[LLVMdev] How to Find Instruction Encoding for a MachineInstr

[Craig Topper]

After a quick glance at the generated description files looks like ARM has
correct size information, but X86 always returns 0. ARM is easy because its
fixed length.

On Tue, Oct 23, 2012 at 5:23 PM, John Criswell <criswell at illinois.edu>wrote:

>  On 10/23/12 7:19 PM, Craig Topper wrote:
>
> 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.
>
>
> The getSize() method of MCInstrDesc which can be fetched from a
> MachineInstr using the getDesc() method:
>
>
> http://llvm.org/doxygen/classllvm_1_1MCInstrDesc.html#ae8a17b854d9787d11797d9334a22647d
>
> Does this method not work as advertised in Doxygen?
>
> -- John T.
>
>
>
>  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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>>
>
>
>
> --
> ~Craig
>
>
>


-- 
~Craig
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