RFR: 8283307: Vectorize unsigned shift right on signed subword types [v3]
Sandhya Viswanathan
sviswanathan at openjdk.java.net
Fri Jun 3 21:52:52 UTC 2022
On Fri, 22 Apr 2022 11:09:09 GMT, Fei Gao <fgao at openjdk.org> wrote:
>> public short[] vectorUnsignedShiftRight(short[] shorts) {
>> short[] res = new short[SIZE];
>> for (int i = 0; i < SIZE; i++) {
>> res[i] = (short) (shorts[i] >>> 3);
>> }
>> return res;
>> }
>>
>> In C2's SLP, vectorization of unsigned shift right on signed subword types (byte/short) like the case above is intentionally disabled[1]. Because the vector unsigned shift on signed subword types behaves differently from the Java spec. It's worthy to vectorize more cases in quite low cost. Also, unsigned shift right on signed subword is not uncommon and we may find similar cases in Lucene benchmark[2].
>>
>> Taking unsigned right shift on short type as an example,
>> 
>>
>> when the shift amount is a constant not greater than the number of sign extended bits, 16 higher bits for short type shown like
>> above, the unsigned shift on signed subword types can be transformed into a signed shift and hence becomes vectorizable. Here is the transformation:
>> 
>>
>> This patch does the transformation in `SuperWord::implemented()` and `SuperWord::output()`. It helps vectorize the short cases above. We can handle unsigned right shift on byte type in a similar way. The generated assembly code for one iteration on aarch64 is like:
>>
>> ...
>> sbfiz x13, x10, #1, #32
>> add x15, x11, x13
>> ldr q16, [x15, #16]
>> sshr v16.8h, v16.8h, #3
>> add x13, x17, x13
>> str q16, [x13, #16]
>> ...
>>
>>
>> Here is the performance data for micro-benchmark before and after this patch on both AArch64 and x64 machines. We can observe about ~80% improvement with this patch.
>>
>> The perf data on AArch64:
>> Before the patch:
>> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
>> urShiftImmByte 1024 3 avgt 5 295.711 ± 0.117 ns/op
>> urShiftImmShort 1024 3 avgt 5 284.559 ± 0.148 ns/op
>>
>> after the patch:
>> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
>> urShiftImmByte 1024 3 avgt 5 45.111 ± 0.047 ns/op
>> urShiftImmShort 1024 3 avgt 5 55.294 ± 0.072 ns/op
>>
>> The perf data on X86:
>> Before the patch:
>> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
>> urShiftImmByte 1024 3 avgt 5 361.374 ± 4.621 ns/op
>> urShiftImmShort 1024 3 avgt 5 365.390 ± 3.595 ns/op
>>
>> After the patch:
>> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
>> urShiftImmByte 1024 3 avgt 5 105.489 ± 0.488 ns/op
>> urShiftImmShort 1024 3 avgt 5 43.400 ± 0.394 ns/op
>>
>> [1] https://github.com/openjdk/jdk/blob/002e3667443d94e2303c875daf72cf1ccbbb0099/src/hotspot/share/opto/vectornode.cpp#L190
>> [2] https://github.com/jpountz/decode-128-ints-benchmark/
>
> Fei Gao has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains five additional commits since the last revision:
>
> - Rewrite the scalar calculation to avoid inline
>
> Change-Id: I5959d035278097de26ab3dfe6f667d6f7476c723
> - Merge branch 'master' into fg8283307
>
> Change-Id: Id3ec8594da49fb4e6c6dcad888bcb1dfc0aac303
> - Remove related comments in some test files
>
> Change-Id: I5dd1c156bd80221dde53737e718da0254c5381d8
> - Merge branch 'master' into fg8283307
>
> Change-Id: Ic4645656ea156e8cac993995a5dc675aa46cb21a
> - 8283307: Vectorize unsigned shift right on signed subword types
>
> ```
> public short[] vectorUnsignedShiftRight(short[] shorts) {
> short[] res = new short[SIZE];
> for (int i = 0; i < SIZE; i++) {
> res[i] = (short) (shorts[i] >>> 3);
> }
> return res;
> }
> ```
> In C2's SLP, vectorization of unsigned shift right on signed
> subword types (byte/short) like the case above is intentionally
> disabled[1]. Because the vector unsigned shift on signed
> subword types behaves differently from the Java spec. It's
> worthy to vectorize more cases in quite low cost. Also,
> unsigned shift right on signed subword is not uncommon and we
> may find similar cases in Lucene benchmark[2].
>
> Taking unsigned right shift on short type as an example,
>
> Short:
> | <- 16 bits -> | <- 16 bits -> |
> | 1 1 1 ... 1 1 | data |
>
> when the shift amount is a constant not greater than the number
> of sign extended bits, 16 higher bits for short type shown like
> above, the unsigned shift on signed subword types can be
> transformed into a signed shift and hence becomes vectorizable.
> Here is the transformation:
>
> For T_SHORT (shift <= 16):
> src RShiftCntV shift src RShiftCntV shift
> \ / ==> \ /
> URShiftVS RShiftVS
>
> This patch does the transformation in SuperWord::implemented() and
> SuperWord::output(). It helps vectorize the short cases above. We
> can handle unsigned right shift on byte type in a similar way. The
> generated assembly code for one iteration on aarch64 is like:
> ```
> ...
> sbfiz x13, x10, #1, #32
> add x15, x11, x13
> ldr q16, [x15, #16]
> sshr v16.8h, v16.8h, #3
> add x13, x17, x13
> str q16, [x13, #16]
> ...
> ```
>
> Here is the performance data for micro-benchmark before and after
> this patch on both AArch64 and x64 machines. We can observe about
> ~80% improvement with this patch.
>
> The perf data on AArch64:
> Before the patch:
> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
> urShiftImmByte 1024 3 avgt 5 295.711 ± 0.117 ns/op
> urShiftImmShort 1024 3 avgt 5 284.559 ± 0.148 ns/op
>
> after the patch:
> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
> urShiftImmByte 1024 3 avgt 5 45.111 ± 0.047 ns/op
> urShiftImmShort 1024 3 avgt 5 55.294 ± 0.072 ns/op
>
> The perf data on X86:
> Before the patch:
> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
> urShiftImmByte 1024 3 avgt 5 361.374 ± 4.621 ns/op
> urShiftImmShort 1024 3 avgt 5 365.390 ± 3.595 ns/op
>
> After the patch:
> Benchmark (SIZE) (shiftCount) Mode Cnt Score Error Units
> urShiftImmByte 1024 3 avgt 5 105.489 ± 0.488 ns/op
> urShiftImmShort 1024 3 avgt 5 43.400 ± 0.394 ns/op
>
> [1] https://github.com/openjdk/jdk/blob/002e3667443d94e2303c875daf72cf1ccbbb0099/src/hotspot/share/opto/vectornode.cpp#L190
> [2] https://github.com/jpountz/decode-128-ints-benchmark/
>
> Change-Id: I9bd0cfdfcd9c477e8905a4c877d5e7ff14e39161
Very nice work! Patch looks good to me.
-------------
Marked as reviewed by sviswanathan (Reviewer).
PR: https://git.openjdk.java.net/jdk/pull/7979
More information about the hotspot-compiler-dev
mailing list