[LLVMdev] LLVM IR is a compiler IR

Talin viridia at gmail.com
Tue Oct 4 22:29:58 PDT 2011

On Tue, Oct 4, 2011 at 5:08 PM, Chris Lattner <clattner at apple.com> wrote:

> On Oct 4, 2011, at 4:56 PM, Talin wrote:
> > LLVM isn't actually a virtual machine. It's widely acknoledged that the
>> > name "LLVM" is a historical artifact which doesn't reliably connote what
>> > LLVM actually grew to be. LLVM IR is a compiler IR.
>> It sounds like you're picking a very specific definition of what a VM is.
>>  LLVM certainly isn't a high level virtual machine like Java, but that's
>> exactly the feature that makes it a practical target for C-family languages.
>>  It isn't LLVM's fault that people want LLVM to magically solve all of C's
>> portability problems.
>> I understand that the official goals of the LLVM project are carefully
> limited. A large number of LLVM users are perfectly happy to live within the
> envelope of what LLVM provides. At the same time, there are also a fair
> number of users who are aiming for things that appear to be just outside
> that envelope. These "near miss" users are looking at Java, at CLR, and
> constantly asking themselves "did I make the right decision betting on LLVM
> rather than these other platforms?" Unfortunately, there are frustratingly
> few choices available in this space, and LLVM happens to be "nearest"
> conceptually to what these users want to accomplish. But bridging the gap
> between where they want to go and where LLVM is headed is often quite a
> challenge, one that is measured in multiple man-years of effort.
> I completely agree, and I'm really interested in LLVM improving to solve
> these sorts of problems.  I'm not sure how this relates to Dan's email or my
> response though.
> Here's my position in a nutshell: The kind of things that Dan wants LLVM to
do should really be a separate sub-project from LLVM proper, built on top of
LLVM. I think it's unrealistic to expect LLVM proper to adopt Dan's stated
objectives - but at the same time, it would be a awful shame if there wasn't
something that could meet his needs, since I think many people other than
Dan would benefit from such a thing.

For example, I don't expect that LLVM IR should suddenly become stable and
usable as an archive format, but I think it entirely reasonable that someone
could come up with a higher-level IR, translatable into LLVM IR, that does
have those qualities. The fact that we have projects that convert JVM
bytecode into LLVM IR is proof that such a thing is possible. (Except that
any language implementer willing to live with the limitations of the JVM
probably wouldn't be on this mailing list to begin with, but I digress.)

The question I am interested in exploring is whether the goals of the "near
miss" users of LLVM are similar enough to each other to be worth having a
conversation about how to achieve those goals collaboratively, or whether it
is better that we should each continue to struggle with our own problems

-- Talin
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