[LLVMdev] How to Find Instruction Encoding for a MachineInstr

Craig Topper craig.topper at gmail.com
Tue Oct 23 17:19:50 PDT 2012

What function provides the encoding length? X86 in particular is so
difficult to encode that only the old style JIT and the MC Code Emitter
could possibly know how many bytes something takes.

On Tue, Oct 23, 2012 at 11:58 AM, John Criswell <criswell at illinois.edu>wrote:

> Dear All,
> I'm enhancing a MachineFunctionPass that enforces control-flow integrity.
>  One of the things I want to do is to set the alignment of an instruction
> (by adding NOPs before it in the MachineBasicBlock or by emitting an
> alignment directive to the assembler) if it causes a specific sequence of
> bytes to be generated at a specific alignment.  The goal is to ensure that
> sequences of bytes used to label valid targets of an indirect branch (e.g.,
> a return instruction) do not appear at a given alignment anywhere in a
> program other than for where I inserted them explicitly.
> It looks like MachineInstr has a method for finding the length of the
> instruction's binary encoding, but I didn't see a method for finding the
> exact bytes that would be emitted from the MachineInstr.  Is there a way to
> do this in the MachineFunctionPass/**MachineInstr infrastructure, or do I
> need to use something like the MC classes?
> Thanks in advance for any help provided.
> -- John T.
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