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dotnet / runtime

.NET is a cross-platform runtime for cloud, mobile, desktop, and IoT apps.
https://docs.microsoft.com/dotnet/core/
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Improve handling of fixed register requirements #7966

Open CarolEidt opened 7 years ago

CarolEidt commented 7 years ago

When a node has a delayed-use register (either due to an RMW def or due to an internalRegDelayFree), if its result has a fixed-use, we can't propagate the register constraint to the def, because there's no associated fixed-reference on the RegRecord to ensure that it will not be used by the delay-free register. So, in those cases we leave the registerAssignment unchanged on the def, although the Interval will be preferenced to that if there is no conflict. See the code in LinearScan::associateRefPosWithInterval(). This means that we may often require a copy at the use of this node's result. This case could be moved to BuildRefPositionsForNode(), at the point where the def RefPosition is created, causing a RefTypeFixedRef to be added at that location. This, however, results in:

There may be an alternative approach to handle this issue, but in any case further investigation is required. This came to my attention while looking at dotnet/runtime#7931 category:cq theme:register-allocator skill-level:expert cost:medium

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ghost commented 1 year ago

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