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<p class="MsoNormal">Still sounds to me as Glue might help (as already proposed by Craig), but maybe I’ve misunderstood something.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Another option is to do a simple lowering into pseudo instructions that you expand after ISel.<o:p></o:p></p>
<p class="MsoNormal">(might be easier than doing something before ISel and then having to bother about chains, glue etc)<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Regards,<o:p></o:p></p>
<p class="MsoNormal">Björn<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><b>From:</b> llvm-dev <llvm-dev-bounces@lists.llvm.org> <b>On Behalf Of
</b>Hendrik Greving via llvm-dev<br>
<b>Sent:</b> den 16 juli 2020 23:35<br>
<b>To:</b> Matt Arsenault <arsenm2@gmail.com><br>
<b>Cc:</b> llvm-dev <llvm-dev@lists.llvm.org><br>
<b>Subject:</b> Re: [llvm-dev] Selection DAG chain question<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Yea. I think AMD chains the node they're expanding into, but they don't chain it into an _existing_ chain. e.g. adding A->B to the DAG is ok. But adding A->B and next C->D with B->C is the problem. I appreciate the input<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Jul 16, 2020 at 2:04 PM Matt Arsenault <<a href="mailto:arsenm2@gmail.com">arsenm2@gmail.com</a>> wrote:<o:p></o:p></p>
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<br>
> On Jul 16, 2020, at 17:00, Hendrik Greving <<a href="mailto:hgreving@google.com" target="_blank">hgreving@google.com</a>> wrote:<br>
> <br>
> > No, non-sideeffecting operations can be legalized as compiler-rt calls<br>
> <br>
> Right, but not as "regular" nodes with side-effects? I guess you could search and analyze the DAG manually but that seems hacky. Maybe something that one day LLVM could support natively.<br>
<br>
<br>
You can’t add arbitrary chains or glue to the regular nodes, but you can define a custom node you select the same way with your chain/glue. You don’t need to preprocess the IR and can do in the custom lowering. This is what AMDGPU does for FDIV (see AMDGPUISD::FMA_W_CHAIN).
GlobalISel avoids these complications by not having nodes or chains, and just instructions with side effects, so in that sense this is a solved problem.<br>
<br>
-Matt<o:p></o:p></p>
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