[PATCH] Fix aliasing issue caused by MemCpyOptimizer (PR18304)

[Philip Reames]

Your change does address a correctness issue and does so in a reasonable 
way.  I wouldn't worry too much about the possible performance regression.

One mandatory change:
- Add a test case which checks the optimization does not apply when 
accessed via capturing parameter.

I'd also prefer that you:
- either use std::distance (rather than iterator subtraction) or 
(preferred) use a argument number indexed loop.
- If you keep the iterator loop, consider a range loop.  Or at least 
move the declaration of B, E into the initializer of the loop to 
restrict their scope.

Philip

On 06/20/2014 06:13 AM, David Wiberg wrote:
> Hello,
>
> The attached patch fixes an aliasing issue caused by MemCpyOptimizer
> by performing the following changes:
> - Added requirement that the callee does not capture the src pointer for
>    the transformation to apply.
> - Updated test cases by adding nocapture attributes where applicable
>
> See http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-June/073819.html
> for some more information. My main concern is performance regressions
> but I haven't been able to get any benchmark results.
>
> Best regards
> David
>
>
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