[InstCombine] try to narrow a truncated load

Authored by spatel on Jul 25 2019, 5:14 AM.


[InstCombine] try to narrow a truncated load

trunc (load X) --> load (bitcast X to narrow type)

We have this transform in DAGCombiner::ReduceLoadWidth(), but the truncated
load pattern can interfere with other instcombine transforms, so I'd like to
allow the fold sooner.

...in that report, we have bitcasts bracketing these ops, so those could get
eliminated too.

We've generally ruled out widening of loads early in IR ( LoadCombine -
http://lists.llvm.org/pipermail/llvm-dev/2016-September/105291.html ), but
that reasoning may not apply to narrowing if we can preserve information
such as the dereferenceable range.

Differential Revision: https://reviews.llvm.org/D64432

llvm-svn: 367011