构建CPython时禁用编译器优化

Question

I've been following along with Philip Guo's excellent series about CPython's internals and have been walking through the interpreter's code with GDB. Unfortunately, GDB seems to skip around quite a bit when I step through the code, executing instructions out of order (a series of assignment operations for example, nothing fancy).

From what I understand, this is because of compiler optimizations. Indeed, CPython's configure file seems to default to -O2 . This is shown to be the case when compiling Python 2.7.15 with ./configure and make :

gcc -pthread -c -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Modules/python.o ./Modules/python.c
gcc -pthread -c -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Parser/acceler.o Parser/acceler.c
gcc -pthread -c -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Parser/grammar1.o Parser/grammar1.c
gcc -pthread -c -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Parser/listnode.o Parser/listnode.c
...

I tried fixing this by configuring without optimizations via ./configure CFLAGS="-g -O0" and compiling with make again, which works (note the change from -fno-strict-aliasing -g -O2 to -fno-strict-aliasing -g -O0 on each line):

gcc -pthread -c -fno-strict-aliasing -g -O0 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Modules/python.o ./Modules/python.c
gcc -pthread -c -fno-strict-aliasing -g -O0 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Parser/acceler.o Parser/acceler.c
gcc -pthread -c -fno-strict-aliasing -g -O0 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Parser/grammar1.o Parser/grammar1.c
gcc -pthread -c -fno-strict-aliasing -g -O0 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes  -I. -IInclude -I./Include   -DPy_BUILD_CORE -o Parser/listnode.o Parser/listnode.c
...

Unfortunately, GDB still jumps around when I step through the code. For completeness, here's the command I'm running with GDB:

gdb --args ./python test.py

The contents of test.py are just some basic arithmetic and print functions, which isn't really relevant.

My understanding of at least one of these steps must be incomplete. Am I setting the optimization flag incorrectly? Is GDB jumping between instructions due to some other reason? Any advice would be greatly appreciated. Thanks!

Answer 1

tl;dr: You want the configure setting --with-pydebug .

You've got an -O3 on the command line that comes after the -O2 , so changing the -O2 to -O0 doesn't do any good; the -O3 still overrides it. You need to figure out where the -O3 is coming from and remove it.

If you look at the generated Makefile , you should see something like this:

# Compiler options
OPT=        -DNDEBUG -g -fwrapv -O3 -Wall
# ...
# Avoid assigning CFLAGS, LDFLAGS, etc. so users can use them on the
# command line to append to these values without stomping the pre-set
# values.
PY_CFLAGS=  $(BASECFLAGS) $(OPT) $(CONFIGURE_CFLAGS) $(CFLAGS) $(EXTRA_CFLAGS)

You can, of course, just edit the generated Makefile to change the -O3 to -O0 . But if you look inside the configure script and Makefile.pre , you can see where this comes from: it's the default value, only replaced in this case:

case $ac_cv_prog_cc_g in
yes)
    if test "$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
    ;;

This also sets the right flags for other compilers besides gcc .

However, the main effect of --with-pydebug is to enable Py_DEBUG —which you probably want if you're source-debugging CPython, but it is a separate thing from -O0 , which is what you were actually asking about. So, if you want only -O0 , as far as I know, the only thing you can do is edit the Makefile (or come up with some complicated set of env variables and configure arguments that tricks it into doing a debug build but then not enabling Py_DEBUG ).