[LLVMdev] Convert fdiv - X/Y -> X*1/Y
Mark Lacey
mark.lacey at apple.com
Thu Aug 8 10:56:25 PDT 2013
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.
>
> 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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>
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