commit dfc1b6b16b99b44901a01cfaa1fba229e8e0c445
author Tess Strickland <sstrickl@google.com> Wed Jun 21 15:24:30 2023 +0000
committer Commit Queue <dart-scoped@luci-project-accounts.iam.gserviceaccount.com> Wed Jun 21 15:24:30 2023 +0000
tree b5f4d109cd9857adea561b7ceb6e0f46ba961312
parent 24ad7f9000ba137a01f5c819d3cb7c1358a209d8

[vm] Store the number of used inputs in SubtypeTestCaches.

Previously, SlowTypeTestStub would use the incoming instance and
destination type to determine which `SubtypeNTestCacheStub` to call,
where `N` is the number of inputs checked for each entry. The code
for generating `InstanceOf` instructions performs similar work,
inlining some checks and then falling back to checking and updating
an appropriate `SubtypeNTestCacheStub` if the inlined checks fail.

Meanwhile, the runtime would use all available information to
create a new entry when needed, depending on the `SubtypeNTestCacheStub`
called at runtime to ignore the parts of the entry it didn't need.

To illustrate a situation where the runtime might use more
information to create an entry than needed by the stub, consider
the following class header:

```
  class C<T> extends D
```

If no subtype of `D` requires knowing the specific type arguments for
an instance to determine whether that instance is an instance of `D`,
then the compiler can generate a `Subtype1TestCacheStub` call, where
only the class id of the instance is checked.

However, in the runtime, when updating the cache, the runtime will
create an entry that includes the specific type arguments used to
instantiate `C` when the initial cache miss occurred, as the runtime
does not know how many inputs the code that checks that SubtypeTestCache
uses. If the cache has grown to the point that it is hash-based, that
means the runtime uses the instance type arguments of the first cache
miss to determine the correct place to create a new entry.

When the stub only performs a linear search over the cache contents,
this works, as every entry is checked until a matching entry is found,
and thus the stub can ignore unneeded information. However, if a cache
is hash-based, this doesn't work: if the first miss was an instance
of `C<int>`, then the runtime includes all that information in a new
entry. This means the stub has to calculate the instance type
arguments, which it did not have to do before, and if a subsequent
check uses different instance type arguments (e.g., an instance of
`C<String>`), the result may still be a cache miss, and the runtime
adds another entry which has the same information as the previous
entry as far as the stub was originally concerned.

Thus, to limit itself to considering only the inputs used by the
appropriate `SubtypeNTestCacheStub`, the runtime needs to be told how
many inputs are used. This CL adds an additional field to
`SubtypeTestCache`s which contains the number of used inputs, and
changes the places where `SubtypeTestCache`s are allocated in the
flow graph compiler and the `DRT_TypeCheck` runtime entry to pass the
appropriate number of inputs. To ensure that the runtime and stubs
agree on how many inputs should be used, the `SlowTypeTestStub` now
uses the value of this field to determine which `SubtypeNTestCacheStub`
to call.

TEST=vm/cc/STC_{Linear,Hash}Lookup

Bug: https://github.com/dart-lang/sdk/issues/48345
Change-Id: Ie16f2d12c4c5eae856b668f28178818203662d1c
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/+/310180
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>
Reviewed-by: Alexander Markov <alexmarkov@google.com>