[Attributor] Simplify offset calculation for a constant GEP

[nikic]

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

1292

For scalable vectors this can still fail even if all indices are constant. Not sure if relevant in this context.

[jdoerfert]

LG, one nit though as pointed out by @nikic .

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

1293

Let's allow failure for now, handling as above.

[sameerds]

Simpler check for constant offset. We don't really care if the indices are constant.

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

auto UsePred = [&](const Use &U, bool &Follow) -> bool {
       if (auto *GEP = dyn_cast<GEPOperator>(Usr)) {
         // Note the order here, the Usr access might change the map, CurPtr is
         // already in it though.
         OffsetInfo &UsrOI = OffsetInfoMap[Usr];
         OffsetInfo &PtrOI = OffsetInfoMap[CurPtr];
         UsrOI = PtrOI;
 
         // TODO: Use range information.
+        APInt GEPOffset(DL.getIndexTypeSizeInBits(GEP->getType()), 0);
         if (PtrOI.Offset == OffsetAndSize::Unknown ||
-            !GEP->hasAllConstantIndices()) {
+            !GEP->accumulateConstantOffset(DL, GEPOffset)) {
           UsrOI.Offset = OffsetAndSize::Unknown;
           Follow = true;
           return true;
         }
 
-        SmallVector<Value *, 8> Indices;
-        for (Use &Idx : GEP->indices()) {
-          if (auto *CIdx = dyn_cast<ConstantInt>(Idx)) {
-            Indices.push_back(CIdx);
-            continue;
-          }
-
-          LLVM_DEBUG(dbgs() << "[AAPointerInfo] Non constant GEP index " << *GEP
-                            << " : " << *Idx << "\n");
-          return false;
-        }
-        UsrOI.Offset = PtrOI.Offset + DL.getIndexedOffsetInType(
-                                          GEP->getSourceElementType(), Indices);
+        UsrOI.Offset = PtrOI.Offset + GEPOffset.getZExtValue();
         Follow = true;
         return true;
       }
       if (isa<CastInst>(Usr) || isa<SelectInst>(Usr) || isa<ReturnInst>(Usr))
         return HandlePassthroughUser(Usr, OffsetInfoMap[CurPtr], Follow);
 
       // For PHIs we need to take care of the recurrence explicitly as the value
       // might change while we iterate through a loop. For now, we give up if
       // the PHI is not invariant.
       if (isa<PHINode>(Usr)) {