[InstCombine] GEPOperator::accumulateConstantOffset does not support scalable vectors

[sdesmalen]

Summary

Avoid transforming:

%0 = bitcast i8* %base to <vscale x 16 x i8>*
%1 = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %0, i64 1

into:

%0 = getelementptr i8, i8* %base, i64 16
%1 = bitcast i8* %0 to <vscale x 16 x i8>*

[efriedma]

Should we add a special case for zero indexes?

[sdesmalen]

• Supported case of index being 0, and added extra test.

[efriedma]

LGTM

llvm/lib/IR/Operator.cpp

 bool GEPOperator::accumulateConstantOffset(const DataLayout &DL,
                                            APInt &Offset) const {
   assert(Offset.getBitWidth() ==
              DL.getIndexSizeInBits(getPointerAddressSpace()) &&
          "The offset bit width does not match DL specification.");
 
   for (gep_type_iterator GTI = gep_type_begin(this), GTE = gep_type_end(this);
        GTI != GTE; ++GTI) {
+    // Scalable vectors have are multiplied by a runtime constant.
+    if (auto *VecTy = dyn_cast<VectorType>(GTI.getIndexedType()))
+      if (VecTy->isScalable())
+        return false;
+
     ConstantInt *OpC = dyn_cast<ConstantInt>(GTI.getOperand());
     if (!OpC)
       return false;
     if (OpC->isZero())
       continue;
 
     // Handle a struct index, which adds its field offset to the pointer.
     if (StructType *STy = GTI.getStructTypeOrNull()) {

llvm/test/Transforms/InstCombine/gep-vector.ll

 ; CHECK-NEXT:    [[TMP1:%.*]] = addrspacecast i32* [[GEP]] to i32 addrspace(3)*
 ; CHECK-NEXT:    ret i32 addrspace(3)* [[TMP1]]
 ;
   %arr_ptr = bitcast <4 x i32>* %x to [4 x i32]*
   %asc = addrspacecast [4 x i32]* %arr_ptr to [4 x i32] addrspace(3)*
   %gep = getelementptr inbounds [4 x i32], [4 x i32] addrspace(3)* %asc, i64 %y, i64 %z
   ret i32 addrspace(3)* %gep
 }
+
+; Negative test - avoid doing bitcast on i8*, because '16' should be scaled by 'vscale'.
+
+define <vscale x 16 x i8> @test_accumulate_constant_offset_vscale(<vscale x 16 x i1> %pg, i8* %base) {
+; CHECK-LABEL: @test_accumulate_constant_offset_vscale
+; CHECK-NEXT:   %bc = bitcast i8* %base to <vscale x 16 x i8>*
+; CHECK-NEXT:   %gep = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %bc, i64 1
+; CHECK-NEXT:   ret <vscale x 16 x i8> %gep
+  %bc = bitcast i8* %base to <vscale x 16 x i8>*
+  %gep = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %bc, i64 1
+  ret <vscale x 16 x i8> %gep
+}