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

[Rui Ueyama via llvm-dev]

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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