[Bug rtl-optimization/77499] [7 Regression] Regression after code-hoisting, due to combine pass failing to evaluate known value range

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77499

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
           Keywords|                            |missed-optimization
   Target Milestone|---                         |7.0
            Summary|Regression after            |[7 Regression] Regression
                   |code-hoisting, due to       |after code-hoisting, due to
                   |combine pass failing to     |combine pass failing to
                   |evaluate known value range  |evaluate known value range

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
on x86 the code is if-converted which might help combine (though there is
nothing to combine on x86).

It looks like nonzero_bits never looks at sets of REGs and thus can only work
on expressions.  Code hoisting generally makes sub-expressions no longer
single-use (in this case the x >> 1) which means they won't appear as part of
a larger expression (and combine doesn't consider re-combining them unless in
some very specific circumstances).  In this case r111:SI is used multiple
times.

It seems with thumb the code is only if-converted after reload for some reason.