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

[Shuxin Yang]

On 8/13/13 5:46 PM, Eric Christopher wrote:
> 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
I remember gcc has a flag to "combine all files as a single monolithic 
entity".
perhaps -combine, which is considered "poor-man's" IPA, and if I remember
correctly, it works only for c.

The standard practice for IPA is
   1. cc -flto-other-other-flags *.c -c
   2) cc -flto *.o

the benefit of this way than the poor-man's ipa is that we don't need to 
modify
makefile with or without IPA/LTO.