[PATCH] D32721: Accept archive files with no symbol table instad of warning on them.

[Rui Ueyama via llvm-commits]

On Wed, May 3, 2017 at 5:58 PM, Mehdi AMINI <joker.eph at gmail.com> wrote:

>
> On Wed, May 3, 2017 at 5:51 PM Rui Ueyama <ruiu at google.com> wrote:
>
>> On Wed, May 3, 2017 at 5:48 PM, Mehdi AMINI <joker.eph at gmail.com> wrote:
>>
>>>
>>> On Wed, May 3, 2017 at 5:44 PM Rui Ueyama <ruiu at google.com> wrote:
>>>
>>>> On Wed, May 3, 2017 at 5:40 PM, Mehdi AMINI <joker.eph at gmail.com>
>>>> wrote:
>>>>
>>>>>
>>>>> On Wed, May 3, 2017 at 5:26 PM Rui Ueyama <ruiu at google.com> wrote:
>>>>>
>>>>>> On Wed, May 3, 2017 at 5:13 PM, Mehdi AMINI <joker.eph at gmail.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Clang is incrementally linking in a matter of a few seconds, so 0.5s
>>>>>>> to read the symbols is a double digit percentage of that.
>>>>>>> And there are over 50 binaries in LLVM, not just one.
>>>>>>>
>>>>>>
>>>>>> We do not support incremental linking,
>>>>>>
>>>>>
>>>>> I'm talking about ThinLTO incremental linking, which we support.
>>>>>
>>>>
>>>> How can that be faster than the regular build?
>>>>
>>>
>>> Not sure what you mean: on my mac ld64 links clang in less than 2s.
>>>
>>
>> But that is ld64. We are talking about LLD, no?
>>
>
> I don't see where you're going: lld is supposed to be fast, isn't it? I
> assume it has to be able to outspeed ld64.
> So I'm giving you a reference of what is "a regular" build time and that
> should explain why you .5s overhead is not trivial.
>

That is hypothetical. If LLD is already able to link Clang with ThinLTO in
2 seconds, it may make sense to warn on 0.5 second loss, but that's in
reality not the case, so I'm not convinced that we should warn on it right
now.


>>
>>>
>>>
>>>
>>>>
>>>>> but even if we support it, we don't need to read archives that haven't
>>>>>> changed since the last build, so the overhead in that hypothetical case
>>>>>> would be much smaller than 0.5s.
>>>>>>
>>>>>
>>>>> So yes we need to read all the archives.
>>>>>
>>>>>
>>>>>> And you still don't address the "principle of least surprise": the
>>>>>>> configuration is *not* what is expected from the user.
>>>>>>>
>>>>>>
>>>>>> As a naive user of LTO, I was surprised that LTO needs llvm-ar, which
>>>>>> is certainly I didn't expect (due to lack of knowledge).
>>>>>>
>>>>>
>>>>> That is why the warning is deserved.
>>>>>
>>>>
>>>> But you no longer need it with this change.
>>>>
>>>> --
>>>>> Mehdi
>>>>>
>>>>>
>>>>>
>>>>>>
>>>>>>> --
>>>>>>> Mehdi
>>>>>>>
>>>>>>>
>>>>>>> 2017-05-03 16:51 GMT-07:00 Rui Ueyama <ruiu at google.com>:
>>>>>>>
>>>>>>>> The cost of reading symbols from object files in archive files is
>>>>>>>> probably much cheaper than you might be thinking. If I strip all symbols
>>>>>>>> from archives from a clang debug build, LLD takes 8.16 seconds to link,
>>>>>>>> while it can usually link it in 7.65 seconds. So the difference is only 0.5
>>>>>>>> seconds, and clang is a fairly large program as a test. That test case uses
>>>>>>>> ELF, but with Peter's patch I believe reading symbols from bitcode files is
>>>>>>>> fast too.
>>>>>>>>
>>>>>>>> To me 0.5 seconds is too small that I want the tool to "just work"
>>>>>>>> instead of annoy me every time I run make/ninja until I change the build
>>>>>>>> configuration to shave off 0.5 seconds from a LTO build.
>>>>>>>>
>>>>>>>> On Wed, May 3, 2017 at 4:23 PM, Mehdi AMINI via Phabricator <
>>>>>>>> reviews at reviews.llvm.org> wrote:
>>>>>>>>
>>>>>>>>> mehdi_amini added a comment.
>>>>>>>>>
>>>>>>>>> I personally  think *not* warn is a terrible thing to do when
>>>>>>>>> there is a configuration issue. Erroring is annoying, but warning should be
>>>>>>>>> intended in such cases!
>>>>>>>>>
>>>>>>>>> > True, but on the other hand, it's pretty much the exact same
>>>>>>>>> work that the archiver would need to do,
>>>>>>>>>
>>>>>>>>> The archiver do it once for potentially a lot of linker
>>>>>>>>> invocations.
>>>>>>>>>
>>>>>>>>> > and asking the user to change their archiver and rebuild would
>>>>>>>>> probably consume even more time.
>>>>>>>>>
>>>>>>>>> This is a one time thing, and the user can live with the warning
>>>>>>>>> (or pass a flag to disable the warning maybe) if they choose to.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Repository:
>>>>>>>>>   rL LLVM
>>>>>>>>>
>>>>>>>>> https://reviews.llvm.org/D32721
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>
>>>>>>>
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