<html><head><meta http-equiv="Content-Type" content="text/html charset=windows-1252"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><br><div><div>On Aug 8, 2013, at 9:56 AM, Jim Grosbach <<a href="mailto:grosbach@apple.com">grosbach@apple.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><meta http-equiv="Content-Type" content="text/html charset=windows-1252"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;">Hi Chad,<div><br></div><div>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.</div></div></blockquote><div><br></div><div>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.</div><br><blockquote type="cite"><div style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div><br></div><div>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.</div><div><br></div><div>-Jim</div><div><br></div><div><br><div><div>On Aug 8, 2013, at 9:25 AM, Chad Rosier <<a href="mailto:chad.rosier@gmail.com">chad.rosier@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="font-family: Helvetica; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: auto; text-align: start; text-indent: 0px; text-transform: none; white-space: normal; widows: auto; word-spacing: 0px; -webkit-text-stroke-width: 0px;">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>LLVM Developers mailing list<br><a href="mailto:LLVMdev@cs.uiuc.edu">LLVMdev@cs.uiuc.edu</a><span class="Apple-converted-space"> </span> <a href="http://llvm.cs.uiuc.edu/">http://llvm.cs.uiuc.edu</a><br><a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a></div></blockquote></div><br></div></div>_______________________________________________<br>LLVM Developers mailing list<br><a href="mailto:LLVMdev@cs.uiuc.edu">LLVMdev@cs.uiuc.edu</a> <a href="http://llvm.cs.uiuc.edu">http://llvm.cs.uiuc.edu</a><br><a href="http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev">http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev</a><br></blockquote></div><br></body></html>