[PATCH] Assign a low branch weight to ASan's slow path.

[Chandler Carruth]

Adding ASan folks...

Didn't we already look at this Kostya? A bit surprised this wasn't already
the case.


On Fri, Aug 29, 2014 at 1:24 AM, Jonas Wagner <jonas.wagner at epfl.ch> wrote:

> Experiments on the SPEC benchmarks show that the slow path is rarely
> taken, and that branch weights reduce branch misses and overall runtime.
> ---
>  lib/Transforms/Instrumentation/AddressSanitizer.cpp | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/lib/Transforms/Instrumentation/AddressSanitizer.cpp
> b/lib/Transforms/Instrumentation/AddressSanitizer.cpp
> index 357b5d5..3ff0a61 100644
> --- a/lib/Transforms/Instrumentation/AddressSanitizer.cpp
> +++ b/lib/Transforms/Instrumentation/AddressSanitizer.cpp
> @@ -870,8 +870,11 @@ void AddressSanitizer::instrumentAddress(Instruction
> *OrigIns,
>    TerminatorInst *CrashTerm = nullptr;
>
>    if (ClAlwaysSlowPath || (TypeSize < 8 * Granularity)) {
> +    // We use branch weights for the slow path check, to indicate that
> the slow
> +    // path is rarely taken. This seems to be the case for SPEC
> benchmarks.
>      TerminatorInst *CheckTerm =
> -        SplitBlockAndInsertIfThen(Cmp, InsertBefore, false);
> +        SplitBlockAndInsertIfThen(Cmp, InsertBefore, false,
> +            MDBuilder(*C).createBranchWeights(1, 100000));
>      assert(dyn_cast<BranchInst>(CheckTerm)->isUnconditional());
>      BasicBlock *NextBB = CheckTerm->getSuccessor(0);
>      IRB.SetInsertPoint(CheckTerm);
> --
> 2.1.0
>
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