[lld] r282633 - Warn on empty archive files.

Fri Sep 30 12:46:57 PDT 2016

Hmm, that's right, and if we compile it for a LTO build, we'll get an
archive file with no symbol containing a bitcode file. That means there's
no reliable way to warn on this thing. Probably I should remove this
warning message.

On Fri, Sep 30, 2016 at 12:35 PM, Nico Weber <thakis at chromium.org> wrote:

> Say you have a .cc file that looks like
>
> #if defined(OS_LINUX)
> void f() {}
> #endif
>
> It sounds like it'll still warn on this on non-linux.
>
> (Anyhow, the "have warnings-as-errors, and a toggle for every warning" bit
> is more important than the behavior on this specific warning.)
>
> On Fri, Sep 30, 2016 at 3:27 PM, Rui Ueyama <ruiu at google.com> wrote:
>
>> Thank you for the example. With my last commit, LLD now warns on an
>> archive file if it has at least one file and has no symbol. So, if an
>> archive file is completely empty, it doesn't warn on it. From your example,
>> it sounds like the warning message is not going to be spammy.
>>
>> On Fri, Sep 30, 2016 at 12:25 PM, Nico Weber <thakis at chromium.org> wrote:
>>
>>> 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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