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

[Eric Christopher]

On Tue, Aug 13, 2013 at 5:29 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:
> Hi, Eric:
>
>  Thank you for the code review.
>
>
> On 8/13/13 5:15 PM, Eric Christopher wrote:
>>
>> On Tue, Aug 13, 2013 at 4:20 PM, Shuxin Yang <shuxin.llvm at gmail.com>
>> wrote:
>>>
>>> Hi,
>>>
>>>      Driver::IsUsingLTO()  returns true when "-emit-llvm" is seen, which
>>> is
>>> quite awkward if we need to differentiate following two commands, where
>>> 1) is just go through regular passes and stop at llc, while 2) needs to
>>> go
>>> through
>>> pre-ipo passes.
>>>
>>>     1) clang -emit-llvm a.c -c , and
>>>     2) clang -flto a.c -c
>>>
>>>   With this tiny patch is to differentiate these two situations.
>>
>> This is a laudable goal, though i'm not sure what we gain here. I.e.
>> Sadly, using -flto to grab bitcode out has been "the way" to do it for
>> long enough that it's fairly baked in.
>
>
> There are couple of reasons for differentiating them.
>
> For instance, vectorization should not be conducted in 2) because it is too
> early to kick in --
> after aggressive inter-procedural optimization, some symbolic trip-count
> will become constant, and some functions are inlined, enabling more
> opportunities etc...
>
> In short,  2) tries to eliminate as much as redundancy without increase code
> size,
> and basically, most loop transformations, no matter it's on a single loop,
> or a loop nest,
> must be disabled in 2).
>
> Thank you again for the code review.
>

I'm not sure this is a good motivation really. I'd almost rather
redefine the interface for "please compile all of these files (*) as
one single monolithic entity" and have the driver handle how to invoke
the various optimizer levels and at what point we actually merge the
modules (and then what passes we run after that).

-eric