[LLVMdev] insert nop instruction

[Stephen Crane]

I've got a pending patch for LLVM (which has been stalled for a while,
totally my fault) to insert NOPs for artificial software diversity.
You can check out the patch at http://reviews.llvm.org/D6983

- stephen

On Fri, Jul 10, 2015 at 2:28 PM, Zahra Marj <zahrafatehimarj at gmail.com> wrote:
> Thanks all. Your comments help me very much.
>
> On Thu, Jul 9, 2015 at 6:57 PM, John Criswell <jtcriswel at gmail.com> wrote:
>>
>> Dear All,
>>
>> To add to this, you can find examples of inserting NOPs for X86 in the CFI
>> pass originally written at Lehigh University that we ported to 64-bit X86
>> for SVA:
>>
>>
>> https://github.com/jtcriswell/SVA/blob/master/llvm/lib/Target/X86/X86CFIOptPass.cpp
>>
>> Alternatively, you could use an InlineAsm call at the LLVM IR level (which
>> I think would be easier to implement).
>>
>> Regards,
>>
>> John Criswell
>>
>>
>> On 7/9/15 8:56 AM, Daniel Sanders wrote:
>>
>> Hi,
>>
>>
>>
>> Given that you are using LLVM-IR, I expect that instruction selection
>> occurs after your pass and instruction selection also applies optimizations.
>> In particular, when the SelectionDAG is built it will eliminate (via
>> constant folding) the '%nop = add i1 0, 0' you added to the LLVM-IR.
>>
>>
>>
>> If you are trying to insert nop's into the resulting assembly then you
>> should probably be trying to insert them at a very late stage of the backend
>> instead. Different IR's based on MachineInstr and MCInst are used in these
>> later stages.
>>
>>
>>
>> From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
>> Behalf Of Zahra Marj
>> Sent: 09 July 2015 12:33
>> To: David Chisnall
>> Cc: LLVM Developers Mailing List
>> Subject: Re: [LLVMdev] insert nop instruction
>>
>>
>>
>> My pass runs after optimization passes.
>>
>>
>>
>> On Thu, Jul 9, 2015 at 1:11 PM, David Chisnall
>> <David.Chisnall at cl.cam.ac.uk> wrote:
>>
>> Hi,
>>
>> What are you trying to achieve?  Inserting NOPs into LLVM IR is likely to
>> be pointless, as optimisations (in the IR or SelectionDAG) will remove them
>> before machine code generation.  If you want to insert NOPs into the
>> generated machine code, then this will not help you (you could insert inline
>> assembly containing nops into the IR).
>>
>> David
>>
>>
>> > On 9 Jul 2015, at 09:25, Zahra Marj <zahrafatehimarj at gmail.com> wrote:
>> >
>> > Hi.
>> > I need to write a function pass that insert nop instruction in function.
>> > Examples of these instructions are: %nop = add i1 0, 0 or %nop = alloca i1,
>> > i1 0. This link couldn't help me:
>> > http://llvm.org/docs/ProgrammersManual.html#creating-and-inserting-new-instructions
>> > I need a clear example about inserting new instruction. Anyone can help
>> > me?
>> > Thanks.
>>
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>>
>>
>>
>>
>>
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>>
>>
>> --
>> John Criswell
>> Assistant Professor
>> Department of Computer Science, University of Rochester
>> http://www.cs.rochester.edu/u/criswell
>
>
>
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