SIL: add the borrowed-from instruction #71176
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meg-gupta
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While working on guaranteed phis, I think I encountered some cases where enclosing value could be adjacent or dominating. Does the borrowed from insertion pass handle when this can happen ?
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I'm not sure what you mean exactly. Yes, the enclosing values (= the operands after from) can be an adjacent phis or dominating borrow introducers.
This is computed iteratively (until convergence) from incoming values in predcessors
…a reverse dependency
It declares from which enclosing values a guaranteed phi argument is borrowed from.
Compute, update and handle borrowed-from instruction in various utilities and passes. Also, used borrowed-from to simplify `gatherBorrowIntroducers` and `gatherEnclosingValues`. Replace those utilities by `Value.getBorrowIntroducers` and `Value.getEnclosingValues`, which return a lazily computed Sequence of borrowed/enclosing values.
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The borrowed-from instruction declares from which enclosing values a guaranteed phi argument is borrowed from. For example:
An enclosing value is either a dominating borrow introducer of the borrowed operand (
%3) or an adjacent phi-argument in the same block (%4).So far, this information was computed dynamically by ownership utilities (e.g.
gatherEnclosingValues). With the borrowed-from instructions those utilities get dramatically simpler.The borrowed-from instruction is similar in spirit to phi-nodes in plain SSA: it "caches" information from predecessor blocks so that optimization passes can easily access this information. If an optimization pass introduces a new guaranteed phi argument it can use the
BorrowedFromUpdaterto generate the required borrowed-from instructions. This is similar toSSAUpdaterwhich inserts required phi-nodes.Beside simplifying the ownership utilities, it's also useful that the adjacent phi relations are now explicitly visible in SIL.
Another nice thing is that this change is (almost) a pure addition to SIL. This means that the existing C++ ownership utilities still work - with and without inserted borrowed-from instructions. The only exception is that they have to look through borrowed-from instructions in some places.