Hi all,
According to the Vector API doc, the `LSHR` operator computes
`a>>>(n&(ESIZE*8-1))`.
However, current implementation is incorrect for negative bytes/shorts.
The background is that one of our customers try to vectorize `urshift` with
`urshiftVector` like the following.
13 public static void urshift(byte[] src, byte[] dst) {
14 for (int i = 0; i < src.length; i++) {
15 dst[i] = (byte)(src[i] >>> 3);
16 }
17 }
18
19 public static void urshiftVector(byte[] src, byte[] dst) {
20 int i = 0;
21 for (; i < spec.loopBound(src.length); i +=spec.length()) {
22 var va = ByteVector.fromArray(spec, src, i);
23 var vb = va.lanewise(VectorOperators.LSHR, 3);
24 vb.intoArray(dst, i);
25 }
26
27 for (; i < src.length; i++) {
28 dst[i] = (byte)(src[i] >>> 3);
29 }
30 }
Unfortunately and to our surprise, code@line28 computes different results with
code@line23.
It took quite a long time to figure out this bug.
The root cause is that current implemenation of Vector API can't compute the
unsigned right shift results as what is done for scalar `>>>` for negative
byte/short elements.
Actually, current implementation will do `(a & 0xFF) >>> (n & 7)` [1] for all
bytes, which is unable to compute the vectorized `>>>` for negative bytes.
So this seems unreasonable and unfriendly to Java developers.
It would be better to fix it.
The key idea to support unsigned right shift of negative bytes/shorts is just
to replace the unsigned right shift operation with the signed right shift
operation.
This logic is:
- For byte elements, unsigned right shift is equal to signed right shift if the
shift_cnt <= 24.
- For short elements, unsigned right shift is equal to signed right shift if
the shift_cnt <= 16.
- For Vector API, the shift_cnt will be masked to shift_cnt <= 7 for bytes and
shift_cnt <= 15 for shorts.
I just learned this idea from https://github.com/openjdk/jdk/pull/7979 .
And many thanks to @fg1417 .
Thanks.
Best regards,
Jie
[1]
https://github.com/openjdk/jdk/blob/master/src/jdk.incubator.vector/share/classes/jdk/incubator/vector/ByteVector.java#L935
-------------
Commit messages:
- Add a space
- 8284932: [Vector API] Incorrect implementation of LSHR operator for negative
byte/short elements
Changes: https://git.openjdk.java.net/jdk/pull/8276/files
Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8276&range=00
Issue: https://bugs.openjdk.java.net/browse/JDK-8284932
Stats: 77 lines in 17 files changed: 19 ins; 12 del; 46 mod
Patch: https://git.openjdk.java.net/jdk/pull/8276.diff
Fetch: git fetch https://git.openjdk.java.net/jdk pull/8276/head:pull/8276
PR: https://git.openjdk.java.net/jdk/pull/8276
