[llvm] r189297 - Add new API lto_codegen_compile_parallel().

[Chandler Carruth]

On Mon, Aug 26, 2013 at 5:53 PM, Shuxin Yang <shuxin.llvm at gmail.com> wrote:

> We certainly need a way to feed multiple resulting objects back to linker.
>  There are couple of ways
> for this end:
>
>    1) 'ld -r all-resulting-obj-on-disk -o result.o"  and feed the only
> object file (i.e. the result.o)
>        back to linker
>
>     2) keep the resulting objects in memory buffer, and feedback to
> buffers back to linker
>         (as proposed by Nick)
>
>     3) As with GNU gold,  save the resulting objects on disk, and feed the
> these disk files back to linker
> one by one.
>
>     I'm big linker nut. I don't know which way work better.  I try to use
> 1) as a workaround for the time being
> before 2) is available. People at Apple disagree my engineering approach.
>
>     From compiler's perspective,
>     o. 1) is not just workaround, 3) is certainly better than 1).
>     o. 2) will win if the program being compiled is small- or medium-sized.
>         With huge programs,  it will be difficult for compiler to decide
> when and how to "spill" some stuff
>         from memory to disk.  Folks in Apple iterate and reiterate we only
> consider the case that the entire
>        program can be loaded in memory. So, the added difficulty for
> compiler dose not seems to be a
>        problem for the workload we care about.


Shuxin, I'm not sure what you're trying to accomplish here, but I don't
think this is the right approach.

First, you seem to be pursuing a partitioning scheme for parallelizing LTO
work despite *no* consensus that this is the correct approach in any of the
community discussions I can find. Please don't commit code toward a design
that the community has expressed serious reservations about without review.

Second, you are committing a new API to the set of the stable C APIs that
libLTO exposes without a thorough discussion on the mailing list. It is
possible I have missed this discussion, but I did look and failed to find
anything that seems to resemble a review, much less an LGTM. If I have
missed it, I apologize and please direct me at the thread. I bring this up
because the specific interface seems surprising to me.

Third, you are justifying the particular approach with a deflection to some
discussion within Apple or with those developers you work with at Apple.
While this may in fact be the motivation for this patch, the open source
community is often not party to these discussions. ;] It would help us if
you would just give the specific basis rather than referencing a discussion
that we weren't involved with. As it happens, I suspect I agree with these
"Folks in Apple" that it is useful to specifically optimize for the case
that an entire program fits into memory, bypassing the filesystem. However,
there are many paths to that end result. From the little information in the
commit log there isn't really enough to tell why *this* is the necessary
path forward (in fact, I'm somewhat confident it isn't).


So, to get back to Eric's original question: what is the motivation for
this API, it's expected actual usage, and the reason why it is important to
stub out in this way now? Better yet, could we have that discussion before
growing the set of stable APIs that we claim to never regress?

-Chandler
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