[LLVMdev] LLVM IR is a compiler IR

[Chris Lattner]

On Oct 4, 2011, at 11:53 AM, Dan Gohman wrote:
> In this email, I argue that LLVM IR is a poor system for building a
> Platform, by which I mean any system where LLVM IR would be a
> format in which programs are stored or transmitted for subsequent
> use on multiple underlying architectures.

Hi Dan,

I agree with almost all of the points you make, but not your conclusion.  Many of the issues that you point out as problems are actually "features" that a VM like Java doesn't provide.  For example, Java doesn't have uninitialized variables on the stack, and LLVM does.  LLVM is capable of expressing the implicit zero initialization of variables that is implicit in Java, it just leaves the choice to the frontend.

Many of the other issues that you raise are true, but irrelevant when compared to other VMs.  For example, LLVM allows a frontend to produce code that is ABI compatible with native C ABIs.  It does this by requiring the frontend to know a lot about the native C ABI.  Java doesn't permit this at all, and so LLVM having "this feature" seems like a feature over-and-above what high-level VMs provide.  Similiarly, the "conditionally" supported features like large and obscurely sized integers simply don't exist in these VMs.

The one key feature that LLVM doesn't have that Java does, and which cannot be added to LLVM "through a small matter of implementation" is verifiable safety. Java bytecode verification is not something that LLVM IR permits, which you can't really do in LLVM (without resorting to techniques like SFI).

With all that said, I do think that we have a real issue here.  The real issue is that we have people struggling to do things that a "hard" and see LLVM as the problem.  For example:

1. The native client folks trying to use LLVM IR as a portable representation that abstracts arbitrary C calling conventions.  This doesn't work because the frontend has to know the C calling conventions of the target.

2. The OpenCL folks trying to turn LLVM into a portable abstraction language by introducing endianness abstractions.  This is hard because C is inherently a non-portable language, and this is only scratching the surface of the issues.  To really fix this, OpenCL would have to be subset substantially, like the EFI C dialect.

> 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.

-Chris