On Sun, Aug 25, 2013 at 11:01 PM, Ben Lippmeier <benl@ouroborus.net> wrote:

On 23/08/2013, at 3:52 AM, Ryan Newton wrote:

> Well, what's the long term plan? Is the LLVM backend going to become the only backend at some point?

I wouldn't argue against ditching the NCG entirely. It's hard to justify fixing NCG performance problems when fixing them won't make the NCG faster than LLVM, and everyone uses LLVM anyway.

This is not true. I don't believe I've ever seen the LLVM backend compile more quickly than the NCG, it usually takes significantly longer, and for at least some (most?) projects produces worse output.

I don't have anything against the LLVM backend in principle*, but at present it's not as good as the NCG for us.

We're going to need more and more SIMD support when processors supporting the Larrabee New Instructions (LRBni) appear on people's desks. At that time there still won't be a good enough reason to implement those instructions in the NCG.

How about that the NCG is better than LLVM? ;)

In all seriousness, I'm quite sympathetic to the desire to support only one backend, and LLVM can offer a lot (SIMD fallbacks, target architectures, etc). But at present, in my experience the LLVM backend doesn't really live up to what I've seen claimed for it. Given that, I think it's a bit premature to talk of dropping the NCG.

My $0.02,
John

[1] Ok, I do have one issue with LLVM. It's always struck me as very brittle, with a lot of breakages between versions. Given that I just tried ghc -fllvm with LLVM-3.3 and the compiler bailed out due to a bad object file, my impression of brittleness doesn't seem likely to change any time soon. Given that LLVM releases major versions predictably often, I don't know that I want ghc devs spending time chasing after them. But in principle it seems the right thing to do.