[PATCH] D39835: [GVN PRE] Clear nsw/nuw for original values in LoadPRE

[Daniel Berlin via llvm-commits]

(and past this, i'm going to leave it alone. I have time for very little
LLVM these days, and deep ingrained GVN issues ain't gonna be what i spend
my little LLVM time on)


On Tue, Dec 5, 2017 at 1:54 PM Daniel Berlin <dberlin at dberlin.org> wrote:

>
>> I can't tell from your example enough detail to know what you're aiming
>> for, but if there's a bug here, it's in replacing the add, not forming the
>> phi node.
>>
>
> I believe you have missed the point, so i'll make it a little later.
>
>
>>
>> > patchAndReplaceAlluses will not remove the nsw from the phi in this
>> case.
>> >  We could teach it to do so.
>> >  The other option is this patch, and fixing load PRE when it creates
>> the phi in the first place.
>>
>> The phi is entirely correct and could be hand written.  If GVN is
>> considering adds with different flags equivalent without appropriate
>> backpatching, that's a bug in GVN's replacement.
>
> End of story.
>>
>
> Sure, but there is nowhere else to fix it.
> When GVN does replacement, it assumes every instruction it is replacing
> with has already been fixed.
> It has no way of saying "hey, somewhere in the chain of this instruction
> may be something you may have to fix later".
>
> It is not recursive in patching at all.
>
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