Check in the experimental GEP splitter pass. This pass splits complex

GEPs (more than one non-zero index) into simple GEPs (at most one
non-zero index).  In some simple experiments using this it's not
uncommon to see 3% overall code size wins, because it exposes
redundancies that can be eliminated, however it's tricky to use
because instcombine aggressively undoes the work that this pass does.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@85144 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 81 insertions, 0 deletions

diff --git a/lib/Transforms/Scalar/GEPSplitter.cpp b/lib/Transforms/Scalar/GEPSplitter.cpp
new file mode 100644
index 0000000000..610a41dae4
--- /dev/null
+++ b/lib/Transforms/Scalar/GEPSplitter.cpp
@@ -0,0 +1,81 @@
+//===- GEPSplitter.cpp - Split complex GEPs into simple ones --------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This function breaks GEPs with more than 2 non-zero operands into smaller
+// GEPs each with no more than 2 non-zero operands. This exposes redundancy
+// between GEPs with common initial operand sequences.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "split-geps"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Constants.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/Pass.h"
+using namespace llvm;
+
+namespace {
+  class GEPSplitter : public FunctionPass {
+    virtual bool runOnFunction(Function &F);
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const;
+  public:
+    static char ID; // Pass identification, replacement for typeid
+    explicit GEPSplitter() : FunctionPass(&ID) {}
+  };
+}
+
+char GEPSplitter::ID = 0;
+static RegisterPass<GEPSplitter> X("split-geps",
+                                   "split complex GEPs into simple GEPs");
+
+FunctionPass *llvm::createGEPSplitterPass() {
+  return new GEPSplitter();
+}
+
+bool GEPSplitter::runOnFunction(Function &F) {
+  bool Changed = false;
+
+  // Visit each GEP instruction.
+  for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
+    for (BasicBlock::iterator II = I->begin(), IE = I->end(); II != IE; )
+      if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(II++)) {
+        unsigned NumOps = GEP->getNumOperands();
+        // Ignore GEPs which are already simple.
+        if (NumOps <= 2)
+          continue;
+        bool FirstIndexIsZero = isa<ConstantInt>(GEP->getOperand(1)) &&
+                                cast<ConstantInt>(GEP->getOperand(1))->isZero();
+        if (NumOps == 3 && FirstIndexIsZero)
+          continue;
+        // The first index is special and gets expanded with a 2-operand GEP
+        // (unless it's zero, in which case we can skip this).
+        Value *NewGEP = FirstIndexIsZero ?
+          GEP->getOperand(0) :
+          GetElementPtrInst::Create(GEP->getOperand(0), GEP->getOperand(1),
+                                    "tmp", GEP);
+        // All remaining indices get expanded with a 3-operand GEP with zero
+        // as the second operand.
+        Value *Idxs[2];
+        Idxs[0] = ConstantInt::get(Type::getInt64Ty(F.getContext()), 0);
+        for (unsigned i = 2; i != NumOps; ++i) {
+          Idxs[1] = GEP->getOperand(i);
+          NewGEP = GetElementPtrInst::Create(NewGEP, Idxs, Idxs+2, "tmp", GEP);
+        }
+        GEP->replaceAllUsesWith(NewGEP);
+        GEP->eraseFromParent();
+        Changed = true;
+      }
+
+  return Changed;
+}
+
+void GEPSplitter::getAnalysisUsage(AnalysisUsage &AU) const {
+  AU.setPreservesCFG();
+}