[InstCombine] Remove redundant evaluateGEPOffsetExpression() fold (NFCI)

If we go through the generic EmitGEPOffset code, the resulting
expression can be (and is) reduced in the same way this code did
manually. There are no changes in lit tests or llvm-test-suite.

This fold predates the time where we started adding nsw to the adds
created by EmitGEPOffset, so it was likely needed back then.

This might not actually be NFC due to worklist order changes etc.

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -404,108 +404,6 @@ Instruction *InstCombinerImpl::foldCmpLoadFromIndexedGlobal(
   return nullptr;
 }
 
-/// Return a value that can be used to compare the *offset* implied by a GEP to
-/// zero. For example, if we have &A[i], we want to return 'i' for
-/// "icmp ne i, 0". Note that, in general, indices can be complex, and scales
-/// are involved. The above expression would also be legal to codegen as
-/// "icmp ne (i*4), 0" (assuming A is a pointer to i32).
-/// This latter form is less amenable to optimization though, and we are allowed
-/// to generate the first by knowing that pointer arithmetic doesn't overflow.
-///
-/// If we can't emit an optimized form for this expression, this returns null.
-///
-static Value *evaluateGEPOffsetExpression(User *GEP, InstCombinerImpl &IC,
-                                          const DataLayout &DL) {
-  gep_type_iterator GTI = gep_type_begin(GEP);
-
-  // Check to see if this gep only has a single variable index.  If so, and if
-  // any constant indices are a multiple of its scale, then we can compute this
-  // in terms of the scale of the variable index.  For example, if the GEP
-  // implies an offset of "12 + i*4", then we can codegen this as "3 + i",
-  // because the expression will cross zero at the same point.
-  unsigned i, e = GEP->getNumOperands();
-  int64_t Offset = 0;
-  for (i = 1; i != e; ++i, ++GTI) {
-    if (ConstantInt *CI = dyn_cast<ConstantInt>(GEP->getOperand(i))) {
-      // Compute the aggregate offset of constant indices.
-      if (CI->isZero()) continue;
-
-      // Handle a struct index, which adds its field offset to the pointer.
-      if (StructType *STy = GTI.getStructTypeOrNull()) {
-        Offset += DL.getStructLayout(STy)->getElementOffset(CI->getZExtValue());
-      } else {
-        uint64_t Size = DL.getTypeAllocSize(GTI.getIndexedType());
-        Offset += Size*CI->getSExtValue();
-      }
-    } else {
-      // Found our variable index.
-      break;
-    }
-  }
-
-  // If there are no variable indices, we must have a constant offset, just
-  // evaluate it the general way.
-  if (i == e) return nullptr;
-
-  Value *VariableIdx = GEP->getOperand(i);
-  // Determine the scale factor of the variable element.  For example, this is
-  // 4 if the variable index is into an array of i32.
-  uint64_t VariableScale = DL.getTypeAllocSize(GTI.getIndexedType());
-
-  // Verify that there are no other variable indices.  If so, emit the hard way.
-  for (++i, ++GTI; i != e; ++i, ++GTI) {
-    ConstantInt *CI = dyn_cast<ConstantInt>(GEP->getOperand(i));
-    if (!CI) return nullptr;
-
-    // Compute the aggregate offset of constant indices.
-    if (CI->isZero()) continue;
-
-    // Handle a struct index, which adds its field offset to the pointer.
-    if (StructType *STy = GTI.getStructTypeOrNull()) {
-      Offset += DL.getStructLayout(STy)->getElementOffset(CI->getZExtValue());
-    } else {
-      uint64_t Size = DL.getTypeAllocSize(GTI.getIndexedType());
-      Offset += Size*CI->getSExtValue();
-    }
-  }
-
-  // Okay, we know we have a single variable index, which must be a
-  // pointer/array/vector index.  If there is no offset, life is simple, return
-  // the index.
-  Type *IntPtrTy = DL.getIntPtrType(GEP->getOperand(0)->getType());
-  unsigned IntPtrWidth = IntPtrTy->getIntegerBitWidth();
-  if (Offset == 0) {
-    // Cast to intptrty in case a truncation occurs.  If an extension is needed,
-    // we don't need to bother extending: the extension won't affect where the
-    // computation crosses zero.
-    if (VariableIdx->getType()->getPrimitiveSizeInBits().getFixedSize() >
-        IntPtrWidth) {
-      VariableIdx = IC.Builder.CreateTrunc(VariableIdx, IntPtrTy);
-    }
-    return VariableIdx;
-  }
-
-  // Otherwise, there is an index.  The computation we will do will be modulo
-  // the pointer size.
-  Offset = SignExtend64(Offset, IntPtrWidth);
-  VariableScale = SignExtend64(VariableScale, IntPtrWidth);
-
-  // To do this transformation, any constant index must be a multiple of the
-  // variable scale factor.  For example, we can evaluate "12 + 4*i" as "3 + i",
-  // but we can't evaluate "10 + 3*i" in terms of i.  Check that the offset is a
-  // multiple of the variable scale.
-  int64_t NewOffs = Offset / (int64_t)VariableScale;
-  if (Offset != NewOffs*(int64_t)VariableScale)
-    return nullptr;
-
-  // Okay, we can do this evaluation.  Start by converting the index to intptr.
-  if (VariableIdx->getType() != IntPtrTy)
-    VariableIdx = IC.Builder.CreateIntCast(VariableIdx, IntPtrTy,
-                                            true /*Signed*/);
-  Constant *OffsetVal = ConstantInt::get(IntPtrTy, NewOffs);
-  return IC.Builder.CreateAdd(VariableIdx, OffsetVal, "offset");
-}
-
 /// Returns true if we can rewrite Start as a GEP with pointer Base
 /// and some integer offset. The nodes that need to be re-written
 /// for this transformation will be added to Explored.
@@ -846,14 +744,7 @@ Instruction *InstCombinerImpl::foldGEPICmp(GEPOperator *GEPLHS, Value *RHS,
   if (PtrBase == RHS && GEPLHS->isInBounds() &&
       !GEPLHS->getType()->isVectorTy()) {
     // ((gep Ptr, OFFSET) cmp Ptr)   ---> (OFFSET cmp 0).
-    // This transformation (ignoring the base and scales) is valid because we
-    // know pointers can't overflow since the gep is inbounds.  See if we can
-    // output an optimized form.
-    Value *Offset = evaluateGEPOffsetExpression(GEPLHS, *this, DL);
-
-    // If not, synthesize the offset the hard way.
-    if (!Offset)
-      Offset = EmitGEPOffset(GEPLHS);
+    Value *Offset = EmitGEPOffset(GEPLHS);
     return new ICmpInst(ICmpInst::getSignedPredicate(Cond), Offset,
                         Constant::getNullValue(Offset->getType()));
   }