[llvm] Add a pass "SinkGEPConstOffset" (PR #140657)

[Juan Manuel Martinez Caamaño via llvm-commits]

================
@@ -0,0 +1,219 @@
+//===- SinkGEPConstOffset.cpp -------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "llvm/Transforms/Scalar/SinkGEPConstOffset.h"
+#include "llvm/ADT/APInt.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Analysis/MemoryBuiltins.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
+#include "llvm/Analysis/TargetTransformInfo.h"
+#include "llvm/Analysis/ValueTracking.h"
+#include "llvm/IR/BasicBlock.h"
+#include "llvm/IR/Constant.h"
+#include "llvm/IR/Constants.h"
+#include "llvm/IR/DataLayout.h"
+#include "llvm/IR/DerivedTypes.h"
+#include "llvm/IR/Dominators.h"
+#include "llvm/IR/Function.h"
+#include "llvm/IR/GetElementPtrTypeIterator.h"
+#include "llvm/IR/IRBuilder.h"
+#include "llvm/IR/InstrTypes.h"
+#include "llvm/IR/Instruction.h"
+#include "llvm/IR/Instructions.h"
+#include "llvm/IR/Module.h"
+#include "llvm/IR/PassManager.h"
+#include "llvm/IR/PatternMatch.h"
+#include "llvm/IR/Type.h"
+#include "llvm/IR/User.h"
+#include "llvm/IR/Value.h"
+#include "llvm/InitializePasses.h"
+#include "llvm/Pass.h"
+#include "llvm/Support/Casting.h"
+#include "llvm/Support/CommandLine.h"
+#include "llvm/Support/ErrorHandling.h"
+#include "llvm/Support/raw_ostream.h"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Transforms/Utils/Local.h"
+#include <cassert>
+#include <cstdint>
+#include <string>
+
+using namespace llvm;
+using namespace llvm::PatternMatch;
+
+namespace {
+
+/// A pass that tries to sink const offset in GEP chain to tail.
+/// It is a FunctionPass because searching for the constant offset may inspect
+/// other basic blocks.
+class SinkGEPConstOffsetLegacyPass : public FunctionPass {
+public:
+  static char ID;
+
+  SinkGEPConstOffsetLegacyPass() : FunctionPass(ID) {
+    initializeSinkGEPConstOffsetLegacyPassPass(
+        *PassRegistry::getPassRegistry());
+  }
+
+  void getAnalysisUsage(AnalysisUsage &AU) const override {
+    AU.addRequired<DominatorTreeWrapperPass>();
+    AU.setPreservesCFG();
+  }
+
+  bool runOnFunction(Function &F) override;
+};
+
+} // end anonymous namespace
+
+char SinkGEPConstOffsetLegacyPass::ID = 0;
+
+INITIALIZE_PASS_BEGIN(
+    SinkGEPConstOffsetLegacyPass, "sink-gep-const-offset",
+    "Sink const offsets down the GEP chain to the tail for reduction of "
+    "register usage", false, false)
+INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
+INITIALIZE_PASS_END(
+    SinkGEPConstOffsetLegacyPass, "sink-gep-const-offset",
+    "Sink const offsets down the GEP chain to the tail for reduction of "
+    "register usage", false, false)
+
+FunctionPass *llvm::createSinkGEPConstOffsetPass() {
+  return new SinkGEPConstOffsetLegacyPass();
+}
+
+/// The purpose of this function is to sink the constant offsets in the base
+/// GEP to current GEP.
+///
+/// A simple example is given:
+///
+/// %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 512
+/// %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 %ofst0
+/// %data = load half, ptr addrspace(3) %gep1, align 2
+/// ==>
+/// %gep0 = getelementptr half, ptr addrspace(3) %ptr, i32 %ofst0
+/// %gep1 = getelementptr half, ptr addrspace(3) %gep0, i32 512
+/// %data = load half, ptr addrspace(3) %gep1, align 2
+static bool sinkGEPConstantOffset(Value *Ptr, const DataLayout *DL) {
+  GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(Ptr);
+  if (!GEP)
+    return false;
+
+  if (GEP->getNumIndices() != 1)
+    return false;
+
+  GetElementPtrInst *BaseGEP =
+      dyn_cast<GetElementPtrInst>(GEP->getPointerOperand());
+  if (!BaseGEP)
+    return false;
+
+  if (BaseGEP->getNumIndices() != 1)
+    return false;
+
+  Value *Idx = GEP->getOperand(1);
+  Value *BaseIdx = BaseGEP->getOperand(1);
+
+  ConstantInt *C = nullptr;
+  if (!match(BaseIdx, m_ConstantInt(C)))
+    return false;
----------------
jmmartinez wrote:

Since the pattern is just matching against a `ConstantInt`, I'd rather use `dyn_cast`.

```suggestion
  ConstantInt *BaseIdxAsCI = dyn_cast<ConstantInt>(BaseIdx);
  if (!BaseIdxAsCI)
    return false;
```

You can later reuse this variable to avoid having a `cast<ConstantInt>` every time you want to access the `ConstantInt` methods.

For example, instead of
```cpp
      NewIdxValue = cast<ConstantInt>(BaseIdx)->getSExtValue() +
                    cast<ConstantInt>(Idx)->getSExtValue();
```
I'd prefer
```cpp
      NewIdxValue = BaseIdxAsCI->getSExtValue() +
                    IdxAsCI->getSExtValue();
```

https://github.com/llvm/llvm-project/pull/140657