[llvm-dev] Passing inormation from pass to lowering

[Felix Berlakovich via llvm-dev]

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 *another* function.

Is DAG.getNode(ISD::BR, dl, MVT::Other, ???) the right approach?
And why isn’t there a X86::BR, but only a X86::BRCOND?

The only implementation I could find that actually creates a jump to a function is Readactor (https://github.com/securesystemslab/multicompiler), but it does it in the AsmPrinter.

Thanks for your help!

Regards

Felix

Von: Reid Kleckner <rnk at google.com>
Gesendet: Samstag, 28. März 2020 17:42
An: Felix Berlakovich <felix at berlakovich.at>
Cc: llvm-dev at lists.llvm.org
Betreff: Re: [llvm-dev] Passing inormation from pass to lowering

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.

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.

The implementation for cfguard is mostly here:
http://github.com/llvm/llvm-project/commit/d157a9bc8ba1085cc4808c6941412322a7fd884e

On Fri, Mar 27, 2020 at 1:28 AM Felix Berlakovich via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Hi!

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?

Regards

Felix
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