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

[Shuxin Yang]

I remember why I didn't implement this rule in Instcombine. It add one 
instruction. So,
this xform should be driven by a redundancy eliminator if you care code 
size.

On 8/8/13 10:13 AM, Shuxin Yang wrote:
> 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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>

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