[PATCH] D30309: CodeGen: BlockPlacement: Precompute layout for chains of triangles.

[Xinliang David Li via llvm-commits]

On Thu, Mar 2, 2017 at 3:43 PM, Kyle Butt via Phabricator <
reviews at reviews.llvm.org> wrote:

> iteratee added inline comments.
>
>
> ================
> Comment at: lib/CodeGen/MachineBlockPlacement.cpp:152
> +             "triangle tail duplication heuristic to kick in. 0 to
> disable."),
> +    cl::init(2),
> +    cl::Hidden);
> ----------------
> davidxl wrote:
> > Should the default be 3?
> If I had found anywhere where it was likely to cause problems in my
> benchmarking, I would bump it to 3, but I haven't.
>

My advice is to start with a slightly more conservative setting initially.

David


>
>
> ================
> Comment at: lib/CodeGen/MachineBlockPlacement.cpp:1061
> +///    longer chains)
> +/// 2) The chains are statically correlated. Branch probabilities have a
> very
> +///    U-shaped distribution. If the branches in a chain are all from the
> same
> ----------------
> davidxl wrote:
> > 2) This is not entirely true --  it depends on the length the chain
> (e.g, when branch proabiity to Pdom blocks are low).  Perhaps remove 2).
> No, It's accurate. If you know that the frequencies are correlated, but
> not whether they're high or low, than this is profitable. Even if the
> individual branch decisions are independent.
>
> If by the length of the chain, you mean that the only correlations that
> matter are neighbors, then that is correct and I can add a note about that.
>
>
> https://reviews.llvm.org/D30309
>
>
>
>
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