[cfe-dev] Compiling with both -flto and -g -- is it supposed to work ?

[David Blaikie via cfe-dev]

Not sure how this works on MacOS where object files are needed for
debugging - on Linux lto works with -g because debug info is linked
into the final binary. (CC'd some Apple folks who might know how this
is supposed to work)

On Tue, Jun 9, 2020 at 12:15 AM Philippe Blain via cfe-dev
<cfe-dev at lists.llvm.org> wrote:
>
> Hello,
>
> I'm wondering if compiling with both `-flto` and `-g` is supported. I could not find any open bug on the bugzilla and the documentation does not mention it should or should not work.
>
> When I do that (I also use `-flto` at the link step), and open the generated executable in GDB (tried both 8.3.1 and 9.2), I get the following:
> ```
> $ gdb --args python testing/process_to_debug.py
> GNU gdb (GDB) 8.3.1
> Reading symbols from <executable>...
>
> warning: `/tmp/lto.o': can't open to read symbols: No such file or directory.
> (No debugging symbols found in <executable>)
> (gdb) r
> Starting program: /Users/Philippe/Logiciels/miniconda3/envs/gdb/bin/python testing/process_to_debug.py
> [New Thread 0x1103 of process 75303]
> warning: `/tmp/lto.o': can't open to read symbols: No such file or directory.
> ```
>
> I get set breakpoints at symbol names, but can't `list` or see function arguments...
>
> This is on macOS 10.11.6, using the pre-compiled clang from https://releases.llvm.org/download.html:
> $ clang --version
> clang version 9.0.0 (git://github.com/llvm/llvm-project.git 0399d5a9682b3cef71c653373e38890c63c4c365)
>
> and the system linker (I do use the above clang as the driver for the link step) :
> $ ld -v
> @(#)PROGRAM:ld  PROJECT:ld64-274.2
> configured to support archs: armv6 armv7 armv7s arm64 i386 x86_64 x86_64h armv6m armv7k armv7m armv7em (tvOS)
> LTO support using: LLVM version 8.0.0, (clang-800.0.42.1)
> TAPI support using: Apple TAPI version 1.30
>
> For reference I tried with <executable> being Git and Python..
> I also tried debugging with the system LLDB (lldb-360.1.70) and the latter does not complain when loading the executable, but I get pretty much the same behaviour, nothing more than function names are available...
>
> Also, dwarfdump outputs:
> $ dwarfdump git
> ----------------------------------------------------------------------
>  File: git (x86_64)
> ----------------------------------------------------------------------
> .debug_info contents:
> < EMPTY >
>
> Is this normal behaviour ?
>
> Thanks,
> Philippe.
> P.S.: this is my first time posting here so don't hesitate if more information is needed.
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