[LLVMdev] insert nop instruction (original) (raw)

Stephen Crane sjcrane at uci.edu
Fri Jul 10 14:40:52 PDT 2015

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

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. _> ________________________ > LLVM Developers mailing list > LLVMdev at cs.uiuc.edu http://llvm.cs.uiuc.edu > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

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