[llvm-dev] [RFC] better link error messages

[Rui Ueyama via llvm-dev]

On Thu, Mar 23, 2017 at 5:16 PM, Mehdi Amini <mehdi.amini at apple.com> wrote:

>
> On Mar 23, 2017, at 3:17 PM, Rui Ueyama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Folks,
>
> I'd like propose a new error message format for LLD so that error message
> for undefined or duplicated symbols are more informative and easy to read.
>
> Below are examples of the current error messages (note that characters in
> red are actually red on terminal):
>
> *Undefined symbols*
> /ssd/clang/bin/ld.lld: error: /ssd/llvm-project/lld/ELF/Writer.cpp:207:
> undefined symbol 'lld::elf::EhFrameSection<llvm
> ::object::ELFType<(llvm::support::endianness)0, true>
> >::addSection(lld::elf::InputSectionBase*)'
>
> *Conflicting symbols*
> /ssd/clang/bin/ld.lld: error: /ssd/llvm-project/lld/ELF/Writer.cpp:38:
> duplicate symbol 'lld::elf::MipsGotSection::addEntry(lld::elf::SymbolBody&,
> long, lld::elf::RelExpr)'
> /ssd/clang/bin/ld.lld: error: /ssd/llvm-project/lld/ELF/SyntheticSections.cpp:673:
> previous definition was here
>
>
> For each error, we want to print out information about 1) symbol name, 2)
> source file name(s) and source location(s) if available and 3) source
> object file name(s) and archive file name(s) if available. Currently, these
> messages lack object file names, and I think the above error messages are a
> bit hard to grasp because too much information is packed into each line.
>
> I'm thinking of changing the format to something like the following:
>
> *Undefined symbols*
> /ssd/clang/bin/ld.lld: error: undefined symbol:
> lld::elf::EhFrameSection<llvm::object::ELFType<(llvm::support::endianness)0,
> true> >::addSection(lld::elf::InputSectionBase*)
>   Source: /ssd/llvm-project/lld/ELF/Writer.cpp:207
>   Object: lib/liblldELF.a(Writer.cpp.o)
>
>
> As a point of comparison, here is what ld64 displays:
>
> Undefined symbols for architecture x86_64:
>   "lld::elf::EhFrameSection<llvm::object::ELFType<(llvm::support::endianness)0,
> true> >::addSection(lld::elf::InputSectionBase*)", referenced from:
>       foo() in lib/liblldELF.a(Writer.cpp.o)
>
> Being able to display source location when available is a really nice
> idea!!
>
>
>
> *Conflicting symbols*
> /ssd/clang/bin/ld.lld: error: duplicate symbol:
> lld::elf::MipsGotSection::addEntry(lld::elf::SymbolBody&, long,
> lld::elf::RelExpr)
>   Source 1: /ssd/llvm-project/lld/ELF/Writer.cpp:38
>   Source 2: /ssd/llvm-project/lld/ELF/SyntheticSections.cpp:673
>   Object 1: lib/liblldELF.a(Writer.cpp.o)
>   Object 2 : lib/liblldELF.a(SyntheticSections.cpp.o)
>
>
> The new error messages contain complete information that the linker is
> able to gather, and it uses more vertical space to improve readability.
>
>
> I feel it is using too much vertical space, and the matching between
> source and object is lost. What about something like:
>
> /ssd/clang/bin/ld.lld: error: duplicate symbol: lld::elf::MipsGotSection::
> addEntry(lld::elf::SymbolBody&, long, lld::elf::RelExpr)
>   - Defined in lib/liblldELF.a(Writer.cpp.o) (from
> /ssd/llvm-project/lld/ELF/Writer.cpp:38)
>   - Defined in lib/liblldELF.a(SyntheticSections.cpp.o) (from
> /ssd/llvm-project/lld/ELF/SyntheticSections.cpp:673)
>

What would you print out for undefined symbols? `- Undefined in foo.o (from
...)` looks odd.
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