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computecpp-sdk/samples/subgroup-scan.cpp at master · codeplaysoftware/computecpp-sdk · GitHub
This repository was archived by the owner on Aug 11, 2023. It is now read-only.
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/*
**************************************************************************
*
* Copyright (C) Codeplay Software Limited
* Licensed 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.
*
* Codeplay's ComputeCpp SDK
*
* scan-subgroups.cpp
*
* Description:
* Example of a parallel inclusive scan in SYCL using subgroup operations.
*
*************************************************************************
*/
#
include
<
sycl/sycl.hpp
>
#
include
<
algorithm
>
#
include
<
iostream
>
#
include
<
numeric
>
#
include
<
vector
>
//
Dummy struct to generate unique kernel name types
template
<
typename
T,
typename
U,
typename
V>
struct
kernel_name
{};
/*
Performs an inclusive scan with the given associative binary operation `Op`
* on the data in the `in` buffer. Runs in parallel on the provided accelerated
* hardware queue. Modifies the input buffer to contain the results of the scan.
* Input size has to be a power of two. If the size isn't so, the input can
* easily be padded to the nearest power of two with any values, and the scan on
* the meaningful part of the data will stay the same.
*/
template
<
typename
T,
typename
Op>
void
par_scan
(sycl::buffer<T,
1
> in, sycl::queue q) {
if
((in.
get_count
() & (in.
get_count
() -
1
)) !=
0
|| in.
get_count
() ==
0
) {
throw
std::runtime_error
(
"
Given input size is not a power of two.
"
);
}
auto
dev = q.
get_device
();
//
Check if there is enough global memory.
size_t
global_mem_size = dev.
get_info
<sycl::info::device::global_mem_size>();
if
(in.
get_count
() > global_mem_size) {
throw
std::runtime_error
(
"
Input size exceeds device global memory size.
"
);
}
//
Obtain device limits.
size_t
max_wgroup_size =
dev.
get_info
<sycl::info::device::max_work_group_size>();
size_t
local_mem_size = dev.
get_info
<sycl::info::device::local_mem_size>();
/*
Find a work-group size that is guaranteed to fit in local memory and is
* below the maximum work-group size of the device.
*/
size_t
wgroup_size_lim =
sycl::min
(max_wgroup_size, local_mem_size /
sizeof
(T));
size_t
input_size = in.
get_count
();
size_t
wgroup_size =
0
;
for
(
size_t
pow =
size_t
(
1
) << (
sizeof
(
size_t
) *
8
-
1
); pow >
0
; pow >>=
1
) {
if
((input_size / pow) * pow == input_size && pow <= wgroup_size_lim) {
wgroup_size = pow;
break
;
}
}
if
(wgroup_size ==
0
) {
throw
std::runtime_error
(
"
Could not find an appropriate work-group size for the given input.
"
);
}
using
namespace
sycl
::access
;
q.
submit
([&](sycl::handler& cgh) {
auto
data = in.
template
get_access
<mode::read_write>(cgh);
auto
temp = sycl::local_accessor<T,
1
>(sycl::range<
1
>(wgroup_size), cgh);
cgh.
parallel_for
<kernel_name<T, Op,
class
scan_segments
>>(
sycl::nd_range<
1
>(input_size, wgroup_size), [=](sycl::nd_item<
1
> item) {
size_t
gid = item.
get_global_linear_id
();
auto
sub_group = item.
get_sub_group
();
auto
scan_res =
inclusive_scan_over_group
(sub_group, data[gid], Op{});
if
(sub_group.
get_local_id
() == sub_group.
get_local_range
() -
1
) {
temp[sub_group.
get_group_linear_id
()] = scan_res;
}
item.
barrier
(sycl::access::fence_space::local_space);
for
(
auto
i =
1u
; i < sub_group.
get_group_linear_range
(); i++) {
scan_res += sub_group.
get_group_linear_id
() >= i ? temp[i -
1
] :
0
;
}
data[gid] = scan_res;
});
});
//
At this point we have computed the inclusive scans of this many segments.
size_t
n_segments = input_size / wgroup_size;
if
(n_segments ==
1
) {
//
If all of the data is in one segment, we're done.
return
;
}
//
Store the last element of each segment in a temporary buffer
sycl::buffer<T,
1
> ends{sycl::range<
1
>(n_segments)};
q.
submit
([&](sycl::handler& cgh) {
auto
scans = in.
template
get_access
<mode::read>(cgh);
auto
elems = ends.
template
get_access
<mode::discard_write>(cgh);
cgh.
parallel_for
<kernel_name<T, Op,
class
copy_ends
>>(
sycl::range<
1
>(n_segments), [=](sycl::item<
1
> item) {
auto
id = item.
get_linear_id
();
//
Offset into the last element of each segment.
elems[item] = scans[(id +
1
) * wgroup_size -
1
];
});
});
//
Recursively scan the array of last elements.
par_scan<T, Op>(ends, q);
//
Add the results of the scan to each segment.
q.
submit
([&](sycl::handler& cgh) {
auto
ends_scan = ends.
template
get_access
<mode::read>(cgh);
auto
data = in.
template
get_access
<mode::read_write>(cgh);
cgh.
parallel_for
<kernel_name<T, Op,
class
add_ends
>>(
//
Work with one less work-group, since the first segment is correct.
sycl::nd_range<
1
>(input_size - wgroup_size, wgroup_size),
[=](sycl::nd_item<
1
> item) {
auto
group = item.
get_group_linear_id
();
auto
off_gid = item.
get_global_linear_id
() + wgroup_size;
data[off_gid] = Op{}(data[off_gid], ends_scan[group]);
});
});
}
/*
Tests the scan with an addition operation, which is its most common use.
* Returns 0 if successful, a nonzero value otherwise.
*/
int
test_sum
(sycl::queue& q) {
constexpr
size_t
size =
8192
;
std::vector<
int32_t
>
in
(size);
std::iota
(in.
begin
(), in.
end
(),
1
);
std::vector<
int32_t
>
sum
(in.
size
());
{
sycl::buffer<
int32_t
,
1
>
buf
(sycl::range<
1
>(in.
size
()));
buf.
set_final_data
(sum.
data
());
q.
submit
([&](sycl::handler& cgh) {
auto
acc = buf.
get_access
<sycl::access::mode::write>(cgh);
cgh.
copy
(in.
data
(), acc);
});
par_scan<
int32_t
, sycl::plus<
int32_t
>>(buf, q);
}
std::vector<
int32_t
>
test_sum
(in.
size
());
std::partial_sum
(in.
begin
(), in.
end
(), test_sum.
begin
());
auto
equal =
std::equal
(sum.
begin
(), sum.
end
(), test_sum.
begin
());
if
(!equal) {
std::cout <<
"
SYCL sum computation incorrect! CPU Results:
\n
"
;
for
(
auto
a : test_sum) {
std::cout << a <<
"
\n
"
;
}
std::cout <<
"
\n
SYCL results:
\n
"
;
for
(
auto
a : sum) {
std::cout << a <<
"
\n
"
;
}
std::cout << std::endl;
return
1
;
}
return
0
;
}
int
main
() {
sycl::queue q{sycl::default_selector{}};
if
(
SYCL_LANGUAGE_VERSION
<
202000
) {
std::cout <<
"
This sample must be compiled with SYCL 2020 support
\n
"
;
return
0
;
}
auto
ret =
test_sum
(q);
if
(ret !=
0
) {
std::cout <<
"
Results are not correct.
\n
"
;
return
ret;
}
std::cout <<
"
Results are correct.
\n
"
;
return
0
;
}
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