[LLVMdev] How to Find Instruction Encoding for a MachineInstr

[Jim Grosbach]

On Oct 23, 2012, at 6:22 PM, Joshua Cranmer <pidgeot18 at gmail.com> wrote:

> On 10/23/2012 1:58 PM, John Criswell 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?
>> 
> 
> As I recall (I haven't played this deep with MachineInstrs for close to a year), it's not necessarily knowable what the length is or the exact bytes that would be emitted since some of them depend on information not known until the final assembly emission pass. An example here is the x86 jmp instruction: the choice between near and long jumps (and hence 2 bytes or 5 bytes on x86-64) is not made until the actual conversion to MCInst and after applying all of the fixups--which only happens deep within the bowels of the AsmPrinter pass.

Right. See X86AsmBackend::mayNeedRelaxation() and friends for the gory details.

-jim

> 
> -- 
> Joshua Cranmer
> News submodule owner
> DXR coauthor
> 
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