[PATCH] D40304: [InstCombine] PR35354: Convert load bitcast (select (Cond, &V1, &V2)) --> select(Cond, load bitcast &V1, load bitcast &V2)

[Daniel Berlin via Phabricator via llvm-commits]

dberlin added a comment.

So, what Sanjoy has pointed out is that canonicalization like this to select, breaks the ability to remove redundant loads and stores and do PRE, compared to the equivalent control flow version, in general.

While it's probably true that 
IE your canonicalization of:
load bitcast (select (Cond, &V1, &V2))
to
select(Cond, load bitcast &V1, load bitcast &V2)

is not likely to break this more, and will make things a little better,  all of our redundancy elimination passes and knowledge propagation passes would be happier with the control flow non-select version.

This is not possible to fix in those passes sanely (you'd need a fake cfg with fake instructions and fake operands to simulate the control flow).
Otherwise, the only way around this is to not canonicalize to select this way that early.


https://reviews.llvm.org/D40304