[llvm-dev] Passing inormation from pass to lowering

[Reid Kleckner via llvm-dev]

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> 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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