[New-bugs-announce] [issue38350] ./configure --with-pydebug should use -O0 rather than -Og

[STINNER Victor]

New submission from STINNER Victor <vstinner at python.org>:

In Python 2.7, when using ./configure --with-pydebug, Python is built using -O0 compiler optimization level: disable all optimizations. The comment is quite explicit: "Optimization messes up debuggers, so turn it off for debug builds".

if test "$Py_DEBUG" = 'true' ; then
    # Optimization messes up debuggers, so turn it off for
    # debug builds.
    OPT="-g -O0 -Wall $STRICT_PROTO"
else
    OPT="-g $WRAP -O3 -Wall $STRICT_PROTO"
fi

In Python 3, -Og is preferred over -O0 for pydebug, if -Og is available:

if test "$Py_DEBUG" = 'true' ; then
    # Optimization messes up debuggers, so turn it off for
    # debug builds.
    if "$CC" -v --help 2>/dev/null |grep -- -Og > /dev/null; then
        OPT="-g -Og -Wall"
    else
        OPT="-g -O0 -Wall"
    fi
else
    OPT="-g $WRAP -O3 -Wall"
fi

Problem: in my experience, gdb traceback doesn't make sense sometimes, and gdb fails to inspect some function arguments and some variables which are displayed as <optimized out>.

See a very concrete example with a test_gdb failure on x86-64 when Python is compiled using gcc -Og:
https://bugzilla.redhat.com/show_bug.cgi?id=1734327#c22

My colleague who is working on gdb suggests to use -O0:
https://bugzilla.redhat.com/show_bug.cgi?id=1734327#c27

Since I started contributing to Python, I always forced gcc -O0 because any other optimization level caused me many issues in gdb. I'm using -O0 for 10 years with sucess.

The GCC documentation says "It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0." But my experience says the opposite.

Note: instead of -g, we could use -g3 to include debug information for macros and defines.

--

I'm proposing to change the *default* compiler flags from -Og to -O0. Obviously, Linux distributions and developers are free to override the compiler optimization level. For example: ./configure --with-pydebug CFLAGS="-Og" ensures that Python is always built using -Og.

I propose to modify 3.7, 3.8 and master branches.

----------
components: Build
messages: 353746
nosy: vstinner
priority: normal
severity: normal
status: open
title: ./configure --with-pydebug should use -O0 rather than -Og
versions: Python 3.7, Python 3.8, Python 3.9

_______________________________________
Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue38350>
_______________________________________