We use specialized type testing stubs in `as` checks to quickly answer
whether the type of an object is assignable to a given type. However,
perfect specialization of the type testing stub is not possible in all
cases. Thus, we use SubtypeTestCaches (STCs) to cache the result of
checking assignability in the runtime to avoid repeated runtime calls
for the same check.
Previously, the STC was linear and capped at 100 entries. This cap was
for two reasons:
* STC are created per `as` check, not per checked type, so there could
be many STCs created for a given type if assignability to that type
is checked many times throughout the code.
* STC were strictly linear, and thus eventually the cost of
traversing the cache outweighs the cost of returning a false
negative that requires going to the runtime and doing the check there.
This CL provides a partial solution to the second reason by performing
similar work to that done for instantiation caches in TypeArguments
(4f925105), where the STC is linear up to a certain threshold, and then
converted to a hash-based STC. Like that CL, only the runtime currently
handles the lookup of cached results in hash-based STCs, with the
changes needed for the `SubtypeNTestCacheStub`s coming in a followup CL.
Since the stubs have not yet been adjusted, we keep the maximum size of
linear caches the same at 100, and allow hash-based STCs to grow to a
maximum size of 1000 since the STCs are still created per `as` check.
This means once the STC has grown over 100 entries, all STC checks
require a runtime call, but for checks cached in the STC, the answer is
found and provided without either doing the slower full runtime check
for assignability and without an attempt to respecialize the type
testing stub when applicable.
Our benchmarks generally see at most a ~2-5% performance regression for
cases where the STC remains linear (except for on the Raspberry Pi 4, where the hit ranges from ~25%-40%), but a performance improvement of
~500-1300% for cases where the STC is converted to a hash table.
TEST=vm/cc/TTS_STC_{Some,Many}Asserts
Change-Id: Ie8629708b5c12fa8464359a2f77c6da56ba46c0c
Cq-Include-Trybots: luci.dart.try:vm-aot-linux-debug-simriscv64-try,vm-aot-linux-debug-x64-try,vm-aot-linux-release-x64-try,vm-aot-linux-product-x64-try,vm-aot-linux-release-simarm64-try,vm-aot-linux-release-simarm_x64-try,vm-aot-linux-debug-x64c-try,vm-aot-mac-release-arm64-try,vm-kernel-precomp-linux-release-x64-try,vm-kernel-precomp-linux-debug-x64-try,vm-aot-dwarf-linux-product-x64-try,vm-linux-release-ia32-try,vm-linux-debug-x64c-try,vm-linux-debug-x64-try,vm-linux-debug-simriscv64-try,vm-linux-release-simarm64-try,vm-linux-release-simarm-try,vm-mac-release-arm64-try,vm-mac-release-x64-try,vm-tsan-linux-release-x64-try,vm-reload-rollback-linux-release-x64-try,vm-reload-linux-release-x64-try,vm-ffi-qemu-linux-release-arm-try
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/284682
Reviewed-by: Alexander Markov <alexmarkov@google.com>
Reviewed-by: Martin Kustermann <kustermann@google.com>
Tess Strickland
c93c5bdd88