[Bug rtl-optimization/85059] New: Compiling with -Og should preserve variable values

[fredrik at dolda2000 dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85059

            Bug ID: 85059
           Summary: Compiling with -Og should preserve variable values
           Product: gcc
           Version: 8.0.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fredrik at dolda2000 dot com
  Target Milestone: ---

One of the most valuable resources when debugging is being able to see the
values of variables (including function arguments), even when those values are
technically dead in the program. If the purpose of the `-Og' optimization
option is, as the manual states, to "optimize the debugging experience", then I
think it would make a lot of sense for it to ensure that variable values are
not thrown away just because they're dead in the dataflow. In practice, this is
the main and foremost reason why I never use `-Og' when debugging a program.

Just as a practical example, consider this C file:

    int *b(void *);

    int a(void *obj)
    {
        return(b(obj)[0]);
    }

Consider the case where b() returns an invalid pointer and a() crashes on
dereferencing it. In such a case, it would likely be enormously helpful to have
the parameter value `obj' to figure out why b() failed. However, when this file
is compiled with `-Og', that value is no longer present after the call to b().
To illustrate:

$ gcc -ggdb -Og -c -o test.o test.c
$ objdump -dr test.o

test.o:     file format elf64-x86-64


Disassembly of section .text:

0000000000000000 <a>:
   0:   48 83 ec 08             sub    $0x8,%rsp
   4:   e8 00 00 00 00          callq  9 <a+0x9>
                        5: R_X86_64_PLT32       b-0x4
   9:   8b 00                   mov    (%rax),%eax
   b:   48 83 c4 08             add    $0x8,%rsp
   f:   c3                      retq   

`obj' only lives in %rdi, and its value is not preserved over the call to b.

If it matters, I've tested this on GCC versions 4.7.2, 6.3.0 and 7.2.0 (the
default versions in various Debian releases), and the latest 8.0.1 from SVN
right now.