llvm dev - Feb 2014 - [LLVMdev] How to implement register allocation constraints to guide allocator dispatching different registers for certain instruction?

Hi Kevin,

I concur Tim’s advice, you should use the @earlyclobber.

Cheers,
-Quentin

On Feb 19, 2014, at 3:27 AM, Tim Northover <t.p.northover at gmail.com>
wrote:
> Hi Kevin,
> 
>> I suppose to add some extra rules on register allocator to avoid it
>> allocating register causing any unpredictable behavior, but I don't
have
>> experience to this part. Can anybody tell me how to implement this or
have
>> better ideas to solve this problem? Thanks in advance.
> 
> I think this is what the "@earlyclobber" constraint is for: if
you
> mark the status output register as @earlyclobber then LLVM will think
> it gets written early on during the instruction (i.e. before the CPU
> would get a chance to read the other two) and make sure it gets
> allocated to a different register.
> 
> Cheers.
> 
> Tim.
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev

Hi Tim and Quentin,

Thanks a lot for your help. This solution is quite simple and works well!
Patch is in http://llvm-reviews.chandlerc.com/D2844, please review:)


2014-02-20 1:35 GMT+08:00 Quentin Colombet <qcolombet at apple.com>:
> Hi Kevin,
>
> I concur Tim's advice, you should use the @earlyclobber.
>
> Cheers,
> -Quentin
>
> On Feb 19, 2014, at 3:27 AM, Tim Northover <t.p.northover at
gmail.com>
> wrote:
>
> > Hi Kevin,
> >
> >> I suppose to add some extra rules on register allocator to avoid
it
> >> allocating register causing any unpredictable behavior, but I
don't have
> >> experience to this part. Can anybody tell me how to implement this
or
> have
> >> better ideas to solve this problem? Thanks in advance.
> >
> > I think this is what the "@earlyclobber" constraint is for:
if you
> > mark the status output register as @earlyclobber then LLVM will think
> > it gets written early on during the instruction (i.e. before the CPU
> > would get a chance to read the other two) and make sure it gets
> > allocated to a different register.
> >
> > Cheers.
> >
> > Tim.
> > _______________________________________________
> > LLVM Developers mailing list
> > LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> > http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>
>

-- 
Best Regards,

Kevin Qin
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Hi,
To generate a call graph for a file(say foo.c), I can do:

clang -S -emit-llvm foo.c -o - | opt -analyze -dot-callgraph

However with this approach I have to run this command and pipe to opt
every single time I need to generate a callgraph.

For a project with many files and a complex build system, it would be
preferable for me to be able to invoke this transform as part of normal
build process by the use of some clang flag. Can I do something like:

clang -run-dot-callgraph-transform-flag foo.c ?
This would allow me to just modify the CFLAGS in Makefile and get results
as part of normal build process?

If this can't be done with stock clang, what can I modify in llvm source
to get something like this?

Thanks,
Harsh