New submission from STINNER Victor <[email protected]>:
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
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Python tracker <[email protected]>
<https://bugs.python.org/issue38350>
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