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

Hi,

To fix this bug(http://llvm.org/bugs/show_bug.cgi?id=18881), we need to add
more register constraints that for STLXR , Ws and Wt should not be the same
register. Because these unpredictable instructions are valid instructions
in MC layer, we couldn't just  simply treat them as unallocated encoding.
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.

-- 
Best Regards,

Kevin Qin
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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.

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.
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