[PATCH][Clang Driver] Driver::IsUsingLTO no longer return true when seeing -emit-llvm

[Eric Christopher]

On Thu, Aug 22, 2013 at 12:14 PM, Rafael Espíndola
<rafael.espindola at gmail.com> wrote:
>> Right. My comment on this change is, in essence, that people have
>> depended upon the fact that -flto does what you want -emit-llvm to do
>> for that compile line for a while. I don't think it's necessarily
>> right, but that you're going to be breaking current use.
>
> So, the possible changes that people using clang will have to adapt to is
>
> 1) Using -flto instead of -emit-llvm when doing an lto build.
> 2) Using -emit-llvm instead of -flto when fetching IL to be passed to
> bugpoint or similar.
>
> I don't think 2 is a big concern, since it is a dev option and we
> change those as needed. Issue 1 is a bit more serious, but the change
> is really simple. The -flto option is also what gcc uses.
>

The concerns I've seen are that -emit-llvm doesn't work as well as
-flto for the task of grabbing IR for any purpose. Nick is the one who
brought this up within hearing range the other day, maybe he can
explain. FWIW I only think -flto should be used to enable lto mode of
everything on the command line and not for grabbing IR at all. I.e. if
there's no link step then -flto shouldn't do anything.

This would be a change in behavior though.

-eric

> Another way to do this would be to keep -emit-llvm == -flto and change
> both to the behavior we want during a LTO build. This would require
> adding a new -dump-ir-before-codegen or similar and people using
> -emit-llvm for debugging would have to switch to that. Overall this
> seems like a worse state than having -flto != -emit-llvm to me.
>
> Cheers,
> Rafael