
   /// MaxUsesToExplore specifies how many uses the analysis should explore for
   /// one value before giving up due too "too many uses". If MaxUsesToExplore
   /// is zero, a default value is assumed.
+  bool PointerMayBeCaptured(const Value *V, bool ReturnCaptures,
+                            bool StoreCaptures, unsigned MaxUsesToExplore = 0);
+
+  /// Variant of the above function which accepts a set of Values that are
+  /// ephemeral and cannot cause pointers to escape.
   bool PointerMayBeCaptured(const Value *V, bool ReturnCaptures,
                             bool StoreCaptures,
+                            const SmallPtrSetImpl<const Value *> &EphValues,
                             unsigned MaxUsesToExplore = 0);
 
   /// PointerMayBeCapturedBefore - Return true if this pointer value may be

llvm/lib/Analysis/CaptureTracking.cpp

@@ -16,6 +16,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallSet.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/Statistic.h"
@@ -74,19 +75,26 @@ bool CaptureTracker::isDereferenceableOrNull(Value *O, const DataLayout &DL) {
 
 namespace {
   struct SimpleCaptureTracker : public CaptureTracker {
-    explicit SimpleCaptureTracker(bool ReturnCaptures)
-        : ReturnCaptures(ReturnCaptures) {}
+    explicit SimpleCaptureTracker(
+
+        const SmallPtrSetImpl<const Value *> &EphValues, bool ReturnCaptures)
+        : EphValues(EphValues), ReturnCaptures(ReturnCaptures) {}
 
     void tooManyUses() override { Captured = true; }
 
     bool captured(const Use *U) override {
       if (isa<ReturnInst>(U->getUser()) && !ReturnCaptures)
         return false;
 
+      if (EphValues.contains(U->getUser()))
+        return false;
+
       Captured = true;
       return true;
     }
 
+    const SmallPtrSetImpl<const Value *> &EphValues;
+
     bool ReturnCaptures;
 
     bool Captured = false;
@@ -212,8 +220,18 @@ namespace {
 /// counts as capturing it or not.  The boolean StoreCaptures specified whether
 /// storing the value (or part of it) into memory anywhere automatically
 /// counts as capturing it or not.
-bool llvm::PointerMayBeCaptured(const Value *V,
-                                bool ReturnCaptures, bool StoreCaptures,
+bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures,
+                                bool StoreCaptures, unsigned MaxUsesToExplore) {
+  SmallPtrSet<const Value *, 1> Empty;
+  return PointerMayBeCaptured(V, ReturnCaptures, StoreCaptures, Empty,
+                              MaxUsesToExplore);
+}
+
+/// Variant of the above function which accepts a set of Values that are
+/// ephemeral and cannot cause pointers to escape.
+bool llvm::PointerMayBeCaptured(const Value *V, bool ReturnCaptures,
+                                bool StoreCaptures,
+                                const SmallPtrSetImpl<const Value *> &EphValues,
                                 unsigned MaxUsesToExplore) {
   assert(!isa<GlobalValue>(V) &&
          "It doesn't make sense to ask whether a global is captured.");
@@ -224,7 +242,7 @@ bool llvm::PointerMayBeCaptured(const Value *V,
   // take advantage of this.
   (void)StoreCaptures;
 
-  SimpleCaptureTracker SCT(ReturnCaptures);
+  SimpleCaptureTracker SCT(EphValues, ReturnCaptures);
   PointerMayBeCaptured(V, &SCT, MaxUsesToExplore);
   if (SCT.Captured)
     ++NumCaptured;

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -38,7 +38,9 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/StringRef.h"
 #include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/AssumptionCache.h"
 #include "llvm/Analysis/CaptureTracking.h"
+#include "llvm/Analysis/CodeMetrics.h"
 #include "llvm/Analysis/GlobalsModRef.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
@@ -762,6 +764,9 @@ struct DSEState {
   // Post-order numbers for each basic block. Used to figure out if memory
   // accesses are executed before another access.
   DenseMap<BasicBlock *, unsigned> PostOrderNumbers;
+  // Values that are only used with assumes. Used to refine pointer escape
+  // analysis.
+  SmallPtrSet<const Value *, 32> EphValues;
 
   /// Keep track of instructions (partly) overlapping with killing MemoryDefs per
   /// basic block.
@@ -776,8 +781,8 @@ struct DSEState {
   DSEState &operator=(const DSEState &) = delete;
 
   DSEState(Function &F, AliasAnalysis &AA, MemorySSA &MSSA, DominatorTree &DT,
-           PostDominatorTree &PDT, const TargetLibraryInfo &TLI,
-           const LoopInfo &LI)
+           PostDominatorTree &PDT, AssumptionCache &AC,
+           const TargetLibraryInfo &TLI, const LoopInfo &LI)
       : F(F), AA(AA), EI(DT, LI), BatchAA(AA, &EI), MSSA(MSSA), DT(DT),
         PDT(PDT), TLI(TLI), DL(F.getParent()->getDataLayout()), LI(LI) {
     // Collect blocks with throwing instructions not modeled in MemorySSA and
@@ -809,6 +814,8 @@ struct DSEState {
     AnyUnreachableExit = any_of(PDT.roots(), [](const BasicBlock *E) {
       return isa<UnreachableInst>(E->getTerminator());
     });
+
+    CodeMetrics::collectEphemeralValues(&F, &AC, EphValues);
   }
 
   /// Return 'OW_Complete' if a store to the 'KillingLoc' location (by \p
@@ -955,7 +962,7 @@ struct DSEState {
       if (!isInvisibleToCallerOnUnwind(V)) {
         I.first->second = false;
       } else if (isNoAliasCall(V)) {
-        I.first->second = !PointerMayBeCaptured(V, true, false);
+        I.first->second = !PointerMayBeCaptured(V, true, false, EphValues);
       }
     }
     return I.first->second;
@@ -974,7 +981,7 @@ struct DSEState {
       // with the killing MemoryDef. But we refrain from doing so for now to
       // limit compile-time and this does not cause any changes to the number
       // of stores removed on a large test set in practice.
-      I.first->second = PointerMayBeCaptured(V, false, true);
+      I.first->second = PointerMayBeCaptured(V, false, true, EphValues);
     return !I.first->second;
   }
 
@@ -1929,12 +1936,13 @@ struct DSEState {
 
 static bool eliminateDeadStores(Function &F, AliasAnalysis &AA, MemorySSA &MSSA,
                                 DominatorTree &DT, PostDominatorTree &PDT,
+                                AssumptionCache &AC,
                                 const TargetLibraryInfo &TLI,
                                 const LoopInfo &LI) {
   bool MadeChange = false;
 
   MSSA.ensureOptimizedUses();
-  DSEState State(F, AA, MSSA, DT, PDT, TLI, LI);
+  DSEState State(F, AA, MSSA, DT, PDT, AC, TLI, LI);
   // For each store:
   for (unsigned I = 0; I < State.MemDefs.size(); I++) {
     MemoryDef *KillingDef = State.MemDefs[I];
@@ -2114,9 +2122,10 @@ PreservedAnalyses DSEPass::run(Function &F, FunctionAnalysisManager &AM) {
   DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
   MemorySSA &MSSA = AM.getResult<MemorySSAAnalysis>(F).getMSSA();
   PostDominatorTree &PDT = AM.getResult<PostDominatorTreeAnalysis>(F);
+  AssumptionCache &AC = AM.getResult<AssumptionAnalysis>(F);
   LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
 
-  bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI, LI);
+  bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, AC, TLI, LI);
 
 #ifdef LLVM_ENABLE_STATS
   if (AreStatisticsEnabled())
@@ -2156,9 +2165,11 @@ class DSELegacyPass : public FunctionPass {
     MemorySSA &MSSA = getAnalysis<MemorySSAWrapperPass>().getMSSA();
     PostDominatorTree &PDT =
         getAnalysis<PostDominatorTreeWrapperPass>().getPostDomTree();
+    AssumptionCache &AC =
+        getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
     LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
 
-    bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, TLI, LI);
+    bool Changed = eliminateDeadStores(F, AA, MSSA, DT, PDT, AC, TLI, LI);
 
 #ifdef LLVM_ENABLE_STATS
     if (AreStatisticsEnabled())
@@ -2182,6 +2193,7 @@ class DSELegacyPass : public FunctionPass {
     AU.addPreserved<MemorySSAWrapperPass>();
     AU.addRequired<LoopInfoWrapperPass>();
     AU.addPreserved<LoopInfoWrapperPass>();
+    AU.addRequired<AssumptionCacheTracker>();
   }
 };
 
@@ -2199,6 +2211,7 @@ INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
 INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
 INITIALIZE_PASS_END(DSELegacyPass, "dse", "Dead Store Elimination", false,
                     false)
 

llvm/test/Transforms/DeadStoreElimination/assume.ll

@@ -8,7 +8,6 @@ define void @f() {
 ; CHECK-NEXT:    [[TMP1:%.*]] = call noalias i8* @_Znwm(i64 32)
 ; CHECK-NEXT:    [[TMP2:%.*]] = icmp ugt i8* [[TMP1]], @global
 ; CHECK-NEXT:    call void @llvm.assume(i1 [[TMP2]])
-; CHECK-NEXT:    store i8 0, i8* [[TMP1]], align 1
 ; CHECK-NEXT:    ret void
 ;
   %tmp1 = call noalias i8* @_Znwm(i64 32)