[PATCH][instcombine] Remove preservesCFG from instcombine

[Mark Heffernan]

On Tue, Nov 4, 2014 at 12:15 PM, Chandler Carruth <chandlerc at google.com>
wrote:
>
>  Should LoopSimplifyPass be marked as IsCFGOnlyPass?  It clearly modifies
>> the CFG.  From the "writing an llvm pass" manual:  "if a pass walks CFG
>> without modifying it then the third argument [isCFGOnly] is set to true",
>> however, I don't know how definitive that is and the logic of that
>> statement doesn't exactly imply that a CFG-modifying pass can't be marked
>> isCFGOnly.  If LoopSimplifyPass is changed to !isCFGOnly, then preservesCFG
>> can be added back to instcombine (at least for the purposes of the bug I
>> ran into).
>
>
> This seems like the correct fix to me. I strongly disagree with the
> original approach (although I understand how you got there).
>

Yep.  Slowly figuring things out here :-)

There is absolutely no reason why preserving the CFG should be equivalent
> to preserving LoopSimplify.
>
> In fact, preserving LoopSimplify never *really* makes sense because it is
> a transformation pass, not an analysis pass. But in the current pass
> manager we have no effective alternative. =[
>

If only someone was working on a new pass manager... ;-)

If no one has any objections, I'll make the change restoring setpreservecfg
to instcombine and changing loopsimplify to !isCfgOnly.

Mark
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