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

[Jim Grosbach]

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.

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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