llvm dev - Jun 2011 - [LLVMdev] New Configure Option for LLVM Builds

On Jun 20, 2011, at 11:47 AM, John Criswell wrote:
> On 6/20/11 10:39 AM, Justin Holewinski wrote:
>> 
>> For the PTX back-end, I would like to introduce a configure-time option
to determine the number of architectural registers available to LLVM register
allocation during code generation.  The motivation for this is PTX is a virtual
instruction set and the number of registers is configurable in the output file. 
Thus, the number of registers specified via tablegen is arbitrary.  For
different use-cases, it may be desirable to change this number.
> 
> Is it possible to make this option a command-line option to llc instead of
a configure-time option?
Ideally, that would be the way to go.  However, as far as I know, LLVM requires
static register data that is generated from tablegen files.  The only way I can
think of changing this data is by automatically generating the tablegen files at
configure time.
> 
> In my experience, it's better to make these options command-line
options to tools when possible; that way, when someone wants to experiment with
changing the parameter, they do not need to recompile.
I completely agree.  Unfortunately, I do not believe that is possible in this
case (without major LLVM back-end changes).
> 
> -- John T.
> 
Thanks,

Justin Holewinski

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On Jun 20, 2011, at 8:53 AM, Justin Holewinski wrote:
> However, as far as I know, LLVM requires static register data that is
generated from tablegen files.
TableGen is not required, you can provide your own TargetRegisterInfo sub-class.
That's a lot of work, though, and I wouldn't recommend it.

/jakob

On Jun 20, 2011, at 1:08 PM, Jakob Stoklund Olesen wrote:
> 
> On Jun 20, 2011, at 8:53 AM, Justin Holewinski wrote:
> 
>> However, as far as I know, LLVM requires static register data that is
generated from tablegen files.
> 
> TableGen is not required, you can provide your own TargetRegisterInfo
sub-class. That's a lot of work, though, and I wouldn't recommend it.
That's an interesting idea.  Since the PTX register file is fairly simple
(no overlapping, etc.), the generated register info does not seem too
complicated.

My main concern would be if the register description structures change, which
would require manual changes to the PTX back-end instead of just relying on
TableGen being updated to use the new format.
> 
> /jakob
> 
Thanks,

Justin Holewinski

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