When available, take advantage of precomputed protocol conformances in the shared cache on Darwin platforms to accelerate conformance lookups and cut down on memory usage.
We consult the precomputed conformances before doing the runtime's standard conformance lookup. When a conformance is precomputed, this avoids the slow linear scan of all loaded conformances for the first access, and it avoids the memory cost of storing the conformance in the cache.
When the shared cache has no images overridden (the normal case), then we can also skip scanning conformances in the shared cache in all circumstances, because the precomputed conformances will always cover those results. This greatly speeds up the slow linear scan for the initial lookup of anything that doesn't have a conformance in the shared cache, including lookups with conformances in apps or app frameworks, and negative lookups.
A validation mode is available by setting the SWIFT_DEBUG_VALIDATE_SHARED_CACHE_PROTOCOL_CONFORMANCES environment variable. When enabled, results from the precomputed conformances are compared with the results from a slow scan of the conformances in the shared cache. This extremely slow, but good at catching bugs in the system.
When the calls for precomputed conformances are unavailable, the new code is omitted and we remain in the same situation as before, with the runtime performing all lookups by scanning conformance sections in all loaded Swift images.
rdar://71128983
Instead of doing the sanity checks by default (with an assert-build of the stdlib), only do the checks if the environment variable SWIFT_DEBUG_ENABLE_COW_SANITY_CHECKS is set to true.
The checks can give false alarms in case a binary is built against a no-assert stdlib but run with an assert-stdlib.
Therefore only do the checks if it's explicitly enabled at runtime.
rdar://problem/65475776
Previously, when NDEBUG was not defined, the allocations made for value
metadata records were filled with 0xAA bytes. Here, that behavior is
both expanded to all metadata records and enabled in release builds when
the environment variable SWIFT_DEBUG_ENABLE_MALLOC_SCRIBBLE is set.
There are a few environment variables used to enable debugging options in the
runtime, and we'll likely add more over time. These are implemented with
scattered getenv() calls at the point of use. This is inefficient, as most/all
OSes have to do a linear scan of the environment for each call. It's also not
discoverable, since the only way to find these variables is to inspect the
source.
This commit places all of these variables in a central location.
stdlib/public/runtime/EnvironmentVariables.def defines all of the debug
variables including their name, type, default value, and a help string. On OSes
which make an `environ` array available, the entire array is scanned in a single
pass the first time any debug variable is requested. By quickly rejecting
variables that do not start with `SWIFT_`, we optimize for the common case where
no debug variables are set. We also have a fallback to repeated `getenv()` calls
when a full scan is not possible.
Setting `SWIFT_HELP=YES` will print out all available debug variables along with
a brief description of what they do.
