[llvm-dev] SROA and volatile memcpy/memset

[Krzysztof Parzyszek via llvm-dev]

On 11/11/2015 9:28 AM, Chandler Carruth wrote:
> So, here is the model that LLVM is using: a volatile memcpy is lowered
> to a loop of loads and stores of indeterminate width. As such, splitting
> a memcpy is always valid.
>
> If we want a very specific load and store width for volatile accesses, I
> think that the frontend should generate concrete loads and stores of a
> type with that width. Ultimately, memcpy is a pretty bad model for
> *specific* width accesses, it is best at handling indeterminate sized
> accesses, which is exactly what doesn't make sense for device backed
> volatile accesses.
>

Yeah, the remark about devices I made in my post was a result of a 
"last-minute" thought to add some rationale.  It doesn't actually apply 
to SROA, since there are no devices that are mapped to the stack, which 
is what SROA is interested in.

The concern with the testcase I attached is really about performance. 
Would it be reasonable to control the splitting in SROA via TTI?

-Krzysztof

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