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

[Shuxin Yang]

I did few transformation in Instruction *InstCombiner::visitFDiv() in an 
attempt to remove some divs.
I may miss this case.  If you need to implement this rule, it is better 
done in Instcombine than in DAG combine.
Doing such xform early expose the redundancy of 1/y, which have positive 
impact to neighboring code,
while DAG combine is bit blind.

You should be very careful, reciprocal is very very very imprecise 
transformation. Sometimes you will see big different
with and without this xform.

On 8/8/13 9:25 AM, Chad Rosier 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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