[Bug tree-optimization/21485] [5/6/7 Regression] missed load PRE, PRE makes i?86/7 suck

["law at redhat dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=21485

Jeffrey A. Law <law at redhat dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |law at redhat dot com

--- Comment #62 from Jeffrey A. Law <law at redhat dot com> ---
So one thing that seems to ever so slightly improve this code is to realize
that as we come around to the top of the loop the test _14 < j_20 can be
rewritten as _14 != j_20 (we already know that _14 <= j_20).

That's quite a surprise as all that seems to do is allow propagation of j_20
for _14 in a later PHI node and we do less use work in PRE -- so it seems like
a step backwards at this point.

But then sink comes along and moves two address computations and a load into a
more control dependent block.  LIM4 later pulls the two address computations
totally out of the loop.

But that all seems to be relatively accidental improvements based on PRE not
seeing a transformation.