[LLVMdev] Convert fdiv - X/Y -> X*1/Y

[Chandler Carruth]

On Thu, Aug 8, 2013 at 2:07 PM, Chad Rosier <chad.rosier at gmail.com> wrote:

> On Thu, Aug 8, 2013 at 1:56 PM, Mark Lacey <mark.lacey at apple.com> wrote:
>
>>
>> On Aug 8, 2013, at 9:56 AM, Jim Grosbach <grosbach at apple.com> wrote:
>>
>> Hi Chad,
>>
>> This is a great transform to do, but you’re right that it’s only safe
>> under fast-math. This is particularly interesting when the original divisor
>> is a constant so you can materialize the reciprocal at compile-time. You’re
>> right that in either case, this optimization should only kick in when there
>> is more than one divide instruction that will be changed to a mul.
>>
>>
>> It can be worthwhile to do this even in the case where there is only a
>> single divide since 1/Y might be loop invariant, and could then be hoisted
>> out later by LICM. You just need to be able to fold it back together when
>> there is only a single use, and that use is not inside a more deeply nested
>> loop.
>>
>
> Ben's patch does exactly this, so perhaps that is the right approach.
>

Just to be clear of what is being proposed (which I rather like):

1) Canonical form is to use the reciprocal when allowed (by the fast math
flags, whichever we decide are appropriate).
2) The backend folds a single-use reciprocal into a direct divide.

Did I get it right? If so, I think this is a really nice way to capture all
of the potential benefits of forming reciprocals without pessimizing code
where it isn't helpful.
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