[PATCH] D73832: Ignore/Drop droppable uses for code-sinking in InstCombine

[Tyker via Phabricator via llvm-commits]

Tyker marked an inline comment as done.
Tyker added inline comments.


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Comment at: llvm/lib/IR/AsmWriter.cpp:2557
 
+    /// Bundles containing dropped values should be ignored.
+    if (any_of(BU.Inputs, [](const Use &U) { return !U.get(); }))
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Tyker wrote:
> lebedev.ri wrote:
> > Would it be better to ensure that we can't end up in such
> > seemingly-degenerate state in the first place,
> > by cleaning up bundles?
> i agree than not having that state would be better but i don't think it is possible without rebuilding instruction after we dropped uses in them.
> 
> an other solution could be to mark dropped uses with an undef value instead of a nullptr.
i tried to find a better solution to represent "dropped" values. but not solution seems to be good.
 - nullptr is problematic because many place assume that operands are never null.
 - rebuilding instructions seems like too expensive for what we are trying to do.

maybe adding a new kind of value to represent it, that is ignored everywhere. but changing the IR for this seems like overkill.

any thought ?


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  https://reviews.llvm.org/D73832/new/

https://reviews.llvm.org/D73832