[Loads] Support dereferenceable assumption with variable size. (#128436)

Update isDereferenceableAndAlignedPointer to make use of dereferenceable
assumptions with variable sizes via SCEV.

To do so, factor out the logic to check via an assumption to a helper,
and use SE to check if the access size is less than the dereferenceable
size.

PR: llvm/llvm-project#128436

Author: fhahn

llvm/include/llvm/Analysis/AssumeBundleQueries.h

@@ -102,10 +102,14 @@ LLVM_ABI void fillMapFromAssume(AssumeInst &Assume,
 struct RetainedKnowledge {
   Attribute::AttrKind AttrKind = Attribute::None;
   uint64_t ArgValue = 0;
+  Value *IRArgValue = nullptr;
   Value *WasOn = nullptr;
+  RetainedKnowledge(Attribute::AttrKind AttrKind = Attribute::None,
+                    uint64_t ArgValue = 0, Value *WasOn = nullptr)
+      : AttrKind(AttrKind), ArgValue(ArgValue), WasOn(WasOn) {}
   bool operator==(RetainedKnowledge Other) const {
     return AttrKind == Other.AttrKind && WasOn == Other.WasOn &&
-           ArgValue == Other.ArgValue;
+           ArgValue == Other.ArgValue && IRArgValue == Other.IRArgValue;
   }
   bool operator!=(RetainedKnowledge Other) const { return !(*this == Other); }
   /// This is only intended for use in std::min/std::max between attribute that

llvm/lib/Analysis/AssumeBundleQueries.cpp

@@ -114,6 +114,9 @@ llvm::getKnowledgeFromBundle(AssumeInst &Assume,
   };
   if (BOI.End - BOI.Begin > ABA_Argument)
     Result.ArgValue = GetArgOr1(0);
+  Result.IRArgValue = bundleHasArgument(BOI, ABA_Argument)
+                          ? getValueFromBundleOpInfo(Assume, BOI, ABA_Argument)
+                          : nullptr;
   if (Result.AttrKind == Attribute::Alignment)
     if (BOI.End - BOI.Begin > ABA_Argument + 1)
       Result.ArgValue = MinAlign(Result.ArgValue, GetArgOr1(1));

llvm/lib/Analysis/Loads.cpp

@@ -31,6 +31,40 @@ static bool isAligned(const Value *Base, Align Alignment,
   return Base->getPointerAlignment(DL) >= Alignment;
 }
 
+static bool isDereferenceableAndAlignedPointerViaAssumption(
+    const Value *Ptr, Align Alignment,
+    function_ref<bool(const RetainedKnowledge &RK)> CheckSize,
+    const DataLayout &DL, const Instruction *CtxI, AssumptionCache *AC,
+    const DominatorTree *DT) {
+  // Dereferenceable information from assumptions is only valid if the value
+  // cannot be freed between the assumption and use. For now just use the
+  // information for values that cannot be freed in the function.
+  // TODO: More precisely check if the pointer can be freed between assumption
+  // and use.
+  if (!CtxI || Ptr->canBeFreed())
+    return false;
+  /// Look through assumes to see if both dereferencability and alignment can
+  /// be proven by an assume if needed.
+  RetainedKnowledge AlignRK;
+  RetainedKnowledge DerefRK;
+  bool IsAligned = Ptr->getPointerAlignment(DL) >= Alignment;
+  return getKnowledgeForValue(
+      Ptr, {Attribute::Dereferenceable, Attribute::Alignment}, *AC,
+      [&](RetainedKnowledge RK, Instruction *Assume, auto) {
+        if (!isValidAssumeForContext(Assume, CtxI, DT))
+          return false;
+        if (RK.AttrKind == Attribute::Alignment)
+          AlignRK = std::max(AlignRK, RK);
+        if (RK.AttrKind == Attribute::Dereferenceable)
+          DerefRK = std::max(DerefRK, RK);
+        IsAligned |= AlignRK && AlignRK.ArgValue >= Alignment.value();
+        if (IsAligned && DerefRK && CheckSize(DerefRK))
+          return true; // We have found what we needed so we stop looking
+        return false;  // Other assumes may have better information. so
+                       // keep looking
+      });
+}
+
 /// Test if V is always a pointer to allocated and suitably aligned memory for
 /// a simple load or store.
 static bool isDereferenceableAndAlignedPointer(
@@ -169,38 +203,12 @@ static bool isDereferenceableAndAlignedPointer(
                                               Size, DL, CtxI, AC, DT, TLI,
                                               Visited, MaxDepth);
 
-  // Dereferenceable information from assumptions is only valid if the value
-  // cannot be freed between the assumption and use. For now just use the
-  // information for values that cannot be freed in the function.
-  // TODO: More precisely check if the pointer can be freed between assumption
-  // and use.
-  if (CtxI && AC && !V->canBeFreed()) {
-    /// Look through assumes to see if both dereferencability and alignment can
-    /// be proven by an assume if needed.
-    RetainedKnowledge AlignRK;
-    RetainedKnowledge DerefRK;
-    bool IsAligned = V->getPointerAlignment(DL) >= Alignment;
-    if (getKnowledgeForValue(
-            V, {Attribute::Dereferenceable, Attribute::Alignment}, *AC,
-            [&](RetainedKnowledge RK, Instruction *Assume, auto) {
-              if (!isValidAssumeForContext(Assume, CtxI, DT))
-                return false;
-              if (RK.AttrKind == Attribute::Alignment)
-                AlignRK = std::max(AlignRK, RK);
-              if (RK.AttrKind == Attribute::Dereferenceable)
-                DerefRK = std::max(DerefRK, RK);
-              IsAligned |= AlignRK && AlignRK.ArgValue >= Alignment.value();
-              if (IsAligned && DerefRK &&
-                  DerefRK.ArgValue >= Size.getZExtValue())
-                return true; // We have found what we needed so we stop looking
-              return false;  // Other assumes may have better information. so
-                             // keep looking
-            }))
-      return true;
-  }
-
-  // If we don't know, assume the worst.
-  return false;
+  return AC && isDereferenceableAndAlignedPointerViaAssumption(
+                   V, Alignment,
+                   [Size](const RetainedKnowledge &RK) {
+                     return RK.ArgValue >= Size.getZExtValue();
+                   },
+                   DL, CtxI, AC, DT);
 }
 
 bool llvm::isDereferenceableAndAlignedPointer(
@@ -317,8 +325,8 @@ bool llvm::isDereferenceableAndAlignedInLoop(
     return false;
 
   const SCEV *MaxBECount =
-      Predicates ? SE.getPredicatedConstantMaxBackedgeTakenCount(L, *Predicates)
-                 : SE.getConstantMaxBackedgeTakenCount(L);
+      Predicates ? SE.getPredicatedSymbolicMaxBackedgeTakenCount(L, *Predicates)
+                 : SE.getSymbolicMaxBackedgeTakenCount(L);
   const SCEV *BECount = Predicates
                             ? SE.getPredicatedBackedgeTakenCount(L, *Predicates)
                             : SE.getBackedgeTakenCount(L);
@@ -339,9 +347,11 @@ bool llvm::isDereferenceableAndAlignedInLoop(
 
   Value *Base = nullptr;
   APInt AccessSize;
+  const SCEV *AccessSizeSCEV = nullptr;
   if (const SCEVUnknown *NewBase = dyn_cast<SCEVUnknown>(AccessStart)) {
     Base = NewBase->getValue();
     AccessSize = MaxPtrDiff;
+    AccessSizeSCEV = PtrDiff;
   } else if (auto *MinAdd = dyn_cast<SCEVAddExpr>(AccessStart)) {
     if (MinAdd->getNumOperands() != 2)
       return false;
@@ -365,12 +375,20 @@ bool llvm::isDereferenceableAndAlignedInLoop(
       return false;
 
     AccessSize = MaxPtrDiff + Offset->getAPInt();
+    AccessSizeSCEV = SE.getAddExpr(PtrDiff, Offset);
     Base = NewBase->getValue();
   } else
     return false;
 
   Instruction *HeaderFirstNonPHI = &*L->getHeader()->getFirstNonPHIIt();
-  return isDereferenceableAndAlignedPointer(Base, Alignment, AccessSize, DL,
+  return isDereferenceableAndAlignedPointerViaAssumption(
+             Base, Alignment,
+             [&SE, AccessSizeSCEV](const RetainedKnowledge &RK) {
+               return SE.isKnownPredicate(CmpInst::ICMP_ULE, AccessSizeSCEV,
+                                          SE.getSCEV(RK.IRArgValue));
+             },
+             DL, HeaderFirstNonPHI, AC, &DT) ||
+         isDereferenceableAndAlignedPointer(Base, Alignment, AccessSize, DL,
                                             HeaderFirstNonPHI, AC, &DT);
 }
 

llvm/lib/IR/Verifier.cpp

@@ -5604,6 +5604,15 @@ void Verifier::visitIntrinsicCall(Intrinsic::ID ID, CallBase &Call) {
                 "third argument should be an integer if present", Call);
         continue;
       }
+      if (Kind == Attribute::Dereferenceable) {
+        Check(ArgCount == 2,
+              "dereferenceable assumptions should have 2 arguments", Call);
+        Check(Call.getOperand(Elem.Begin)->getType()->isPointerTy(),
+              "first argument should be a pointer", Call);
+        Check(Call.getOperand(Elem.Begin + 1)->getType()->isIntegerTy(),
+              "second argument should be an integer", Call);
+        continue;
+      }
       Check(ArgCount <= 2, "too many arguments", Call);
       if (Kind == Attribute::None)
         break;