[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 5:48 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?


>
>
>
>>
>>> 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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