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[ValueTracking] don't delete assumes of side-effectful instructions
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Authored by arielb1 on Aug 10 2017, 12:54 PM.

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Summary

ValueTracking has to strike a balance when attempting to propagate information backwards from assumes, because if the information is trivially propagated backwards, it can appear to LLVM that the assumption is known to be true, and therefore can be removed.

This is sound (because an assumption has no semantic effect except for causing UB), but prevents the assume from allowing further optimizations.

The isEphemeralValueOf check exists to try and prevent this issue by not removing the source of an assumption. This tries to make it a little bit more general to handle the case of side-effectful instructions, such as in

%0 = call i1 @get_val()
%1 = xor i1 %0, true
call void @llvm.assume(i1 %1)

FIX: try to make llvm-commits see this

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rL LLVM

Event Timeline

hfinkel accepted this revision.Aug 10 2017, 8:09 PM

LGTM

This revision is now accepted and ready to land.Aug 10 2017, 8:09 PM
This revision was automatically updated to reflect the committed changes.