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<div class="moz-cite-prefix">On 8/8/13 10:40 AM, Michael Ilseman
wrote:<br>
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I believe we were under the impression that InstCombine, as a
canonicalized/optimizer, should not increase code size but only
reduce it.
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<div>Minor aside, but you don't need all of fast-math for the IR,
just the "arcp" flag, which allows for replacement of division
with reciprocal-multiply.<br>
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I vaguely remember we are not quite happy with the arcp relax rank
because the it is not very relax, while <br>
the the reciprocal is very imprecise. I guess we are better off
using unsafe-math to guard such xform?<br>
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<div>On Aug 8, 2013, at 10:21 AM, Shuxin Yang <<a
moz-do-not-send="true"
href="mailto:shuxin.llvm@gmail.com">shuxin.llvm@gmail.com</a>>
wrote:</div>
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<div bgcolor="#FFFFFF" text="#000000"> 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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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>
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