[llvm-dev] Python build dependency in LLVM and/or clang?

[David Zarzycki via llvm-dev]

Hi Simon,

Just a reminder: TableGen is a huge build bottleneck on some machines. I worry that dumping JSON and then pushing that through Python would make this bottleneck much worse. Consider this: TableGen performance is so critical that there is an entire build option just to force it to be built with optimizations when creating debug builds: LLVM_OPTIMIZED_TABLEGEN

Dave

> On Jul 5, 2019, at 12:43 PM, Simon Tatham via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hello llvm-devs,
> 
> I'm currently starting to look at implementing compiler intrinsics for the Arm MVE vector instruction set.
> 
> In a similar sort of style to the existing NEON intrinsics, this is going to involve describing the set of functions needed in Tablegen, and then processing that data into a header file and some bits and pieces to compile into clang (the list of builtins, data tables needed by Sema or CodeGen, etc). But the differences between MVE and NEON are large enough - particularly the predication system - that I'm going to need new Tablegen backends, rather than reusing the NEON ones.
> 
> I would like, if possible, to write these Tablegen backends in Python, by consuming the output of the 'llvm-tblgen -dump-json' feature I added to Tablegen last year. I think they'll be simpler and more concise that way than in C++, not to mention quicker to develop.
> 
> But before I do that, I wanted to check whether there would be any objection on grounds of build dependencies. Is it acceptable to require Python as part of the normal build process for LLVM? Or is Python something that can be used in places like the test suite and developer utilities, but has to be kept out of the main build to make bootstrapping as easy as possible? I looked for a written policy on this, but didn't find one.
> 
> Cheers,
> Simon
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