llvm dev - Mar 2019 - PHI / Select Simplification

Hi all,

I'm working on some LLVM 7 code and I was wondering if there exists an
optimization pass (perhaps that exists in a later version of LLVM) that
does the following. Or if not, a good pass for me to implement this inside
of may be.

Suppose you have the following loop:

for:                ; preds = %entry, %for
  %phi = phi double [ %entry_value, %entry ], [ %selected_value, %for ]
  %condition = ...
  %selected_value = select i1 %condition, double %inner_value, double
%any_value
  br i1 %condition, label %for, label %loop_exit

In this case, (and similarly cases where the loop condition is the same as
the select condition), we may simply replace %selected_value in the for
loop with %inner_value from the select (and therefore remove the select).

This type of structure comes up frequently in some codes I'm looking at and
figured I'd check if anyone has something to deal with it already before I
implement my own (and try to upstream).

Cheers,
Billy Moses
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On Sat, 16 Mar 2019 at 04:28, William Moses via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> This type of structure comes up frequently in some codes I'm looking at
and figured I'd check if anyone has something to deal with it already before
I implement my own (and try to upstream).
This seems to be done by a combination of Correlated Value Propagation
(which forwards %inner_value to the phi) and InstCombine (which
removes the now-dead select) on a simple example I fleshed out
(attached here). You can try it out with "opt -correlated-propagation
-instcombine -S simple.ll".

It's possible the value propagation isn't clever enough for your real
code, in which case it's probably still the best place to look at
fixing things in LLVM rather than adding a new pass.

Cheers.

Tim.
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