llvm dev - Mar 2015 - [LLVMdev] Clang flag to either print/omit nop instruction in llvm backend

I have created custom Clang flag -no_nop_optimise which can be passes with
clang on command line.

I am planning to implement someting like:
when -no_nop_optimise flag is passed on the command line do not emit a nop
instruction.

i.e. inside SelLowering class
if ( !OPT_no_nop_optimise)
{
      BuildMI(*BB, MI, DL, TII->get(Mips::NOP));
}

Any clues on how can I pass the flag to the LLVM backend from clang?

Thanks,
Ambuj
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One possibility is adding the option to the backend instead and using
'-mllvm –no_nop_optimise' on the clang command. This approach is good
for options you don't intend end-users to use.

Just to check: The NOP's you're trying to eliminate aren't related
to branch delay slots are they? The reason I ask is because removing those
without putting another instruction in their place will change the behaviour of
your code.

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Ambuj Agrawal
Sent: 19 March 2015 15:45
To: llvmdev at cs.uiuc.edu; cfe-dev at cs.uiuc.edu
Subject: [LLVMdev] Clang flag to either print/omit nop instruction in llvm
backend

I have created custom Clang flag -no_nop_optimise which can be passes with clang
on command line.
I am planning to implement someting like:
when -no_nop_optimise flag is passed on the command line do not emit a nop
instruction.
i.e. inside SelLowering class
if ( !OPT_no_nop_optimise)
{
      BuildMI(*BB, MI, DL, TII->get(Mips::NOP));
}

Any clues on how can I pass the flag to the LLVM backend from clang?

Thanks,
Ambuj
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Thanks Daniel for your help. It works as required.

Also, I am working with the custom defined instructions for MIPS and are
not related to branch delay slots.

Thanks,
Ambuj

On Thu, Mar 19, 2015 at 4:56 PM, Daniel Sanders <Daniel.Sanders at
imgtec.com>
wrote:
>  One possibility is adding the option to the backend instead and using
> '-mllvm –no_nop_optimise' on the clang command. This approach is
good for
> options you don't intend end-users to use.
>
>
>
> Just to check: The NOP's you're trying to eliminate aren't
related to
> branch delay slots are they? The reason I ask is because removing those
> without putting another instruction in their place will change the
> behaviour of your code.
>
>
>
> *From:* llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at
cs.uiuc.edu] *On
> Behalf Of *Ambuj Agrawal
> *Sent:* 19 March 2015 15:45
> *To:* llvmdev at cs.uiuc.edu; cfe-dev at cs.uiuc.edu
> *Subject:* [LLVMdev] Clang flag to either print/omit nop instruction in
> llvm backend
>
>
>
> I have created custom Clang flag -no_nop_optimise which can be passes with
> clang on command line.
>
> I am planning to implement someting like:
>
> when -no_nop_optimise flag is passed on the command line do not emit a nop
> instruction.
>
> i.e. inside SelLowering class
>
> if ( !OPT_no_nop_optimise)
>
> {
>       BuildMI(*BB, MI, DL, TII->get(Mips::NOP));
> }
>
>
>
> Any clues on how can I pass the flag to the LLVM backend from clang?
>
>
>
> Thanks,
>
> Ambuj
>
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