[llvm] [CGP][AArch64] Rebase the common base offset for better ISel (PR #74046)

[David Green via llvm-commits]

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@@ -6121,6 +6121,55 @@ bool CodeGenPrepare::splitLargeGEPOffsets() {
     int64_t BaseOffset = LargeOffsetGEPs.begin()->second;
     Value *NewBaseGEP = nullptr;
 
+    auto createNewBase = [&](int64_t BaseOffset, Value *OldBase,
+                             GetElementPtrInst *GEP) {
+      LLVMContext &Ctx = GEP->getContext();
+      Type *PtrIdxTy = DL->getIndexType(GEP->getType());
+      Type *I8PtrTy =
+          PointerType::get(Ctx, GEP->getType()->getPointerAddressSpace());
+      Type *I8Ty = Type::getInt8Ty(Ctx);
+
+      BasicBlock::iterator NewBaseInsertPt;
+      BasicBlock *NewBaseInsertBB;
+      if (auto *BaseI = dyn_cast<Instruction>(OldBase)) {
+        // If the base of the struct is an instruction, the new base will be
+        // inserted close to it.
+        NewBaseInsertBB = BaseI->getParent();
+        if (isa<PHINode>(BaseI))
+          NewBaseInsertPt = NewBaseInsertBB->getFirstInsertionPt();
+        else if (InvokeInst *Invoke = dyn_cast<InvokeInst>(BaseI)) {
+          NewBaseInsertBB =
+              SplitEdge(NewBaseInsertBB, Invoke->getNormalDest(), DT.get(), LI);
+          NewBaseInsertPt = NewBaseInsertBB->getFirstInsertionPt();
+        } else
+          NewBaseInsertPt = std::next(BaseI->getIterator());
+      } else {
+        // If the current base is an argument or global value, the new base
+        // will be inserted to the entry block.
+        NewBaseInsertBB = &BaseGEP->getFunction()->getEntryBlock();
+        NewBaseInsertPt = NewBaseInsertBB->getFirstInsertionPt();
+      }
+      IRBuilder<> NewBaseBuilder(NewBaseInsertBB, NewBaseInsertPt);
+      // Create a new base.
+      Value *BaseIndex = ConstantInt::get(PtrIdxTy, BaseOffset);
+      NewBaseGEP = OldBase;
+      if (NewBaseGEP->getType() != I8PtrTy)
+        NewBaseGEP = NewBaseBuilder.CreatePointerCast(NewBaseGEP, I8PtrTy);
+      NewBaseGEP =
+          NewBaseBuilder.CreateGEP(I8Ty, NewBaseGEP, BaseIndex, "splitgep");
+      NewGEPBases.insert(NewBaseGEP);
+      return;
+    };
+
+    // Check whether all the offsets can be encoded with perfered common base.
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davemgreen wrote:

perfered -> prefered

https://github.com/llvm/llvm-project/pull/74046