zeroshade commented on code in PR #1389:
URL: https://github.com/apache/arrow-go/pull/1389#discussion_r4328409638
##########
arrow/bitutil/bitmaps_test.go:
##########
@@ -214,6 +214,58 @@ func TestCopyBitmap(t *testing.T) {
}
}
+func TestTransferBitmapOverlapping(t *testing.T) {
+ tests := []struct {
+ name string
+ srcSlice, dstSlice int
+ srcOffset int
+ dstOffset int
+ length int
+ }{
+ {name: "aligned right partial", srcOffset: 0, dstOffset: 8,
length: 31},
+ {name: "aligned left partial", srcOffset: 8, dstOffset: 0,
length: 31},
+ {name: "aligned full bytes", srcOffset: 0, dstOffset: 16,
length: 64},
+ {name: "unaligned right", srcOffset: 3, dstOffset: 11, length:
145},
+ {name: "unaligned left", srcOffset: 11, dstOffset: 3, length:
145},
+ {name: "same bit range", srcOffset: 5, dstOffset: 5, length:
145},
+ {name: "aliased slice views", srcSlice: 2, dstSlice: 3,
srcOffset: 3, dstOffset: 2, length: 145},
+ {name: "adjacent bits", srcOffset: 5, dstOffset: 6, length: 1},
Review Comment:
Could you add a case where source and destination start in the same byte at
different bit offsets and span several bytes, for example 0→1 with length 9?
Skipping the copy when both start in the same byte still passes all 18
subtests, but corrupts that case.
##########
arrow/bitutil/bitmaps_transfer_benchmark_test.go:
##########
@@ -0,0 +1,93 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package bitutil_test
+
+import (
+ "strconv"
+ "testing"
+
+ "github.com/apache/arrow-go/v18/arrow/bitutil"
+)
+
+// BenchmarkTransferBitmapAliasing measures the overlap snapshot path and the
+// allocation-free path, including disjoint ranges in the same backing buffer.
+func BenchmarkTransferBitmapAliasing(b *testing.B) {
+ operations := []struct {
+ name string
+ transfer func([]byte, int, int, []byte, int)
+ }{
+ {name: "copy", transfer: bitutil.CopyBitmap},
+ {name: "invert", transfer: bitutil.InvertBitmap},
+ }
+ offsets := []struct {
+ name string
+ src, dst int
+ }{
+ {name: "aligned"},
+ {name: "unaligned", src: 3, dst: 5},
+ }
+ layouts := []struct {
+ name string
+ separate, disjoint bool
+ srcByte, dstByte int
+ }{
+ {name: "separate", separate: true},
+ {name: "disjoint-same-buffer", disjoint: true},
+ {name: "overlap-forward", dstByte: 1},
+ {name: "overlap-backward", srcByte: 1},
+ {name: "same-range"},
+ }
+
+ for _, operation := range operations {
+ b.Run(operation.name, func(b *testing.B) {
+ for _, nbytes := range []int{32, 1024, 64 * 1024} {
+ b.Run(strconv.Itoa(nbytes), func(b *testing.B) {
+ for _, offset := range offsets {
+ b.Run(offset.name, func(b
*testing.B) {
+ for _, layout := range
layouts {
+
b.Run(layout.name, func(b *testing.B) {
+ var
src, dst []byte
+ if
layout.separate {
+
src = make([]byte, nbytes+8)
+
dst = make([]byte, nbytes+8)
+ } else {
+
backing := make([]byte, 2*nbytes+32)
+
srcByte, dstByte := layout.srcByte, layout.dstByte
+
if layout.disjoint {
+
dstByte = nbytes + 16
+
}
+
src, dst = backing[srcByte:], backing[dstByte:]
+ }
+ for i
:= range src {
+
src[i] = byte(i*37 + 0x5b)
+ }
+
+
b.SetBytes(int64(nbytes))
+
b.ReportAllocs()
+
b.ResetTimer()
+ for i
:= 0; i < b.N; i++ {
Review Comment:
Nit: please use `for b.Loop()`. This benchmark could also live in
`bitmaps_test.go` next to the other bitmap benchmarks.
##########
arrow/bitutil/bitmaps.go:
##########
@@ -421,6 +421,25 @@ func transferBitmap(mode transferMode, src []byte,
srcOffset, length int, dst []
return
}
+ // Both transfer paths can overwrite source bytes before reading them
when
+ // the ranges alias. Only snapshot the source when their byte spans
overlap.
+ srcStart := srcOffset / 8
+ dstStart := dstOffset / 8
+ srcBytes := src[srcStart :
srcStart+int(BytesForBits(int64(srcOffset%8+length)))]
+ dstBytes := dst[dstStart :
dstStart+int(BytesForBits(int64(dstOffset%8+length)))]
Review Comment:
On 386, `dstOffset%8+length` overflows before the `int64` conversion. With
256 MiB buffers, `srcOffset=0`, `dstOffset=1` and `length=2147483647`, both
copy and invert panic here with a negative slice bound. Both work on `main`.
Please widen before adding, as `int64(dstOffset%8) + int64(length)`, on the
source line above as well.
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