[LLVMdev] insert nop instruction

[John Criswell]

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 <mailto: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 
> <mailto: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 
> <https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_ProgrammersManual.html-23creating-2Dand-2Dinserting-2Dnew-2Dinstructions&d=AwMFaQ&c=8hUWFZcy2Z-Za5rBPlktOQ&r=Mfk2qtn1LTDThVkh6-oGglNfMADXfJdty4_bhmuhMHA&m=Qp9GWIWeL3Bal7o2P67hp2QbDCZLKBcsuzuT3xzMMUo&s=GZrVdTwGMs5jykSuspIi7GRCIni6kaJDICG65OxeZMc&e=>
> > I need a clear example about inserting new instruction. Anyone can 
> help me?
> > Thanks.
>
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu <mailto:LLVMdev at cs.uiuc.edu> http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>
>
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev


-- 
John Criswell
Assistant Professor
Department of Computer Science, University of Rochester
http://www.cs.rochester.edu/u/criswell

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20150709/20e33063/attachment.html>