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

[Eric Christopher]

On Thu, Aug 22, 2013 at 12:53 PM, Eric Christopher <echristo at gmail.com> wrote:
> 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.
>

Since I've obviously confused Rafael a bit as well let me attempt to
be a bit more clear here:

a) I'm not, ultimately, objecting to the patch pending some horrible
statement from Nick.
b) I agree on principle with -flto doing different things from
-emit-llvm, especially as a flag to modify and change the pass
manager.
c) I think if we make the change on -flto != -emit-llvm then
-emit-llvm should error if it isn't passed as part of -c or -S.
d) We could probably use some documentation. :)

-eric