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

[Kuperstein, Michael M via llvm-dev]

The way I understand -Os is “don’t increase size unless there’s a clear and significant performance gain” – that is, much less aggressive in terms of code size than the definition you gave below.
I’m not advocating this point of view, mind you, it’s just that this has been my understanding.

Since leaving the div as-is can be very expensive compared to the alternative sequence, I thought -Oz was the safe way to go with this.


From: Mehdi Amini [mailto:mehdi.amini at apple.com]
Sent: Thursday, August 20, 2015 20:32
To: escha
Cc: Steve King; llvm-dev; Kuperstein, Michael M
Subject: Re: [llvm-dev] [RFC] Improving integer divide optimization (related to D12082)


On Aug 20, 2015, at 10:22 AM, escha <escha at apple.com<mailto:escha at apple.com>> wrote:


On Aug 20, 2015, at 9:59 AM, Mehdi Amini <mehdi.amini at apple.com<mailto:mehdi.amini at apple.com>> wrote:


On Aug 20, 2015, at 9:46 AM, Steve King <steve at metrokings.com<mailto:steve at metrokings.com>> wrote:

On Wed, Aug 19, 2015 at 10:58 PM, Mehdi Amini <mehdi.amini at apple.com<mailto: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.

Sure, the question is more what is the tradeoff for -Os.
My remember of -Os is somehow “don’t perform transformation that increases code size”.
It seems that it is what is done here: converting an (possibly expensive) division into multiple (less expensive) operations, probably increasing code size (target dependent admittedly).

—
Mehdi

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