[Bug debug/115945] New: DW_LNE_set_discriminator emitted right before Special Opcodes

[blarsen at redhat dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=115945

            Bug ID: 115945
           Summary: DW_LNE_set_discriminator emitted right before Special
                    Opcodes
           Product: gcc
           Version: 14.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: blarsen at redhat dot com
  Target Milestone: ---

Created attachment 58672
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=58672&action=edit
small cpp code that reproduces the issue

The issue:

When generating line table entries for the attached program, GCC generates the
following list of operations at some point (as printed by readelf):

  Copy (view 3)
  Set column to 18
  Extended opcode 4: set Discriminator to 1
  Special opcode 13: advance Address by 0 to 0x401133 and Line by 8 to 11 (view
4)

However, per the DWARF 5 spec section 6.2.5.1, Special Opcodes always clobber
the Discriminator register to be 0, so the extended opcode 4 is superfluous
and, worse, the information is lost, making it harder to unwind the program.

command line:
  g++ -fPIC -Og -g gcc_reproducer.cpp

GCC information:

  gcc version 14.1.1 20240701 (Red Hat 14.1.1-7) (GCC) 

  Configured with: ../configure --enable-bootstrap
--enable-languages=c,c++,fortran,objc,obj-c++,ada,go,d,m2,lto --prefix=/usr
--mandir=/usr/share/man --infodir=/usr/share/info
--with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-shared
--enable-threads=posix --enable-checking=release --enable-multilib
--with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions
--enable-gnu-unique-object --enable-linker-build-id
--with-gcc-major-version-only --enable-libstdcxx-backtrace
--with-libstdcxx-zoneinfo=/usr/share/zoneinfo --with-linker-hash-style=gnu
--enable-plugin --enable-initfini-array
--with-isl=/builddir/build/BUILD/gcc-14.1.1-20240701/obj-x86_64-redhat-linux/isl-install
--enable-offload-targets=nvptx-none,amdgcn-amdhsa --enable-offload-defaulted
--without-cuda-driver --enable-gnu-indirect-function --enable-cet
--with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
--with-build-config=bootstrap-lto --enable-link-serialization=1