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[InstCombine] combineLoadToOperationType(): don't fold int<->ptr cast into load

Authored by lebedev.ri on Oct 11 2020, 10:17 AM.

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[InstCombine] combineLoadToOperationType(): don't fold int<->ptr cast into load

And another step towards transforms not introducing inttoptr and/or
ptrtoint casts that weren't there already.

As we've been establishing (see D88788/D88789), if there is a int<->ptr cast,
it basically must stay as-is, we can't do much with it.

I've looked, and the most source of new such casts being introduces,
as far as i can tell, is this transform, which, ironically,
tries to reduce count of casts..

On vanilla llvm test-suite + RawSpeed, @ -O3, this results in
-33.58% less IntToPtrs (19014 -> 12629)
and +76.20% more PtrToInts (18589 -> 32753),
which is an increase of +20.69% in total.

However just on RawSpeed, where i know there are basically
none IntToPtr in the original source code,
this results in -99.27% less IntToPtrs (2724 -> 20)
and +82.92% more PtrToInts (4513 -> 8255).
which is again an increase of 14.34% in total.

To me this does seem like the step in the right direction,
we end up with strictly less IntToPtr, but strictly more PtrToInt,
which seems like a reasonable trade-off.

See https://reviews.llvm.org/D88860 / https://reviews.llvm.org/D88995
for some more discussion on the subject.

(Eventually, CastInst::isNoopCast()/CastInst::isEliminableCastPair
should be taught about this, yes)

Reviewed By: nlopes, nikic

Differential Revision: https://reviews.llvm.org/D88979