Expanding a bit the EnumSet API to have iterator-based
insert and constructors (like the STL).
This is also a pre-requisite from the capability-trimming pass as
it allows to build a const set from a constexpr std::array easily.
Signed-off-by: Nathan Gauër <brioche@google.com>
The iterator class was initialized by setting the offset
and bucket to 0. Big oversight: what if the first enum is
not valid? Then `*iterator->begin()` would return the wrong
value.
Because the first capacity is Matrix, this bug was not visible by
any SPIRV test.
And this specific case wasn't tested correctly in the new enumset tests.
Signed-off-by: Nathan Gauër <brioche@google.com>
---------
Signed-off-by: Nathan Gauër <brioche@google.com>
This commit adds forward iterator, and renames functions to
it matches the std::unordered_set/std::set better.
This goes against the SPIR-V coding style, but might be better in
the long run, especially when this set is used along real STL
sets.
(Right now, they are not compatible, and requires 2 syntaxes).
This container could in theory handle bidirectional
iterator, but for now, only forward seemed required for
our use-cases.
Signed-off-by: Nathan Gauër <brioche@google.com>
The current EnumSet implementation is only efficient for enums with
values < than 64. The reason is the first 63 values are stored as a
bitmask in a 64 bit unsigned integer, and the other values are stored
in a std::set.
For small enums, this is fine (most SPIR-V enums have IDs < than 64),
but performance starts to drop with larger enums (Capabilities,
opcodes).
Design considerations:
----------------------
This PR changes the internal behavior of the EnumSet to handle enums
with arbitrary values while staying performant.
The idea is to extend the 64-bits buckets sparsely:
- each bucket can store 64 value, starting from a multiplier of 64.
This could be considered as a hashset with linear probing.
- For small enums, there is a slight memory overhead due to the bucket
storage, but lookup is still constant.
- For linearly distributed values, lookup is constant.
- Worse case for storage are for enums with values which are multiples of 64.
But lookup is constant.
- Worse case for lookup are enums with a lot of small ranges scattered in
the space (requires linear probing).
For enums like capabilities/opcodes, this bucketing is useful as values
are usually scatters in distinct, but almost contiguous blocks.
(vendors usually have allocated ranges, like [5000;5500], while [1000;5000]
is mostly unused).
Benchmarking:
-------------
Benchmarking was done in 2 ways:
- a benchmark built for the occasion, which only measure the EnumSet
performance.
- SPIRV-Tools tests, to measure a more realist scenario.
Running SPIR-V tests with both implementations shows the same
performance (delta < noise). So seems like we have no regressions.
This method is noisy by nature (I/O, etc), but the most representative
of a real-life scenario.
Protocol:
- run spirv-tests with no stdout using perf, multiple times.
Result:
- measure noise is larger than the observed difference.
The custom benchmark was testing EnumSet interfaces using SPIRV enums.
Doing thousand of insertion/deletion/lookup, with 2 kind of scenarios:
- add once, lookup many times.
- add/delete/loopkup many time.
For small enums, results are similar (delta < noise). Seems relevant
with the previously observed results as most SPIRV enums are small, and
SPIRV-Tools is not doing that many intensive operations on EnumSets.
Performance on large enums (opcode/capabilities) shows an improvement:
+-----------------------------+---------+---------+---------+
| Metric | Old | New | Delta % |
+-----------------------------+---------+---------+---------+
| Execution time | 27s | 7s | -72% |
| Instruction count | 174b | 129b | -25% |
| Branch count | 28b | 33b | +17% |
| Branch miss | 490m | 26m | -94% |
| Cache-misses | 149k | 26k | -82% |
+-----------------------------+---------+---------+---------+
Future work
-----------
This was by-design an NFC change to compare apples-to-apples.
The next PR aims to add STL-like iterators to the EnumSet to allow
using it with STL algorithms, and range-based for loops.
Signed-off-by: Nathan Gauër <brioche@google.com>
* Remove use of deprecated googletest macro
INSTANTIATE_TEST_CASE_P has been deprecated. We need to use
INSTANTIATE_TEST_SUITE_P instead.
* Remove extra commas from test suites.
Re-formatted the source tree with the command:
$ /usr/bin/clang-format -style=file -i \
$(find include source tools test utils -name '*.cpp' -or -name '*.h')
This required a fix to source/val/decoration.h. It was not including
spirv.h, which broke builds when the #include headers were re-ordered by
clang-format.
