PR18102, PATCH + test: wrong code on x86_64-linux-gnu (affecting trunk and 3.3)

[Stepan Dyatkovskiy]

Fixed test

Stepan Dyatkovskiy wrote:
> Hi all.
>
> I think, trouble was in DAGCombiner::MergeConsequtiveStores.
>
> Consider, how it works for next example:
>
> store i8 1, a[0]
> store i8 2, a[1]
> store i8 3, a[1]   ; a[1] again.
> return   ; DAG starts here
>
> 1. Method will collect all the 3 stores.
> 2. It sorts them by distance from the base pointer (farthest with
> highest index).
> 3. It takes first consecutive non-overlapping stores and (if possible)
> replaces them with a single store instruction.
>
> The point is, we can't determine here which 'store' instruction
> would be the second after sorting ('store 2' or 'store 3').
> It happens that 'store 3' would be the second, and 'store 2' would be
> the third.
>
> So after merging we have the next result:
>
> store i16 (1 | 3 << 8), base   ; is a[0] but bit-casted to i16
> store i8 2, a[1]
>
> So actually we swapped 'store 3' and 'store 2' and got wrong contents in
> a[1].
>
> Fix: In sort routine just also take into account mem-op sequence number.
>
> -Stepan
>
>
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