llvm dev - Dec 2011 - [LLVMdev] Adding option to LLVM opt to disable a specific pass from command line

Hi all,

I would like to add an option for LLVM 'opt' to disable a specific
optimization pass from command line.

The idea is to have something like:

opt -O2 -disable-pass=[passname,...]

Do you think it could be useful ? How should I proceed to develop it and
commit changes to LLVM trunk ?
Thanks for your advices and recommandations.
Best Regards
Seb
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Hello,

On Dec 7, 2011, at 2:07 AM, Seb wrote:
> Hi all,
> 
> I would like to add an option for LLVM 'opt' to disable a specific
optimization pass from command line.
> 
> The idea is to have something like:
> 
> opt -O2 -disable-pass=[passname,...]
> 
> Do you think it could be useful ?
I have few questions :

- Why (and when) would you us this ?
- Some of the passes are executed multiple times, how would you select which
invocation to disable ? Or would you disable all invocation of such passes ?
- Some passes are required by another passes. In such cases PassManager will
insist on running them, which may conflict with the user request from command
line. Who wins?
- Why not update the list passes run as part of -O2 (use --debug-pass=Arguments
to get it) to remove selected passes and run opt <my list of passes> ... ?

> How should I proceed to develop it and commit changes to LLVM trunk ?
> Thanks for your advices and recommandations.
> Best Regards
> Seb

-
Devang

Hello Devang,

answers are interleaved

2011/12/7 Devang Patel <dpatel at apple.com>
> Hello,
>
> On Dec 7, 2011, at 2:07 AM, Seb wrote:
>
> > Hi all,
> >
> > I would like to add an option for LLVM 'opt' to disable a
specific
> optimization pass from command line.
> >
> > The idea is to have something like:
> >
> > opt -O2 -disable-pass=[passname,...]
> >
> > Do you think it could be useful ?
>
> I have few questions :
>
> - Why (and when) would you us this ?
>
I woudl use this to selectively disable passes in opt that are either
redundant with passes performed by the fron-end I'm working on our exhibit
a BUG in opt. For instance, I figured out that loop-idiom pass has a BUG in
LLVM 2.9, a llvm.memcpy is generated for an overlapping memory region and
then x86 backend reorder loads/store thus generating a BUG.

So my use would be to disable all loop-idiom invocations.

> - Some of the passes are executed multiple times, how would you select
> which invocation to disable ? Or would you disable all invocation of such
> passes ?
>
By default I guess I would like to disable all invocation of such passes.
We could also imagine to have an option -disable-pass=[pass_name:inv_num,
...] to deal with multiple invocation.

- Some passes are required by another passes. In such cases
PassManager
> will insist on running them, which may conflict with the user request from
> command line. Who wins?
>
How to I know that a pass  is required by another ?
Let's say give priority to pass manager and issue a warning like like
'option -disable-pass=<pass_name> ignored because required by pass
<other_pass>.'

- Why not update the list passes run as part of -O2 (use
> --debug-pass=Arguments to get it) to remove selected passes and run opt
<my
> list of passes> ... ?
>
>
 I also thought about this but it can lead to long command lines and source
of errors.

> > How should I proceed to develop it and commit changes to LLVM trunk ?
> > Thanks for your advices and recommandations.
> > Best Regards
> > Seb
>
>
> -
> Devang
>
>
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