Stephen Adams
e66a0cbde8
- Add SSA HArrayFlags{Check,Get,Set} instructions to check for
fixed-length and unmodifiable JSArray and JavaScript typed data
instances.
- Add optimizations to remove redundant checks.
- Added HOutputConstrained interface for instructions that must have
the input and output in the same JavaScript variable. This
generalizes some code generation logic already applied to HCheck.
Bug: #53785
Change-Id: I61750dd03aa3a964eed3bc76e1656c5f60f77109
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372002
Commit-Queue: Stephen Adams <sra@google.com>
Reviewed-by: Mayank Patke <fishythefish@google.com>
64 lines
2.8 KiB
Dart
64 lines
2.8 KiB
Dart
// Copyright (c) 2024, the Dart project authors. Please see the AUTHORS file
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// for details. All rights reserved. Use of this source code is governed by a
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// BSD-style license that can be found in the LICENSE file.
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/// Values for the 'array flags' property that is attached to native JSArray and
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/// typed data objects to encode restrictions.
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///
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/// We encode the restrictions as two bits - one for potentially growable
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/// objects that are restricted to be fixed length, and a second for objects
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/// with potentially modifiable contents that are restricted to have
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/// unmodifiable contents. We only use three combinations (00 = growable, 01 =
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/// fixed-length, 11 = unmodifiable), since the core List class does not have a
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/// variant that is an 'append-only' list (would be 10).
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///
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/// We use the same scheme for typed data so that the same check works for
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/// modifiability checks for `a[i] = v` when `a` is a JSarray or Uint8Array.
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///
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/// `const` JSArray values are tagged with an additional bit for future use in
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/// avoiding copying. We could also use the `const` bit for unmodifiable typed
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/// data for which there in no modifable view of the underlying buffer.
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///
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/// Given the absense of append-only lists, we could have used a more
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/// numerically compact scheme (0 = growable, 1 = fixed-length, 2 =
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/// unmodifiable, 3 = const). This compact scheme requires comparison
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///
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/// if (x.flags > 0)
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///
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/// rather than mask-testing
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///
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/// if (x.flags & 1)
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///
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/// The unrestricted flags (0) are usually encoded as a missing property, i.e.,
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/// `undefined`, which is converted to NaN for '>', but converted to the more
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/// comfortable '0' for '&'. We hope that there is less to go potentially go
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/// wrong with JavaScript engine slow paths with the bitmask.
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class ArrayFlags {
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/// Value of array flags that marks a JSArray as being fixed-length. This is
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/// not used on typed data since that is always fixed-length.
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static const int fixedLength = fixedLengthCheck;
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/// Value of array flags that marks a JSArray or typed-data object as
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/// unmodifiable. Includes the 'fixed-length' bit, since all unmodifiable
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/// lists are also fixed length.
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static const int unmodifiable = fixedLengthCheck | unmodifiableCheck;
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/// Value of array flags that marks a JSArray as a constant (transitively
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/// unmodifiable).
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static const int constant =
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fixedLengthCheck | unmodifiableCheck | constantCheck;
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/// Default value of array flags when there is no flags property. This value
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/// is not stored on the flags property.
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static const int none = 0;
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/// Bit to check for fixed-length JSArray.
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static const int fixedLengthCheck = 1;
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/// Bit to check for unmodifiable JSArray and typed data.
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static const int unmodifiableCheck = 2;
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/// Bit to check for constant JSArray.
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static const int constantCheck = 4;
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}
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