[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 5:40 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?


> 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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