[lld] r282633 - Warn on empty archive files.

[Nico Weber via llvm-commits]

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.
>>>>
>>>>
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>>>>
>>>
>>>
>
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