[vm, compiler] Unoptimized megamorphic calls.

When an instance call in unoptimized code creates more than FLAG_max_polymorphic_checks cases, switch the call to use a MegamorphicCache instead of ICData. The prevents unbounded collection of type feedback, and gives improvements on microbenchmarks in the 3-8% range for unoptimized code.

It also leads to a loss of target frequency information for the optimizer, leading to different ordering for range checks in polymorphic inlining. This leads to changes on megamorphic microbenchmarks from -31% to +60%, weighted toward the negative end.

In practice the frequency information seems unimportant, as dart2js has 4.01% geomean improvement.

This is a step toward direct monomorphic calls in unoptimized code, which will also make use of the patching and type feedback extraction added here.

Bug: https://github.com/dart-lang/sdk/issues/26780
Bug: https://github.com/dart-lang/sdk/issues/36409
Bug: https://github.com/dart-lang/sdk/issues/36731
Change-Id: I29f53f23b6794c5f5f0db8b8184788cee16fd9c5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/99270
Reviewed-by: Alexander Markov <alexmarkov@google.com>
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A client-optimized language for fast apps on any platform

Dart is:

  • Optimized for UI: Develop with a programming language specialized around the needs of user interface creation

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Dart has flexible compiler technology lets you run Dart code in different ways, depending on your target platform and goals:

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