[LLVMdev] Proposal: add intrinsics for safe division

[Philip Reames]

On 04/25/2014 04:18 PM, Eric Christopher wrote:
> On Fri, Apr 25, 2014 at 3:55 PM, Philip Reames
> <listmail at philipreames.com> wrote:
>> On 04/25/2014 02:21 PM, Eric Christopher wrote:
>>>> In short, I agree with your observations that these intrinsics are not an
>>>> obvious slam-dunk compared to making the explicit control flow, but I
>>>> think
>>>> that the intrinsics do give enough flexibility on the LLVM side that it
>>>> would be great if front-ends used them rather than rolling the control
>>>> flow
>>>> themselves.
>>>>
>>> The problem is that then we have 2 problems: All targets (except for
>>> arm64) then have to lower the intrinsic as the first thing they do
>>> (giving us a TTI pass as the first thing in the pipeline) to take
>>> advantage of the information later during optimization, _and_ we have
>>> to plumb all of the work optimizing the intrinsic as well giving us a
>>> situation where we've now split our optimization efforts as well as
>>> the pain of maintaining an intrinsic that's useful for a single
>>> target.
>>>
>>> I really think that this is just solidifying my position that the
>>> intrinsic is a bad idea and that this should be done as later
>>> optimizations.
>>>
>>> -eric
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>> Does anyone have performance data on ARM for the early vs late lowering
>> approaches?  Having data would make the discussion much more concrete about
>> the tradeoffs of the two approaches.
>>
>> To put it another way, how much do we loose performance-wise in missed
>> pattern matching?
>>
> There were no performance or optimization numbers provided with the
> patch or proposal :)
>
> -eric
Agreed.  That was my point.  :)

Philip