[llvm-dev] [RFC] Improving integer divide optimization (related to D12082)

escha via llvm-dev llvm-dev at lists.llvm.org
Thu Aug 20 10:22:01 PDT 2015

> On Aug 20, 2015, at 9:59 AM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>> On Aug 20, 2015, at 9:46 AM, Steve King <steve at metrokings.com> wrote:
>> On Wed, Aug 19, 2015 at 10:58 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:
>>> Isn’t the problem the fact that the patch makes it harder for a target to
>>> get the generic code to reach its custom hook?
>>> Now the "cheap pow2 sdiv” is merged with the generic “cheap div” you can’t
>>> distinguish anymore.
>> Yes and also the issue of needing more information to make a smart
>> isIntDivCheap() decision.
>> In visitSDIV(), checking looks like this when the denominator is a power of 2.
>>   if (TLI.isIntDivCheap(N->getValueType(0), MinSize))
>>     return SDValue();
>>   // Target-specific implementation of sdiv x, pow2.
>>   if (SDValue Res = BuildSDIVPow2(N))
>>     return Res;
>> How about this for isIntDivCheap?
>> 1) pass Function* to allow the target to decide for itself which
>> attributes matter, and
> If you want the attributes, I think you should pass the attributes and not the whole Function.
> I’m not sure why MinSize does not trigger with -Os by default, Michael is it intended?

I think -Oz might be MinSize?  -Oz being absolute minimum size at the cost of significant speed, IIRC.

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