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

[Mehdi AMINI via llvm-commits]

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.



>
>>
>>
>>
>>>
>>>> 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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