[LLVMdev] insert nop instruction

[Daniel Sanders]

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.
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