commit 477d965a3b6c8ffaf0d43367ff258593d7d07e38
author gregce <gregce@google.com> Wed Jun 05 13:40:21 2019 -0700
committer Copybara-Service <copybara-worker@google.com> Wed Jun 05 13:43:03 2019 -0700
tree 493ab5717837a3abdf7c5bdd6bbb921c944fecaf
parent 267df73bfd00fdd3e20d7b5b12b1b8624262b341

Manual trimming: trim all feature flags from toolchain deps.

This is a performance optimization following up on https://bazel-review.googlesource.com/c/bazel/+/101952.

That change reduced the number of BuildOptions hash key computations on a simple manually trimming android_binary build from 809 to 475, compared to 138 when manual trimming is disabled (--enforce_transitive_configs_for_config_feature_flag=false). But I wanted to dig further into the remaining (+337) difference.

Profiling showed most of these were deps on platform and toolchain rules. For example, transitions from @bazel_tools//platforms:[host|target]_platform to their constituent constraint_values accounted for 192 of the difference. Transitions from toolchain rules to their deps made up most of the rest.

This highlights the following inefficiency in the intersection of manual trimming and toolchain resolution:

Given "rule -> unloaded toolchain context -> all toolchains and platforms and their dependencies", the current logic uses the rule's configuration for the toolchain context and top-level platform/toolchain definitions. If this happens when "rule" is an android_binary setting transitive_configs, this means those definitions inherit its feature flags in *their* configs.

Then *their* deps, which don't use feature flags, go through the standard manual trimming transition, which removes those flags which requires new BuildOptions for each one.

In other words:

 <@bazel_tools//platforms:target_platform, featureFlags=[foo, bar]>
    --> <some_platform_constraint1, featureFlags=[]>   # Remove flags: creates a new BuildOptions
    --> <some_platform_constraint2, featureFlags=[]>   # Remove flags: creates a new BuildOptions
    --> <some_platform_constraint3, featureFlags=[]>   # Remove flags: creates a new BuildOptions
    ...

and so on.

There's no need to do that for all those dependencies. Instead, we can apply the transition directly on the "rule -> unloaded toolchain context" dependency, of which there's only one. As long as toolchains and platforms don't actually need feature flags (which they shouldn't), this is safe.

So this PR does that. With that change, we get:

Manual trimming disabled:
--------------------------------------------------------------------------
BuildOptions hashkey calls:                                                       137
ConfigurationResolver transition.apply() calls:                                   447
Manual trimming calls creating a new BuildOptions instance:                       0
ConfiguredTargetFunction total deps evaluated:                                    442,617
ConfiguredTargetFunction non-trivial (not NOOP, NULL, Transition) deps evaluated: 1,419
ConfiguredTargetFunction non-trivial transition.apply calls:                      445

Manual trimming enabled, before this change:
--------------------------------------------------------------------------
BuildOptions hashkey calls:                                                       494
ConfigurationResolver transition.apply() calls:                                   588
Manual trimming calls creating a new BuildOptions instance:                       227
ConfiguredTargetFunction total deps evaluated:                                    443,126
ConfiguredTargetFunction non-trivial (not NOOP, NULL, Transition) deps evaluated: 1,932
ConfiguredTargetFunction non-trivial transition.apply calls:                      586

Manual trimming enabled, after this change:
--------------------------------------------------------------------------
BuildOptions hashkey calls:                                                       294
ConfigurationResolver transition.apply() calls:                                   466
Manual trimming calls creating a new BuildOptions instance:                       78
ConfiguredTargetFunction total deps evaluated:                                    438,699
ConfiguredTargetFunction non-trivial (not NOOP, NULL, Transition) deps evaluated: 1,496
ConfiguredTargetFunction non-trivial transition.apply calls:                      464

PiperOrigin-RevId: 251708695