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

[Rui Ueyama via llvm-dev]

On Fri, Mar 24, 2017 at 2:04 PM, Sean Silva <chisophugis at gmail.com> wrote:

> I lile the idea of having it more structured and I think your suggested
> format is the right direction.
>
> I think one principle should be that we assume that file names and symbol
> names are "really long" (possibly wrapped by the terminal etc.).
>

Right. That's what we should expect.

I believe the current error message format for the existing linkers were
set more than 30 years ago when path names and symbol names were much
shorter than they are today. If they are short, error messages become
something like "src/libfoo/bar.o: undefined symbol: strbar", which is quite
easy to read. That is unfortunately no longer the case because we are
creating significantly larger programs than a few decades ago and C++ name
mangling makes symbol names significantly longer than before.

So I would modify your suggested format to use the "note" color for the
> strings like "Object 1" so that even if those lines are wrapped by the
> terminal then they can still be easily visually parsed. We may also want to
> do something like ">>> Object 1:" so that places without color (like
> google's internal web ui for build logs, or if users are piping the build
> system's output into `less` or `tee`) are still a bit easier to parse in
> the presence of wrapped lines.
>

Makes sense. I tried a few different formats based on suggestions from
Mehdi, Hans and you, and come up with this one. What do you think?

bin/ld.lld: *error:* undefined symbol:
lld::elf::EhFrameSection<llvm::object::ELFType<(llvm::support::endianness)0,
true> >::addSection(lld::elf::InputSectionBase*)
*>>> referenced by*
/home/buildslave/buildslave/clang-cmake-aarch64-39vma/llvm/tools/lld/ELF/Writer.cpp:207
*>>>*               lib/liblldELF.a(Writer.cpp.o)


bin/ld.lld: *error:* duplicate symbol:
lld::elf::MipsGotSection::addEntry(lld::elf::SymbolBody&, long,
lld::elf::RelExpr)
*>>> defined at*
/home/buildslave/buildslave/clang-cmake-aarch64-39vma/llvm/tools/lld/ELF/Writer.cpp:38
*>>>*            lib/liblldELF.a(Writer.cpp.o)
*>>> defined at*
/home/buildslave/buildslave/clang-cmake-aarch64-39vma/llvm/tools/lld/ELF/SyntheticSections.cpp:673
*>>>*            lib/liblldELF.a(SyntheticSections.cpp.o)


This format prints out source file names and object file names in different
lines. I found that that is easier to read than print them in the same
line, as error messages with some amount of whitespace are easier to find
in dense build system outputs.

This principle also suggests (and your suggested format does this) that to
> avoid having to parse past a symbol/file name to get to other information,
> there should be at most one symbol/file name on a given line and there
> should never be anything after it on the line. The ld64 format violates
> this, for example.
>
> -- Sean Silva
>
> On Mar 23, 2017 4:18 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)
>
> *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.
>
> Thoughts?
>
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