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

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>*

Reviewers: efriedma, ctetreau

Reviewed By: efriedma

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76236

Author: sdesmalen-arm

llvm/lib/IR/Operator.cpp

@@ -45,6 +45,11 @@ bool GEPOperator::accumulateConstantOffset(const DataLayout &DL,
     if (OpC->isZero())
       continue;
 
+    // Scalable vectors have are multiplied by a runtime constant.
+    if (auto *VecTy = dyn_cast<VectorType>(GTI.getIndexedType()))
+      if (VecTy->isScalable())
+        return false;
+
     // Handle a struct index, which adds its field offset to the pointer.
     if (StructType *STy = GTI.getStructTypeOrNull()) {
       unsigned ElementIdx = OpC->getZExtValue();

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

@@ -134,3 +134,24 @@ define i32 addrspace(3)* @inbounds_bitcast_vec_to_array_addrspace_matching_alloc
   %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 i8* @test_accumulate_constant_offset_vscale_nonzero(<vscale x 16 x i1> %pg, i8* %base) {
+; CHECK-LABEL: @test_accumulate_constant_offset_vscale_nonzero
+; 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, i64 4
+; CHECK-NEXT:   ret 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, i64 4
+  ret i8* %gep
+}
+
+define i8* @test_accumulate_constant_offset_vscale_zero(<vscale x 16 x i1> %pg, i8* %base) {
+; CHECK-LABEL: @test_accumulate_constant_offset_vscale_zero
+; CHECK-NEXT:   %[[RES:.*]] = getelementptr i8, i8* %base, i64 4
+; CHECK-NEXT:   ret i8* %[[RES]]
+  %bc = bitcast i8* %base to <vscale x 16 x i8>*
+  %gep = getelementptr <vscale x 16 x i8>, <vscale x 16 x i8>* %bc, i64 0, i64 4
+  ret i8* %gep
+}