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

[Chad Rosier]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20130808/6ba77a65/attachment.html>