[LLVMdev] Proposal: add intrinsics for safe division

[Eric Christopher]

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
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>
> 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