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

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.

That said, my question is in regards to how best to implement this considering
that the PTX back-end is a part of upstream LLVM and the new option will be
committed to LLVM trunk.  Creating a new option seems trivial for the CMake
build scripts; just add a special target to automatically generate parts of the
PTXRegisterInfo.td file.  The Autotools build scripts seem to complicate
matters, however.  The makefiles for the different back-ends appear to be
regular makefiles that are not processed by automake, making it difficult to
introduce configure-time variables.

Is the introduction of a configure-time variable in the CMake build scripts an
acceptable addition in LLVM trunk?
Is there any precedent for backend-specific configure options?  Would the
autotools-based build system need to be modified to get something like this to
work?

Thanks,

Justin Holewinski

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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?

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.

-- John T.

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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 8:39 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.
> 
> That said, my question is in regards to how best to implement this
considering that the PTX back-end is a part of upstream LLVM and the new option
will be committed to LLVM trunk.  Creating a new option seems trivial for the
CMake build scripts; just add a special target to automatically generate parts
of the PTXRegisterInfo.td file.  The Autotools build scripts seem to complicate
matters, however.  The makefiles for the different back-ends appear to be
regular makefiles that are not processed by automake, making it difficult to
introduce configure-time variables.
> 
> 	• Is the introduction of a configure-time variable in the CMake build
scripts an acceptable addition in LLVM trunk?
> 	• Is there any precedent for backend-specific configure options?  Would
the autotools-based build system need to be modified to get something like this
to work?
This should be a run-time option.

You should be able to define 'enough' registers in your RegisterInfo.td
file, and then simply reserve the registers you don't want.

/jakob

Justin Holewinski <justin.holewinski at gmail.com> writes:

[snip]
> Is the introduction of a configure-time variable in the CMake build
> scripts an acceptable addition in LLVM trunk?
Yes, but I'm just speaking just as the "official" maintainer of
the
CMake build so I'm not saying that it is the best solution for your
problem.

If you end adding the configure option, please put it in the PTX target
CMakelists.txt file. Extra bonus if you document it on doc/CMake.html
(you may create a new section "Target-specific options.")

Instead of configuring a header file, try passing the option as a
`define' (add_definitions -DPTX_FOO=BAR)
> Is there any precedent for backend-specific configure options?
No, AFAIR, but I don't think it matters if it turns to be the Right
Thing for solving your problem.
> Would the autotools-based build system need to be modified to get
> something like this to work?
Sure, you need to add the option to the scripts, just like CMake, but
don't know the details. I'll look at the svn log for
autoconf/configure.ac and see what modifications required a similar
change. Then submit a patch for review. Once approved, you need to
regenerate the scripts before committing.

HTH