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<div class="moz-cite-prefix">On 8/26/13 11:19 PM, Chandler Carruth
wrote:<br>
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cite="mid:CAGCO0Khk4+Eoy2pmW5yWUKA4KO3moX9wyvBLGOVHSosoUdD5Nw@mail.gmail.com"
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<div dir="ltr">On Mon, Aug 26, 2013 at 5:53 PM, Shuxin Yang <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:shuxin.llvm@gmail.com" target="_blank"
class="cremed">shuxin.llvm@gmail.com</a>></span> wrote:<br>
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<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-left:1ex">We
certainly need a way to feed multiple resulting objects
back to linker. There are couple of ways<br>
for this end:<br>
<br>
1) 'ld -r all-resulting-obj-on-disk -o result.o" and
feed the only object file (i.e. the result.o)<br>
back to linker<br>
<br>
2) keep the resulting objects in memory buffer, and
feedback to buffers back to linker<br>
(as proposed by Nick)<br>
<br>
3) As with GNU gold, save the resulting objects on
disk, and feed the these disk files back to linker<br>
one by one.<br>
<br>
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<br>
before 2) is available. People at Apple disagree my
engineering approach.<br>
<br>
From compiler's perspective,<br>
o. 1) is not just workaround, 3) is certainly better
than 1).<br>
o. 2) will win if the program being compiled is small-
or medium-sized.<br>
With huge programs, it will be difficult for
compiler to decide when and how to "spill" some stuff<br>
from memory to disk. Folks in Apple iterate and
reiterate we only consider the case that the entire<br>
program can be loaded in memory. So, the added
difficulty for compiler dose not seems to be a<br>
problem for the workload we care about.</blockquote>
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<div>Shuxin, I'm not sure what you're trying to accomplish
here, but I don't think this is the right approach.</div>
<div><br>
</div>
<div>First, you seem to be pursuing a partitioning scheme
for parallelizing LTO work despite *no* consensus that
this is the correct approach </div>
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I sent a proposal long time ago, as far as I can understand from the
mailing list. There is no objection at all. <br>
Actually, but my approach is not new at all. It is almost a "std"
way to perform partition. It looks similar to all LTOs I
worked/played before.<br>
It just need some LLVM flavor. But this change has nothing to do
the partition implementation, it just add a interface. <br>
<br>
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<div>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.</div>
<div><br>
</div>
<div>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. </div>
</div>
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Sorry, I thought this is pretty Apple thing, as no other system use
this API. <br>
I will revert tomorrow, and initiate a discussion. <br>
<br>
The APIs are almost divided into two classes. One for Unix+gold, the
other one for OSX + Apple LD.<br>
I don't like the way it is, and I don't like the such APIs at all (I
mean all of them). <br>
I used to argue we are better off having a symbol-related interface
instead of LTO-related API.<br>
But the community dose not buy my point. As I have little
knowledge about LLVM, I have to keep <br>
open mind, and adapter to LLVM-thinking, but it certainly take some
time. <br>
<br>
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<div>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.</div>
<div><br>
</div>
<div>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. ;] </div>
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That is true:-)<br>
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<div>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. </div>
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You bet!. <br>
<br>
I debate with them. No chance to win. Why don't you suspect in the
first place:-). <br>
But "folks in Apple" argue that is plan in the future. It dose not
seems to be pretty lame argument, <br>
as current implement of LTO bring everything in memory. <br>
<br>
| 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).<br>
<br>
In concept, there is only one alternative : compile the the merged
module into multiple objects, and feed the object back to linker. <br>
<br>
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<div>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? </div>
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The motivation is: the existing LTO compile the merged module into
*single* object, <br>
with this new API, it enable the way to compile merged module into
*multiple* objects. <br>
I'm wondering if this is clear now. <br>
<br>
for instance, suppose the command line is "clang -flto a.o b.bc
c.o d.bc" (*.o is real object, and *.bc are bitcode), <br>
existing LTO will merge b.bc and d.dc into t.bc (merged module),
LTO will compile the merged t.bc into t.o, <br>
and feed the t.o back the linker which combine a.o c.o t.o into
a.out. <br>
<br>
The new API will trigger the compiler convert t.o into p1.o and
p2.o ...., and feed these p*.o back to linker, which <br>
combine a.o and c.o into a.out. <br>
<br>
<br>
<br>
<br>
<br>
<br>
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<div>Better yet, could we have that discussion before
growing the set of stable APIs that we claim to never
regress?</div>
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<br>
Sure. Sorry about that. I actually don't what to touch the lto_xxx()
API for now. I just want to do some workaround <br>
on the limitation on the linker, and wait for new ld. But Bob didn't
buy my argument:-).<br>
<br>
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