Shifts deopt on negative constants #34072
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area-vm
Use area-vm for VM related issues, including code coverage, and the AOT and JIT backends.
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Aug 9, 2018
Rationale: This improves JIT and AOT performance of unary minus and also improves constant folding and range analysis on negative constant (viz. x / -3 is often x / - (3)). The SHIFT operator needed some special treatment, since we have to avoid converting a NON-speculative shifts back into a deopt. #34072 Change-Id: I230c9cfda98297f683bbba53688e57c2cc659360 Reviewed-on: https://dart-review.googlesource.com/68434 Reviewed-by: Alexander Markov <[email protected]> Reviewed-by: Vyacheslav Egorov <[email protected]> Commit-Queue: Aart Bik <[email protected]>
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The dart compiler current replaces negative shifts (when they will throw) with a deopt, which seems an acceptable detour for JIT but is devastating for AOT. Apparently this was not an issue until now since the dart compiler does not deal very well with negative constants (viz -3 really is -(3)).
A recent improvement in recognizing static calls to unary neg as a native operation exposes this shortcoming, since it improves constant folding (and range analysis) and changes some of the shifts in the tests to deopts, a big nono for AOT.
Before I can improve the negate, this needs to be dealt with.
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