[llvm-commits] PATCH: Teach BranchProbabilityInfo to read branch metadata

[Andrew Trick]

On Oct 17, 2011, at 1:29 AM, Chandler Carruth wrote:

> On Mon, Oct 17, 2011 at 12:32 AM, Chandler Carruth <chandlerc at google.com> wrote:
> This patch teaches the BranchProbabilityInfo pass to use metadata encoded probabilities when present on branch instructions. Switch instructions still need to be handled.
> 
> And here is a patch which generalizes this logic to support switch instructions. It should be applied on top of the previous patch.
> 
> BTW, Chris mentioned an interest in getting this fixed and potentially merged onto the 3.0 release branch, so prompt review would be appreciated. =] 
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Chandler,

Your patches look great to me.  I don't suggest any changes. I just have 
some general comments for the record:

In calcMetaDataWeight, if we really wanted to recover from overflow, we
would scale back all edge weights (all by the same divisor) until the
sum is less than UINT32_MAX. But I don't think this is an expected
case, so your patch should be fine--it's simpler this way, and seems
like it will handle the output of LowerExpect.

As you're aware, the biggest problem with the branch weight metadata
is CFG transforms invalidating it. You've already fixed the obvious
swapSuccessors issue. That should at least make it usable, but we
really have no way to verify this stuff currently.

It's also quite silly for opt to run a LowerExpectIntrinsics
pass. That's a language-specific front-end implementation hack that
should be part of the clang driver. Given that we're standardizing on
metadata for branch weights, we should never see those intrinsics in
bitcode.


-Andy
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