RFR: 8293198: [vectorapi] Improve the implementation of VectorMask.indexInRange()
Xiaohong Gong
xgong at openjdk.org
Wed Jan 18 09:08:14 UTC 2023
The Vector API `"indexInRange(int offset, int limit)"` is used
to compute a vector mask whose lanes are set to true if the
index of the lane is inside the range specified by the `"offset"`
and `"limit"` arguments, otherwise the lanes are set to false.
There are two special cases for this API:
1) If `"offset >= 0 && offset >= limit"`, all the lanes of the
generated mask are false.
2) If` "offset >= 0 && limit - offset >= vlength"`, all the
lanes of the generated mask are true. Note that `"vlength"` is
the number of vector lanes.
For such special cases, we can simply use `"maskAll(false|true)"`
to implement the API. Otherwise, the original comparison with
`"iota" `vector is needed. And for further optimization, we have
optimal instruction supported by SVE (i.e. whilelo [1]), which
can implement the API directly if the `"offset >= 0"`.
As a summary, to optimize the API, we can use the if-else branches
to handle the specific cases in java level and intrinsify the
remaining case by C2 compiler:
public VectorMask<E> indexInRange(int offset, int limit) {
if (offset < 0) {
return this.and(indexInRange0Helper(offset, limit));
} else if (offset >= limit) {
return this.and(vectorSpecies().maskAll(false));
} else if (limit - offset >= length()) {
return this.and(vectorSpecies().maskAll(true));
}
return this.and(indexInRange0(offset, limit));
}
The last part (i.e. `"indexInRange0"`) in the above implementation
is expected to be intrinsified by C2 compiler if the necessary IRs
are supported. Otherwise, it will fall back to the original API
implementation (i.e. `"indexInRange0Helper"`). Regarding to the
intrinsifaction, the compiler will generate `"VectorMaskGen"` IR
with "limit - offset" as the input if the current platform supports
it. Otherwise, it generates `"VectorLoadConst + VectorMaskCmp"` based
on `"iota < limit - offset"`.
For the following java code which uses `"indexInRange"`:
static final VectorSpecies<Double> SPECIES =
DoubleVector.SPECIES_PREFERRED;
static final int LENGTH = 1027;
public static double[] da;
public static double[] db;
public static double[] dc;
private static void func() {
for (int i = 0; i < LENGTH; i += SPECIES.length()) {
var m = SPECIES.indexInRange(i, LENGTH);
var av = DoubleVector.fromArray(SPECIES, da, i, m);
av.lanewise(VectorOperators.NEG).intoArray(dc, i, m);
}
}
The core code generated with SVE 256-bit vector size is:
ptrue p2.d ; maskAll(true)
...
LOOP:
...
sub w11, w13, w14 ; limit - offset
cmp w14, w13
b.cs LABEL-1 ; if (offset >= limit) => uncommon-trap
cmp w11, #0x4
b.lt LABEL-2 ; if (limit - offset < vlength)
mov p1.b, p2.b
LABEL-3:
ld1d {z16.d}, p1/z, [x10] ; load vector masked
...
cmp w14, w29
b.cc LOOP
...
LABEL-2:
whilelo p1.d, x16, x10 ; VectorMaskGen
...
b LABEL-3
...
LABEL-1:
uncommon-trap
Please note that if the array size `LENGTH` is aligned with
the vector size 256 (i.e. `LENGTH = 1024`), the branch "LABEL-2"
will be optimized out by compiler and it becomes another
uncommon-trap.
For NEON, the main CFG is the same with above. But the compiler
intrinsification is different. Here is the code:
sub x10, x10, x12 ; limit - offset
scvtf d16, x10
dup v16.2d, v16.d[0] ; replicateD
mov x8, #0xd8d0
movk x8, #0x84cb, lsl #16
movk x8, #0xffff, lsl #32
ldr q17, [x8], #0 ; load the "iota" const vector
fcmgt v18.2d, v16.2d, v17.2d ; mask = iota < limit - offset
Here is the performance data of the new added benchmark on an ARM
SVE 256-bit platform:
Benchmark (size) Before After Units
IndexInRangeBenchmark.byteIndexInRange 1024 11203.697 41404.431 ops/ms
IndexInRangeBenchmark.byteIndexInRange 1027 2365.920 8747.004 ops/ms
IndexInRangeBenchmark.doubleIndexInRange 1024 1227.505 6092.194 ops/ms
IndexInRangeBenchmark.doubleIndexInRange 1027 351.215 1156.683 ops/ms
IndexInRangeBenchmark.floatIndexInRange 1024 1468.876 11032.580 ops/ms
IndexInRangeBenchmark.floatIndexInRange 1027 699.645 2439.671 ops/ms
IndexInRangeBenchmark.intIndexInRange 1024 2842.187 11903.544 ops/ms
IndexInRangeBenchmark.intIndexInRange 1027 689.866 2547.424 ops/ms
IndexInRangeBenchmark.longIndexInRange 1024 1394.135 5902.973 ops/ms
IndexInRangeBenchmark.longIndexInRange 1027 355.621 1189.458 ops/ms
IndexInRangeBenchmark.shortIndexInRange 1024 5521.468 21578.340 ops/ms
IndexInRangeBenchmark.shortIndexInRange 1027 1264.816 4640.504 ops/ms
And the performance data with ARM NEON:
Benchmark (size) Before After Units
IndexInRangeBenchmark.byteIndexInRange 1024 4026.548 15562.880 ops/ms
IndexInRangeBenchmark.byteIndexInRange 1027 305.314 576.559 ops/ms
IndexInRangeBenchmark.doubleIndexInRange 1024 289.224 2244.080 ops/ms
IndexInRangeBenchmark.doubleIndexInRange 1027 39.740 76.499 ops/ms
IndexInRangeBenchmark.floatIndexInRange 1024 675.264 4457.470 ops/ms
IndexInRangeBenchmark.floatIndexInRange 1027 79.918 144.952 ops/ms
IndexInRangeBenchmark.intIndexInRange 1024 740.139 4014.583 ops/ms
IndexInRangeBenchmark.intIndexInRange 1027 78.608 147.903 ops/ms
IndexInRangeBenchmark.longIndexInRange 1024 400.683 2209.551 ops/ms
IndexInRangeBenchmark.longIndexInRange 1027 41.146 69.599 ops/ms
IndexInRangeBenchmark.shortIndexInRange 1024 1821.736 8153.546 ops/ms
IndexInRangeBenchmark.shortIndexInRange 1027 158.810 243.205 ops/ms
The performance improves about `3.5x ~ 7.5x` on the vector size aligned
(1024 size) benchmarks both with NEON and SVE. And it improves about
`3.5x/1.8x` on the vector size not aligned (1027 size) benchmarks with
SVE/NEON respectively. We can also observe the similar improvement on
the x86 platforms.
[1] https://developer.arm.com/documentation/ddi0596/2020-12/SVE-Instructions/WHILELO--While-incrementing-unsigned-scalar-lower-than-scalar-
-------------
Commit messages:
- 8293198: [vectorapi] Improve the implementation of VectorMask.indexInRange()
Changes: https://git.openjdk.org/jdk/pull/12064/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12064&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8293198
Stats: 665 lines in 41 files changed: 622 ins; 0 del; 43 mod
Patch: https://git.openjdk.org/jdk/pull/12064.diff
Fetch: git fetch https://git.openjdk.org/jdk pull/12064/head:pull/12064
PR: https://git.openjdk.org/jdk/pull/12064
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