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

[Chad Rosier]

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.


>
> I don’t have a strong preference for instcombine vs. dagcombine, though I
> lean slightly towards later when we’ll have more target information
> available if we want to apply a more complicated cost function for some
> targets.
>
> -Jim
>
>
> On Aug 8, 2013, at 9:25 AM, Chad Rosier <chad.rosier at gmail.com> wrote:
>
> I would like to transform X/Y -> X*1/Y.  Specifically, I would like to
> convert:
>
> define void @t1a(double %a, double %b, double %d) {
> entry:
>   %div = fdiv fast double %a, %d
>   %div1 = fdiv fast double %b, %d
>   %call = tail call i32 @foo(double %div, double %div1)
>   ret void
> }
>
> to:
>
> define void @t1b(double %a, double %b, double %d) {
> entry:
>   %div = fdiv fast double 1.000000e+00, %d
>   %mul = fmul fast double %div, %a
>   %mul1 = fmul fast double %div, %b
>   %call = tail call i32 @foo(double %mul, double %mul1)
>   ret void
> }
>
> Is such a transformation best done as a (target-specific) DAG combine?
>
> A similar instcombine already exists for the X/C->X*1/C case (see the
> CvtFDivConstToReciprocal function in InstCombineMlDivRem.cpp), but I don't
> believe the above can be done as an instcombine as it creates a new
> instruction (in addition to replacing the original).  Also, I only want to
> perform the transformation if there are multiple uses of 1/Y (like in my
> test case).  Otherwise, the transformation replaces a fdiv with a fdiv+fmul
> pair, which I doubt would be profitable.
>
> FWIW, I'm also pretty sure this combine requires -fast-math.
>
> Can someone point me in the right direction?
>
>  Thanks,
>   Chad
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