[LLVMdev] Re: Re: variable sized structs in LLVM

[Ed Cogburn]

Misha Brukman wrote:

> On Tue, Jun 21, 2005 at 02:11:22PM -0400, Ed Cogburn wrote:
>> Reid Spencer wrote:
>> > Its certainly possible to generate .ll files but its probably about
>> > the same amount of work to use the LLVM API and there are
>> > significant speed and validity benefits to doing so.
>> 
>> Does this mean that LLVM is moving away from the idea of a truly
>> abstract IR language, to being a set of development libraries for use
>> by build-time-dependent frontends?
> 
[snip]
>
> What do you mean by "build-time-dependent frontends"?


Sorry, I meant frontends that link to LLVM's libraries, as opposed to
spitting out .ll files for consumption by LLVM's assembler.


> Frontends can be completely separate from the LLVM infrastructure and do
> not even need to be linked with any LLVM classes to create LLVM code from
> source languages.


Unless, apparently, you're trying to implement variable-sized objects in
your language. :)  One of LLVM's powerful (IMHO) selling points was the
concept of a *complete*, abstract IR language which any independent
frontend could write to (sending llvm-as unoptimized, even ugly, but
correct, IR assembly that gets converted to optimized native code), without
being dependent on LLVM's own code, i.e. just write to the IR spec, and be
able to do anything that any frontend linked to LLVM itself, like Reid's
Stacker, could do.

I guess what I'm suggesting is that both the variable struct & data
alignment problems should be at some point abstracted away within LLVM's IR
language, with data alignment issues ideally completely invisible to the
user of LLVM's IR, and with variable structs handled perhaps like GCC & C99
eventually did to support variable sized structs in C.  See 'Zero-length
arrays':

http://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html#Zero-Length