[PATCH] Allow 0-weight branches in BranchProbabilityInfo.

[Hans Wennborg]

On Wed, May 6, 2015 at 10:44 AM, Duncan P. N. Exon Smith
<dexonsmith at apple.com> wrote:
>
>> On 2015 May 6, at 13:37, Hans Wennborg <hans at chromium.org> wrote:
>>
>> On Wed, May 6, 2015 at 10:09 AM, Duncan P. N. Exon Smith
>> <dexonsmith at apple.com> wrote:
>>>>       assert(Weight <= UINT32_MAX / SI.getNumSuccessors());
>>>
>>> Hmm.  I don't think BPI should guarantee this anymore.  This is
>>> unnecessary for keeping the sum under `UINT32_MAX`.
>>
>> It's useful to know that the sum of the weights for an arbitrary set
>> of successors won't overflow. For example, switch lowering will merge
>> adjacent cases with the same successors into clusters and just add the
>> weights, without having to worry about overflow.
>>
>> Are you saying you don't want to guarantee that anymore, or that
>> forcing each weight <= UINT32_MAX / SI.getNumSuccessors() is
>> unnecessarily strict?
>
> No, just loosening it to a guarantee that summing doesn't overflow.
> I outlined a cheap way to do that here:
>
> http://lists.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20150427/274432.html

That sounds great. Thanks!