[cfe-dev] [PATCH] ubsan: Emit branch metadata

[Richard Smith]

On Thu, Dec 13, 2012 at 9:23 AM, Will Dietz <willdtz at gmail.com> wrote:
> As previously discussed, default block placement for ubsan handler
> blocks (when recoverable) are suboptimal and cause poor icache
> performance.
>
> Attached is a simple patch adding strong branch hints to ensure proper
> block placement.
>
> Also, I've attached an extended version of the previous set of
> experiments that includes results for this change vs without this
> change, as well as the similarly isolated impact of using the ColdCC
> change.
>
> In summary, this minor change brings overall slowdown from 34% to 23%,
> and relative to NoRecover from 20% to 10%.
>
> Easy win :).
>
> Ok to commit?

To match the 'unreachable' weights, please use 1<<20 - 1, not 1<<20
(see lib/Analysis/BranchProbabilityInfo.cpp's UR_NONTAKEN_WEIGHT), and
also add a comment explaining where this number comes from. Please
update one of Clang's ubsan IRGen tests to check that the metadata is
being created. Then feel free to commit. Thanks!