diff --git a/llvm/include/llvm/Transforms/IPO/Attributor.h b/llvm/include/llvm/Transforms/IPO/Attributor.h
--- a/llvm/include/llvm/Transforms/IPO/Attributor.h
+++ b/llvm/include/llvm/Transforms/IPO/Attributor.h
@@ -5067,8 +5067,9 @@
              OAS.getOffset() < getOffset() + getSize();
     }
 
-    /// Constant used to represent unknown offset or sizes.
-    static constexpr int64_t Unknown = 1 << 31;
+    /// Constants used to represent special offsets or sizes.
+    static constexpr int64_t Unassigned = -1;
+    static constexpr int64_t Unknown = -2;
   };
 
   /// Call \p CB on all accesses that might interfere with \p OAS and return
diff --git a/llvm/lib/Transforms/IPO/AttributorAttributes.cpp b/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
--- a/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
+++ b/llvm/lib/Transforms/IPO/AttributorAttributes.cpp
@@ -1226,7 +1226,7 @@
 
   /// Helper struct, will support ranges eventually.
   struct OffsetInfo {
-    int64_t Offset = OffsetAndSize::Unknown;
+    int64_t Offset = OffsetAndSize::Unassigned;
 
     bool operator==(const OffsetInfo &OI) const { return Offset == OI.Offset; }
   };
@@ -1243,6 +1243,8 @@
 
     auto HandlePassthroughUser = [&](Value *Usr, OffsetInfo PtrOI,
                                      bool &Follow) {
+      assert(PtrOI.Offset != OffsetAndSize::Unassigned &&
+             "Cannot pass through if the input Ptr was not visited!");
       OffsetInfo &UsrOI = OffsetInfoMap[Usr];
       UsrOI = PtrOI;
       Follow = true;
@@ -1283,11 +1285,15 @@
         APInt GEPOffset(DL.getIndexTypeSizeInBits(GEP->getType()), 0);
         if (PtrOI.Offset == OffsetAndSize::Unknown ||
             !GEP->accumulateConstantOffset(DL, GEPOffset)) {
+          LLVM_DEBUG(dbgs() << "[AAPointerInfo] GEP offset not constant "
+                            << *GEP << "\n");
           UsrOI.Offset = OffsetAndSize::Unknown;
           Follow = true;
           return true;
         }
 
+        LLVM_DEBUG(dbgs() << "[AAPointerInfo] GEP offset is constant " << *GEP
+                          << "\n");
         UsrOI.Offset = PtrOI.Offset + GEPOffset.getZExtValue();
         Follow = true;
         return true;
@@ -1306,15 +1312,22 @@
         bool IsFirstPHIUser = !OffsetInfoMap.count(Usr);
         OffsetInfo &UsrOI = OffsetInfoMap[Usr];
         OffsetInfo &PtrOI = OffsetInfoMap[CurPtr];
-        // Check if the PHI is invariant (so far).
-        if (UsrOI == PtrOI)
-          return true;
 
         // Check if the PHI operand has already an unknown offset as we can't
         // improve on that anymore.
         if (PtrOI.Offset == OffsetAndSize::Unknown) {
+          LLVM_DEBUG(dbgs() << "[AAPointerInfo] PHI operand offset unknown "
+                            << *CurPtr << " in " << *Usr << "\n");
+          Follow = UsrOI.Offset != OffsetAndSize::Unknown;
           UsrOI = PtrOI;
-          Follow = true;
+          return true;
+        }
+
+        // Check if the PHI is invariant (so far).
+        if (UsrOI == PtrOI) {
+          assert(PtrOI.Offset != OffsetAndSize::Unassigned &&
+                 "Cannot assign if the current Ptr was not visited!");
+          LLVM_DEBUG(dbgs() << "[AAPointerInfo] PHI is invariant (so far)");
           return true;
         }
 
diff --git a/llvm/test/CodeGen/AMDGPU/attributor.ll b/llvm/test/CodeGen/AMDGPU/attributor.ll
new file mode 100644
--- /dev/null
+++ b/llvm/test/CodeGen/AMDGPU/attributor.ll
@@ -0,0 +1,42 @@
+; RUN: llc < %s | FileCheck %s
+
+target triple = "amdgcn-amd-amdhsa"
+
+; The call to intrinsic implicitarg_ptr reaches a load through a phi. The
+; offsets of the phi cannot be determined, and hence the attirbutor assumes that
+; hostcall is in use.
+
+; CHECK: .value_kind:     hidden_hostcall_buffer
+; CHECK: .value_kind:     hidden_multigrid_sync_arg
+
+define amdgpu_kernel void @the_kernel(i32 addrspace(1)* %a, i64 %index1, i64 %index2, i1 %cond)  {
+entry:
+  %tmp7 = tail call i8 addrspace(4)* @llvm.amdgcn.implicitarg.ptr()
+  br i1 %cond, label %old, label %new
+
+old:                                              ; preds = %entry
+  %tmp4 = getelementptr i8, i8 addrspace(4)* %tmp7, i64 %index1
+  br label %join
+
+new:                                              ; preds = %entry
+  %tmp12 = getelementptr inbounds i8, i8 addrspace(4)* %tmp7, i64 %index2
+  br label %join
+
+join:                                             ; preds = %new, %old
+  %.in.in.in = phi i8 addrspace(4)* [ %tmp12, %new ], [ %tmp4, %old ]
+  %.in.in = bitcast i8 addrspace(4)* %.in.in.in to i16 addrspace(4)*
+
+  ;;; THIS USE is where the offset into implicitarg_ptr is unknown
+  %.in = load i16, i16 addrspace(4)* %.in.in, align 2
+
+  %idx.ext = sext i16 %.in to i64
+  %add.ptr3 = getelementptr inbounds i32, i32 addrspace(1)* %a, i64 %idx.ext
+  %tmp16 = atomicrmw add i32 addrspace(1)* %add.ptr3, i32 15 syncscope("agent-one-as") monotonic, align 4
+  ret void
+}
+
+declare i32 @llvm.amdgcn.workitem.id.x()
+
+declare align 4 i8 addrspace(4)* @llvm.amdgcn.implicitarg.ptr()
+
+declare i32 @llvm.amdgcn.workgroup.id.x()
diff --git a/llvm/test/Transforms/Attributor/value-simplify-pointer-info.ll b/llvm/test/Transforms/Attributor/value-simplify-pointer-info.ll
--- a/llvm/test/Transforms/Attributor/value-simplify-pointer-info.ll
+++ b/llvm/test/Transforms/Attributor/value-simplify-pointer-info.ll
@@ -1616,6 +1616,71 @@
   ret i32 %add1
 }
 
+define i8 @local_alloca_not_simplifiable_2(i64 %index1, i64 %index2, i1 %cnd) {
+; TUNIT: Function Attrs: nofree norecurse nosync nounwind willreturn
+; TUNIT-LABEL: define {{[^@]+}}@local_alloca_not_simplifiable_2
+; TUNIT-SAME: (i64 [[INDEX1:%.*]], i64 [[INDEX2:%.*]], i1 [[CND:%.*]]) #[[ATTR3]] {
+; TUNIT-NEXT:  entry:
+; TUNIT-NEXT:    [[BYTES:%.*]] = alloca [1024 x i8], align 16
+; TUNIT-NEXT:    [[GEP0:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[BYTES]], i64 0, i64 0
+; TUNIT-NEXT:    store i8 7, i8* [[GEP0]], align 16
+; TUNIT-NEXT:    br i1 [[CND]], label [[LEFT:%.*]], label [[RIGHT:%.*]]
+; TUNIT:       left:
+; TUNIT-NEXT:    [[GEP1:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[BYTES]], i64 0, i64 [[INDEX1]]
+; TUNIT-NEXT:    br label [[JOIN:%.*]]
+; TUNIT:       right:
+; TUNIT-NEXT:    [[GEP2:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[BYTES]], i64 0, i64 [[INDEX2]]
+; TUNIT-NEXT:    br label [[JOIN]]
+; TUNIT:       join:
+; TUNIT-NEXT:    [[GEP_JOIN:%.*]] = phi i8* [ [[GEP1]], [[LEFT]] ], [ [[GEP2]], [[RIGHT]] ]
+; TUNIT-NEXT:    store i8 9, i8* [[GEP_JOIN]], align 4
+; TUNIT-NEXT:    [[I:%.*]] = load i8, i8* [[GEP0]], align 16
+; TUNIT-NEXT:    ret i8 [[I]]
+;
+; CGSCC: Function Attrs: nofree norecurse nosync nounwind willreturn
+; CGSCC-LABEL: define {{[^@]+}}@local_alloca_not_simplifiable_2
+; CGSCC-SAME: (i64 [[INDEX1:%.*]], i64 [[INDEX2:%.*]], i1 [[CND:%.*]]) #[[ATTR5]] {
+; CGSCC-NEXT:  entry:
+; CGSCC-NEXT:    [[BYTES:%.*]] = alloca [1024 x i8], align 16
+; CGSCC-NEXT:    [[GEP0:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[BYTES]], i64 0, i64 0
+; CGSCC-NEXT:    store i8 7, i8* [[GEP0]], align 16
+; CGSCC-NEXT:    br i1 [[CND]], label [[LEFT:%.*]], label [[RIGHT:%.*]]
+; CGSCC:       left:
+; CGSCC-NEXT:    [[GEP1:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[BYTES]], i64 0, i64 [[INDEX1]]
+; CGSCC-NEXT:    br label [[JOIN:%.*]]
+; CGSCC:       right:
+; CGSCC-NEXT:    [[GEP2:%.*]] = getelementptr inbounds [1024 x i8], [1024 x i8]* [[BYTES]], i64 0, i64 [[INDEX2]]
+; CGSCC-NEXT:    br label [[JOIN]]
+; CGSCC:       join:
+; CGSCC-NEXT:    [[GEP_JOIN:%.*]] = phi i8* [ [[GEP1]], [[LEFT]] ], [ [[GEP2]], [[RIGHT]] ]
+; CGSCC-NEXT:    store i8 9, i8* [[GEP_JOIN]], align 4
+; CGSCC-NEXT:    [[I:%.*]] = load i8, i8* [[GEP0]], align 16
+; CGSCC-NEXT:    ret i8 [[I]]
+;
+entry:
+  %Bytes = alloca [1024 x i8], align 16
+  %gep0 = getelementptr inbounds [1024 x i8], [1024 x i8]* %Bytes, i64 0, i64 0
+  store i8 7, i8* %gep0, align 4
+  br i1 %cnd, label %left, label %right
+
+left:                                             ; preds = %entry
+  %gep1 = getelementptr inbounds [1024 x i8], [1024 x i8]* %Bytes, i64 0, i64 %index1
+  br label %join
+
+right:                                            ; preds = %entry
+  %gep2 = getelementptr inbounds [1024 x i8], [1024 x i8]* %Bytes, i64 0, i64 %index2
+  br label %join
+
+join:                                             ; preds = %right, %left
+  %gep.join = phi i8* [ %gep1, %left ], [ %gep2, %right ]
+  store i8 9, i8* %gep.join, align 4
+
+  ; This load cannot be replaced by the value 7 from %entry, since the previous
+  ; store interferes due to its unknown offset.
+  %i = load i8, i8* %gep0, align 4
+  ret i8 %i
+}
+
 ; We could simplify these if we separate accessed bins wrt. alignment (here mod 4).
 define i32 @unknown_access_mixed_simplifiable(i32 %arg1, i32 %arg2) {
 ; CHECK: Function Attrs: nofree norecurse nosync nounwind readnone willreturn