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I remember why I didn't implement this rule in Instcombine. It add
one instruction. So, <br>
this xform should be driven by a redundancy eliminator if you care
code size. <br>
<br>
<div class="moz-cite-prefix">On 8/8/13 10:13 AM, Shuxin Yang wrote:<br>
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<blockquote cite="mid:5203D1A3.9090506@gmail.com" type="cite">
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I did few transformation in Instruction *InstCombiner::visitFDiv()
in an attempt to remove some divs. <br>
I may miss this case. If you need to implement this rule, it is
better done in Instcombine than in DAG combine. <br>
Doing such xform early expose the redundancy of 1/y, which have
positive impact to neighboring code, <br>
while DAG combine is bit blind. <br>
<br>
You should be very careful, reciprocal is very very very imprecise
transformation. Sometimes you will see big different<br>
with and without this xform. <br>
<br>
<div class="moz-cite-prefix">On 8/8/13 9:25 AM, Chad Rosier wrote:<br>
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<blockquote
cite="mid:CAMo3wbRcDnCo5EYaawWDOMCaAFz5BP9iD=kN8D4My3MRHVX4gw@mail.gmail.com"
type="cite">I would like to transform X/Y -> X*1/Y.
Specifically, I would like to convert:<br>
<br>
define void @t1a(double %a, double %b, double %d) {<br>
entry:<br>
%div = fdiv fast double %a, %d<br>
%div1 = fdiv fast double %b, %d<br>
%call = tail call i32 @foo(double %div, double %div1)<br>
ret void<br>
}<br>
<br>
to:<br>
<br>
define void @t1b(double %a, double %b, double %d) {<br>
entry:<br>
%div = fdiv fast double 1.000000e+00, %d<br>
%mul = fmul fast double %div, %a<br>
%mul1 = fmul fast double %div, %b<br>
%call = tail call i32 @foo(double %mul, double %mul1)<br>
ret void<br>
}<br>
<br>
Is such a transformation best done as a (target-specific) DAG
combine?<br>
<br>
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.<br>
<br>
FWIW, I'm also pretty sure this combine requires -fast-math.<br>
<br>
Can someone point me in the right direction?<br>
<br>
Thanks,<br>
Chad<br>
<br>
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