[lld] r282633 - Warn on empty archive files.

[Nico Weber via llvm-commits]

For example

static_library("foo") {
  if (is_linux) {
    sources = [ "foo_linux.cc" ]
  } else if (is_win) {
    sources = [ "foo_win.cc" ]
  }
}

This is an empty library on mac, android, etc. If static libraries were not
allowed, then everything that depends on this would either have to be
careful to only depend on "foo" on linux and windows, or each of these
libraries would have to be a static_library only on linux and win, and a
dummy group elsewhere. We actually did this for a while (due to xcodebuild
not liking empty static library targets iirc), and while it's arguably
cleaner, it turns out to be annoying to do in practice without any real
benefit. So now we have a bunch of these library targets that end up empty
on some platforms.

On Fri, Sep 30, 2016 at 3:17 PM, Rui Ueyama <ruiu at google.com> wrote:

> I'm OK to revert this patch entirely if it is still annoying, but in the
> first place why Chromium has an archive file that has no symbols?
>
> On Fri, Sep 30, 2016 at 12:11 PM, Nico Weber <thakis at chromium.org> wrote:
>
>> Also, in case it isn't that way yet, please make sure that for warnings:
>>
>> 1.) It's possible to turn warnings into errors
>> 2.) It's possible to disable every warning with some flag
>>
>> (like in clang)
>>
>> For warnings to be enforceable, we need warnings-as-errors. But we might
>> not want to fix all the things that a tool warns about, so we need to be
>> able to disable warnings we think are useful.
>>
>> On Fri, Sep 30, 2016 at 1:03 PM, Rui Ueyama <ruiu at google.com> wrote:
>>
>>> I'd instead make it precise; if an archive file has no symbol table and
>>> it contains a LLVM bitcode file, it is likely that the file was created by
>>> mistake. What do you think?
>>>
>>> On Fri, Sep 30, 2016 at 9:58 AM, Nico Weber <thakis at chromium.org> wrote:
>>>
>>>> libtool (on mac) warns on this, and it's spectacularly unhelpful. We
>>>> filter out that warning in a wrapper script since there's no flag to turn
>>>> it off and the warning is just noise in our build. If lld gets this
>>>> warning, please provide a way to toggle it off.
>>>>
>>>> On Sep 30, 2016 12:31 PM, "Reid Kleckner" <rnk at google.com> wrote:
>>>>
>>>>> Are you sure we should be warning on empty archives? This comes up in
>>>>> Chromium's build, which apparently has an archive with no symbols:
>>>>> https://build.chromium.org/p/chromium.fyi/builders/ClangToTL
>>>>> inuxLLD/builds/1596/steps/compile/logs/stdio
>>>>>
>>>>> That build also doesn't involve LTO, so the long explanatory comment
>>>>> isn't helpful.
>>>>>
>>>>> On Wed, Sep 28, 2016 at 2:10 PM, Rui Ueyama via llvm-commits <
>>>>> llvm-commits at lists.llvm.org> wrote:
>>>>>
>>>>>> Author: ruiu
>>>>>> Date: Wed Sep 28 16:10:54 2016
>>>>>> New Revision: 282633
>>>>>>
>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=282633&view=rev
>>>>>> Log:
>>>>>> Warn on empty archive files.
>>>>>>
>>>>>> Differential Revision: https://reviews.llvm.org/D25044
>>>>>>
>>>>>> Modified:
>>>>>>     lld/trunk/ELF/InputFiles.cpp
>>>>>>     lld/trunk/test/ELF/empty-archive.s
>>>>>>
>>>>>> Modified: lld/trunk/ELF/InputFiles.cpp
>>>>>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/InputFiles
>>>>>> .cpp?rev=282633&r1=282632&r2=282633&view=diff
>>>>>> ============================================================
>>>>>> ==================
>>>>>> --- lld/trunk/ELF/InputFiles.cpp (original)
>>>>>> +++ lld/trunk/ELF/InputFiles.cpp Wed Sep 28 16:10:54 2016
>>>>>> @@ -427,8 +427,17 @@ template <class ELFT> void ArchiveFile::
>>>>>>    File = check(Archive::create(MB), "failed to parse archive");
>>>>>>
>>>>>>    // Read the symbol table to construct Lazy objects.
>>>>>> -  for (const Archive::Symbol &Sym : File->symbols())
>>>>>> +  bool IsEmpty = true;
>>>>>> +  for (const Archive::Symbol &Sym : File->symbols()) {
>>>>>>      Symtab<ELFT>::X->addLazyArchive(this, Sym);
>>>>>> +    IsEmpty = false;
>>>>>> +  }
>>>>>> +
>>>>>> +  if (IsEmpty)
>>>>>> +    warning(getName() + " has no symbol. Chances are you are doing "
>>>>>> +            "an LTO build and forgot to use an ar command that can
>>>>>> create "
>>>>>> +            "a symbol table for LLVM bitcode files. If so, use
>>>>>> llvm-ar or "
>>>>>> +            "GNU ar + plugin.");
>>>>>>  }
>>>>>>
>>>>>>  // Returns a buffer pointing to a member file containing a given
>>>>>> symbol.
>>>>>>
>>>>>> Modified: lld/trunk/test/ELF/empty-archive.s
>>>>>> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/test/ELF/empty
>>>>>> -archive.s?rev=282633&r1=282632&r2=282633&view=diff
>>>>>> ============================================================
>>>>>> ==================
>>>>>> --- lld/trunk/test/ELF/empty-archive.s (original)
>>>>>> +++ lld/trunk/test/ELF/empty-archive.s Wed Sep 28 16:10:54 2016
>>>>>> @@ -1,3 +1,5 @@
>>>>>>  // RUN: llvm-ar rc %t.a
>>>>>>  // RUN: llvm-mc -filetype=obj -triple=x86_64-pc-linux %s -o %t.o
>>>>>> -// RUN: ld.lld -shared %t.o %t.a -o t
>>>>>> +// RUN: ld.lld -shared %t.o %t.a -o t 2>&1 | FileCheck %s
>>>>>> +
>>>>>> +// CHECK: has no symbol.
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> llvm-commits mailing list
>>>>>> llvm-commits at lists.llvm.org
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>>>>>>
>>>>>
>>>>>
>>>
>>
>
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