[LLVMdev] Thinking about "whacky" backends

[Nate Fries]

-------- Original Message --------
Subject: 	Re: [LLVMdev] Thinking about "whacky" backends
Date: 	Fri, 03 Jun 2011 14:44:05 -0400
From: 	Nate Fries <nfries88 at yahoo.com>
To: 	Joachim Durchholz <jo at durchholz.org>



On 6/3/2011 1:38 PM, Joachim Durchholz wrote:
>  Am 01.06.2011 23:25, schrieb Nate Fries:
>>  That said, it seems like it ought to be possible to do the same thing
>>  by emitting bitcode for all supported platform/arch combinations
>  Wait... is bitcode not platform-agnostic?
>  I thought it is.
bitcode is, indeed, platform-agnostic.
However, with any language using a preprocessor (IE, C and C++) or a
similar mechanism of determining the underlying OS and architecture, and
in the case of code using it for such a purpose, LLVM would output
different bitcode. As I said in a previous message, replacing
preprocessor conditionals with runtime ones does not seem like a good
option to me. So this would be the simplest alternative I could think
of. Another option would be meta-data, as I suggested previously as well.
>>  compressing them in an archive,  then decompressing and either
>>  interpreting or JIT-compiling the appropriate bitcode for the
>>  platform. This would just be a more flexible means to that same end.
>  Not sure how that is more flexible - care to elaborate?
More flexible to the programmer, not to the system. There are many
pieces of code where, to port between Windows, OS X, iOS, Android, and
PC Linux/*BSD/etc would require a ton of preprocessor work.

I'm also considering that perhaps the real best option here would be to
add a new language feature to C++ (ie, invent a new language that is a
strict superset of C++) for a simplistic control flow construct that
only executes code on the specified system and architecture.
__os(WinXP | WinVista | Win7, IA32) { [... 32-bit Windows code here ...] },
__os(WinXP | WinVista | Win7, AMD64) { [... 64-bit Windows code here ...] },
__os(Linux24 | Linux26, IA32) { [...32-bit Linux code here...] },
__os(Linux24 | Linux26, AMD64) { [... 64-bit Linux code here ...] },
__os( , ) { throw new not_supported_exception(); };
Of course that's only a rough idea and will need a ton of refining, but
it should make my point fairly clear.

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