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

Rui Ueyama via llvm-commits llvm-commits at lists.llvm.org
Wed May 3 17:26:25 PDT 2017


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, 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.

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).


> --
> 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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