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[ValueTracking][ConstantRange] Distinguish low/high always overflow

Authored by nikic on May 26 2019, 6:49 AM.



In order to fold an always overflowing signed saturating add/sub, we need to know in which direction the always overflow occurs. This patch splits up AlwaysOverflows into AlwaysOverflowsLow and AlwaysOverflowsHigh to pass through this information (but it is not used yet).

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nikic created this revision.May 26 2019, 6:49 AM
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lebedev.ri added inline comments.May 26 2019, 12:33 PM

These now need a comment for each.
While with the enum on the LHS of the diff it is obvious,
here i'm not sure what low/high means.
The direction of the overflow?

nikic updated this revision to Diff 201465.May 26 2019, 12:49 PM

Add comments for enum cases.

nikic marked an inline comment as done.May 26 2019, 12:54 PM
nikic added inline comments.

Yeah, this is the direction of overflow. For AlwaysOverflowLow the real result would be lower than signed/unsigned min, for AlwaysOverflowHigh it would be higher than signed/unsigned max.

lebedev.ri accepted this revision.May 27 2019, 11:54 AM

Looks reasonable.


Thanks. Hmm, still a bit confusing, maybe adding a simple examples could help. Or not.


overflows low

This revision is now accepted and ready to land.May 27 2019, 11:54 AM
nikic marked an inline comment as done.May 27 2019, 12:04 PM
nikic added inline comments.

What do you mean by this comment? The unsigned add is an overflow high case.

This revision was automatically updated to reflect the committed changes.