Simplify the solution by just dropping the dereferenceable attrs

when there is a risk that they would become incorrect if neither
adjusting nor dropping them.

Author: bjope

llvm/include/llvm/IR/InstrTypes.h

@@ -1559,11 +1559,6 @@ class CallBase : public Instruction {
     Attrs = Attrs.addDereferenceableParamAttr(getContext(), i, Bytes);
   }
 
-  /// adds the dereferenceable attribute to the list of attributes.
-  void addDereferenceableOrNullParamAttr(unsigned i, uint64_t Bytes) {
-    Attrs = Attrs.addDereferenceableOrNullParamAttr(getContext(), i, Bytes);
-  }
-
   /// adds the dereferenceable attribute to the list of attributes.
   void addDereferenceableRetAttr(uint64_t Bytes) {
     Attrs = Attrs.addDereferenceableRetAttr(getContext(), Bytes);

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

@@ -563,22 +563,6 @@ static void shortenAssignment(Instruction *Inst, Value *OriginalDest,
   for_each(LinkedDVRAssigns, InsertAssignForOverlap);
 }
 
-// Helper to trim or drop any dereferencable/dereferencable_or_null attributes
-// for a given argument, based on the new access being restricted to derefence
-// bytes in the range [Offset, Offset+Size).
-static void trimDereferencableAttrs(AnyMemIntrinsic *Intrinsic, unsigned Arg,
-                                    uint64_t Offset, uint64_t Size) {
-  uint64_t End = Offset + Size;
-  if (Intrinsic->getParamDereferenceableBytes(Arg) >= End)
-    Intrinsic->addDereferenceableParamAttr(Arg, Size);
-  else
-    Intrinsic->removeParamAttr(Arg, Attribute::Dereferenceable);
-  if (Intrinsic->getParamDereferenceableOrNullBytes(Arg) >= End)
-    Intrinsic->addDereferenceableOrNullParamAttr(Arg, Size);
-  else
-    Intrinsic->removeParamAttr(Arg, Attribute::DereferenceableOrNull);
-}
-
 static bool tryToShorten(Instruction *DeadI, int64_t &DeadStart,
                          uint64_t &DeadSize, int64_t KillingStart,
                          uint64_t KillingSize, bool IsOverwriteEnd) {
@@ -660,7 +644,11 @@ static bool tryToShorten(Instruction *DeadI, int64_t &DeadStart,
         DeadI->getIterator());
     NewDestGEP->setDebugLoc(DeadIntrinsic->getDebugLoc());
     DeadIntrinsic->setDest(NewDestGEP);
-    trimDereferencableAttrs(DeadIntrinsic, 0, ToRemoveSize, NewSize);
+    // Simply drop any dereferenceable attributes (memset with a constant length
+    // already implies dereferenceability by itself, so dropping is simpler than
+    // trying to adjust the dereferenceable size).
+    DeadIntrinsic->removeParamAttr(0, Attribute::Dereferenceable);
+    DeadIntrinsic->removeParamAttr(0, Attribute::DereferenceableOrNull);
   }
 
   // Update attached dbg.assign intrinsics. Assume 8-bit byte.

llvm/test/Transforms/DeadStoreElimination/OverwriteStoreBegin.ll

@@ -403,12 +403,12 @@ entry:
   ret void
 }
 
-; Verify that we adjust the dereferenceable attribute.
+; Verify that we adjust/drop the dereferenceable attribute.
 define void @dereferenceable(ptr nocapture %p) {
 ; CHECK-LABEL: @dereferenceable(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds i8, ptr [[P:%.*]], i64 4
-; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 4 dereferenceable(24) [[TMP0]], i8 0, i64 24, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 4 [[TMP0]], i8 0, i64 24, i1 false)
 ; CHECK-NEXT:    store i32 1, ptr [[P]], align 4
 ; CHECK-NEXT:    ret void
 ;
@@ -418,12 +418,12 @@ entry:
   ret void
 }
 
-; Verify that we adjust the dereferenceable_or_null attribute.
+; Verify that we adjust/drop the dereferenceable_or_null attribute.
 define void @dereferenceable_or_null(ptr nocapture %p) {
 ; CHECK-LABEL: @dereferenceable_or_null(
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr inbounds i8, ptr [[P:%.*]], i64 8
-; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 4 dereferenceable_or_null(20) [[TMP0]], i8 0, i64 20, i1 false)
+; CHECK-NEXT:    call void @llvm.memset.p0.i64(ptr align 4 [[TMP0]], i8 0, i64 20, i1 false)
 ; CHECK-NEXT:    store i64 1, ptr [[P]], align 4
 ; CHECK-NEXT:    ret void
 ;