[PATCH] D125774: [InstCombine] fold float trunc into exact itofp for small constants

[Nikita Popov via Phabricator via llvm-commits]

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I'm not strongly opposed to adding some basic IV reasoning to computeKnownBits(), but the implementation needs to be much more careful. I think there are //at least// three bugs here (missing check for predicate, no check whether this is a loop exit or continue condition, no reasoning about how the start value range relates to the end value). This requires much more extensive testing (and those tests should be simpler, not involving float operations).

If you want to add this, you'd probably want to start over with just handling a canonical IV (step 1 incrementing IV with start smaller and end and ult or ne condition), and expand from there.

Generally, the utility that handles this kind of simplification based on IV ranges is SimplifyIndVars -- that one is based on the full power of SCEV, and will be able to deal with non-trivial recurrences. Extending it to handle this case is probably the most correct approach. Maybe there are complications with looking through float operations there though.


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https://reviews.llvm.org/D125774