[llvm-commits] PATCH: Teach the function cloner to prune dead basic blocks

[Chandler Carruth]

Ping.

On Sun, Mar 25, 2012 at 1:21 PM, Chandler Carruth <chandlerc at gmail.com>wrote:

> On Sun, Mar 25, 2012 at 12:17 PM, Chris Lattner <clattner at apple.com>wrote:
>
>> WeakVH is *substantially* more expensive than AssertingVH.  The later is
>> just a raw pointer when assertions are disabled.
>>
>
> Yep, but that's not what this patch changes. The value map is currently
> using a *TrackingVH*, not an AssertingVH. My understanding is that
> TrackingVH isn't substantially less expensive than WeakVH. Both have to be
> updated on RAUW. WeakVH has to be updated on delete as well, but this patch
> adds the first delete operations.
>
>
>> The former requires densemap operations when a WeakVH is modified.
>>
>> Why is it expensive to update the value mapping?
>>
>
> We have to update everything that maps *to* the deleted instructions.
> That's a full walk of the value map for each instruction deleted.
>
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