<div dir="ltr">I'd recommend looking at how tail call lowering is implemented. The TAILJMP* family of pseudo instructions are modelled as calls throughout CodeGen, but later in X86MCInstLower.cpp they are translated to plain JMP instructions. This avoids branch folding and the like from assuming things about the control flow of blocks ending in JMP_1. Similarly, you probably need to define a family of such call-like pseudo instructions, and lower them later.<div><br></div><div>Alternatively, you could try to get by without new instructions, and instead staple a bunch of extra special operands to the regular CALL* family of instructions. Later, you would change the way such instructions are "printed" depending on the presence of your operand. CALL* is a variadic instruction, so you can put more or less anything in the operand list, such as perhaps metadata. This approach runs the risk of rewriting CALL instructions that didn't carry your special annotations, or having other CodeGen passes fold call instructions in ways that are incompatible with your transformation.</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Mar 30, 2020 at 1:04 PM Felix Berlakovich <<a href="mailto:felix@berlakovich.at">felix@berlakovich.at</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex">
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<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">The one thing I can‘t seem to figure out is how to lower a jump to another function at any stage before AsmPrinter. The methods to construct an unconditional
branch always seem to expect a (machine) basic block and I don’t know how to get the first basic block of *<b>another</b>* function.
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">Is DAG.getNode(ISD::BR, dl, MVT::Other, ???) the right approach?
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">And why isn’t there a X86::BR, but only a X86::BRCOND?
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">The only implementation I could find that actually creates a jump to a function is Readactor (<a href="https://github.com/securesystemslab/multicompiler" target="_blank">https://github.com/securesystemslab/multicompiler</a>),
but it does it in the AsmPrinter.<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">Thanks for your help!<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">Regards<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)">Felix
<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11pt;font-family:Calibri,sans-serif;color:rgb(31,73,125)"><u></u> <u></u></span></p>
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<p class="MsoNormal"><b><span style="font-size:11pt;font-family:Calibri,sans-serif">Von:</span></b><span style="font-size:11pt;font-family:Calibri,sans-serif"> Reid Kleckner <<a href="mailto:rnk@google.com" target="_blank">rnk@google.com</a>>
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<b>Gesendet:</b> Samstag, 28. März 2020 17:42<br>
<b>An:</b> Felix Berlakovich <<a href="mailto:felix@berlakovich.at" target="_blank">felix@berlakovich.at</a>><br>
<b>Cc:</b> llv</span><span lang="DE" style="font-size:11pt;font-family:Calibri,sans-serif"><a href="mailto:m-dev@lists.llvm.org" target="_blank">m-dev@lists.llvm.org</a><br>
<b>Betreff:</b> Re: [llvm-dev] Passing inormation from pass to lowering<u></u><u></u></span></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">I would encourage you to look at the "cfguard" bundle added for Windows Control Flow Integrity. AsmPrinter is probably far too late for the transform you want to do. You probably need to implement it somewhere in call lowering, so assuming
this is for x86, you would implement this in X86ISelLowering::LowerCall.<u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">I think it would be less invasive to attach your metadata to the existing call instructions, and then change code generation later, rather than replacing the IR instructions completely.<br>
<br>
The implementation for cfguard is mostly here:<u></u><u></u></p>
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<p class="MsoNormal"><a href="http://github.com/llvm/llvm-project/commit/d157a9bc8ba1085cc4808c6941412322a7fd884e" target="_blank">http://github.com/llvm/llvm-project/commit/d157a9bc8ba1085cc4808c6941412322a7fd884e</a> <u></u><u></u></p>
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<p class="MsoNormal"><u></u> <u></u></p>
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<p class="MsoNormal">On Fri, Mar 27, 2020 at 1:28 AM Felix Berlakovich via llvm-dev <<a href="mailto:llvm-dev@lists.llvm.org" target="_blank">llvm-dev@lists.llvm.org</a>> wrote:<u></u><u></u></p>
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<p class="MsoNormal">Hi!<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">I have written a ModulePass that calculates various things and adds custom metadata attributes to certain instructions. Depending on the attributes, I would like to change the machine
code of these instructions. For example, I would like to replace certain calls with jumps, but as far as I can tell the IR metadata is not accessible anymore on the level of machine instructions (e.g. in the AsmPrinter). What is the best way to pass the information
calculated by the pass (e.g the attributes) to the part where the target specific instructions are emitted?<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">Regards<u></u><u></u></p>
<p class="MsoNormal"> <u></u><u></u></p>
<p class="MsoNormal">Felix<u></u><u></u></p>
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