[ShrinkBorrowScope] Don't hoist over any applies. #40764
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Previously, the body--of the loop that iterated the instructions in the block--contained the check that the instruction over which hoisting might be done is the introducer. Here, that check is moved into tryHoistOverInstruction. Now that function and its other caller, canHoistOverInstruction, will return the right value (false) if they are ever called with the introducer (though that could not happen without this change).
Previously, hoisting was done over applies that were users of the borrowed value. Here, that is no longer done. That hoisting was done on the theory that multiple distinct lexical scopes were equivalent to a single enclosing lexical scope. Dead borrow elimination, however, means that they are not in fact the same, however: If a callee takes an object as an argument, and in that callee that argument is dead, FSO and inliniing (even with approaches to maintain lexical borrow scopes when the value that is borrowed comes from an owned argument) can result in a dead borrow scope in the caller which could then be eliminated which would then enable the destroy_value to be hoisted over the inlined body, including over barriers.
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@swift-ci please test |
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Build failed |
atrick
approved these changes
Jan 8, 2022
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This change is safe of course but
- the code you're ripping out will still be useful for when we do hoist over applies. You just need to convert the
copy_valuetocopy_value [lexical]to tie the two scopes together
- we'll need to make sure any regressions are fixed when we reenable hoisting... or defer this change until we have a way to hoist safely
@nate-chandler the second half of this comment was nonsense. I had thought that this code was already enabled by default.
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@swift-ci please test macos platform |
nate-chandler
deleted the
copy-propagation/shrink-borrow-scope/dont-hoist-over-any-applies
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Previously, hoisting was done over applies that were users of the borrowed value. Here, that is no longer done.
That hoisting was done on the theory that multiple distinct lexical scopes were equivalent to a single enclosing lexical scope. Dead borrow elimination, however, means that they are not in fact the same: If a callee takes an object as an argument, and in that callee that argument is dead, FSO and inliniing (even with approaches to maintain lexical borrow scopes when the value that is borrowed comes from an owned argument) can result in a dead borrow scope in the caller which could then be eliminated which would then enable the destroy_value to be hoisted over the inlined body, including over barriers.