LAA: clarify loop-variant GEP idx computation

The stripGetElementPtr function is mysteriously named, and calls into
another mysterious getGEPInductionOperand which does something
complicated with GEP indices. The real purpose of the badly-named
stripGetElementPtr function is to get a loop-variant GEP index, if there
is one. The getGEPInductionOperand is totally redundant, as stripping
off zeros from the end of GEP indices has no effect on computing the
loop-variant GEP index, as constant zeros are always loop-invariant.
Moreover, the GEP induction operand is simply the first non-zero index
from the end, which stripGetElementPtr returns when it finds that any of
the GEP indices are loop-variant: this is a completely unrelated value
to the GEP index that is loop-variant. The implicit assumption here is
that there is only ever one loop-variant index, and it is the first
non-zero one from the end.

The logic is unnecessarily complicated for what stripGetElementPtr wants
to achieve, and the header comments are confusing as well. Strip
getGEPInductionOperand, rework and rename stripGetElementPtr.

Author: artagnon

llvm/lib/Analysis/LoopAccessAnalysis.cpp

@@ -42,13 +42,12 @@
 #include "llvm/IR/DiagnosticInfo.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Function.h"
-#include "llvm/IR/GetElementPtrTypeIterator.h"
 #include "llvm/IR/InstrTypes.h"
 #include "llvm/IR/Instruction.h"
 #include "llvm/IR/Instructions.h"
+#include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/PassManager.h"
-#include "llvm/IR/PatternMatch.h"
 #include "llvm/IR/Type.h"
 #include "llvm/IR/Value.h"
 #include "llvm/IR/ValueHandle.h"
@@ -66,7 +65,6 @@
 #include <vector>
 
 using namespace llvm;
-using namespace llvm::PatternMatch;
 
 #define DEBUG_TYPE "loop-accesses"
 
@@ -2809,50 +2807,24 @@ bool LoopAccessInfo::isInvariant(Value *V) const {
   return SE->isLoopInvariant(S, TheLoop);
 }
 
-/// Find the operand of the GEP that should be checked for consecutive
-/// stores. This ignores trailing indices that have no effect on the final
-/// pointer.
-static unsigned getGEPInductionOperand(const GetElementPtrInst *Gep) {
-  const DataLayout &DL = Gep->getDataLayout();
-  unsigned LastOperand = Gep->getNumOperands() - 1;
-  TypeSize GEPAllocSize = DL.getTypeAllocSize(Gep->getResultElementType());
-
-  // Walk backwards and try to peel off zeros.
-  while (LastOperand > 1 && match(Gep->getOperand(LastOperand), m_Zero())) {
-    // Find the type we're currently indexing into.
-    gep_type_iterator GEPTI = gep_type_begin(Gep);
-    std::advance(GEPTI, LastOperand - 2);
-
-    // If it's a type with the same allocation size as the result of the GEP we
-    // can peel off the zero index.
-    TypeSize ElemSize = GEPTI.isStruct()
-                            ? DL.getTypeAllocSize(GEPTI.getIndexedType())
-                            : GEPTI.getSequentialElementStride(DL);
-    if (ElemSize != GEPAllocSize)
-      break;
-    --LastOperand;
-  }
-
-  return LastOperand;
-}
-
-/// If the argument is a GEP, then returns the operand identified by
-/// getGEPInductionOperand. However, if there is some other non-loop-invariant
-/// operand, it returns that instead.
-static Value *stripGetElementPtr(Value *Ptr, ScalarEvolution *SE, Loop *Lp) {
+/// If \p Ptr is a GEP, which has a loop-variant operand, return that operand.
+/// Otherwise, return \p Ptr.
+static Value *getLoopVariantGEPOperand(Value *Ptr, ScalarEvolution *SE,
+                                       Loop *Lp) {
   auto *GEP = dyn_cast<GetElementPtrInst>(Ptr);
   if (!GEP)
     return Ptr;
 
-  unsigned InductionOperand = getGEPInductionOperand(GEP);
+  // There must be exactly one loop-variant operand.
+  if (count_if(GEP->operands(), [&](const Use &U) {
+        return !SE->isLoopInvariant(SE->getSCEV(U), Lp);
+      }) != 1)
+    return Ptr;
 
-  // Check that all of the gep indices are uniform except for our induction
-  // operand.
-  for (unsigned I = 0, E = GEP->getNumOperands(); I != E; ++I)
-    if (I != InductionOperand &&
-        !SE->isLoopInvariant(SE->getSCEV(GEP->getOperand(I)), Lp))
-      return Ptr;
-  return GEP->getOperand(InductionOperand);
+  auto *It = find_if(GEP->operands(), [&](const Use &U) {
+    return !SE->isLoopInvariant(SE->getSCEV(U), Lp);
+  });
+  return It->get();
 }
 
 /// Get the stride of a pointer access in a loop. Looks for symbolic
@@ -2867,7 +2839,7 @@ static const SCEV *getStrideFromPointer(Value *Ptr, ScalarEvolution *SE, Loop *L
   // pointer, otherwise, we are analyzing the index.
   Value *OrigPtr = Ptr;
 
-  Ptr = stripGetElementPtr(Ptr, SE, Lp);
+  Ptr = getLoopVariantGEPOperand(Ptr, SE, Lp);
   const SCEV *V = SE->getSCEV(Ptr);
 
   if (Ptr != OrigPtr)