Debugging Bitcoin Core with gdb - how to turn off compiler optimizations

Question

I ran the configure script with the --enable-debug flag, then ran make. I am running unit tests:

gdb --args src/test/test_bitcoin --log_level=all --run_test=script_standard_tests

I'm trying to step through the execution of the source files using gdb, but I am still getting <optimized out> for many variables, and it skips lines here and there. I thought that the --enable-debug flag would turn off optimization, but am I mistaken? I'm wondering if the optimization is always on for the tests or if there is a way to turn them off.

MCCCS · Accepted Answer · 2018-11-21T19:03:36.870

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Bitcoin Core disables -O2 and enables -Og when --enable-debug is set. -Og is intended for debugging; however, it removes some of the debugging information and is buggy. To disable it and to improve debugging experience, I suggest changing between L256 and L278 to:

CXXFLAGS="-ggdb3 -ftrapv"

-ggdb3 is more powerful than -g3, it uses a GDB-only, LLDB-incompatible dialect for debugging. This will also get rid of -O2, except in libsecp256k1.

Edit: Sipa suggests a more elegant way without altering autoconf files, which is adding args for configuring.

./configure CXXFLAGS="-O0 -ggdb3"

O0 is needed in this case.

edited Nov 21 '18 at 19:03

answered Nov 21 '18 at 16:07

MCCCS

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Nice workaround, thanks. I'm still getting ``, though, I must be missing something. – JBaczuk Nov 21 '18 at 16:24
Now it is showing all variables. I must've not recompiled the file correctly. – JBaczuk Nov 21 '18 at 16:39
Glad that I could help! :) – MCCCS Nov 21 '18 at 16:40
If you want to avoid all ""s, you need to compile with "-O0" to disable all optimizations. – Pieter Wuille Nov 21 '18 at 17:49
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Also there is no need to modify configure.ac, you can override things on the command line by using `./configure CXXFLAGS="-O0 -ggdb3" ...`. – Pieter Wuille Nov 21 '18 at 17:51
@PieterWuille the manual (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html) says O0 is default in GCC, but agree about the second, will edit soon. – MCCCS Nov 21 '18 at 17:51
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@MCCCS But `--enable-debug` will pass `-Og` which overrides the `-O0` default. – Pieter Wuille Nov 21 '18 at 18:00
Why doesn't `--enable-debug` use `-O0`? – JBaczuk Nov 21 '18 at 19:33

score 1 · Answer 2 · answered Sep 24 '19 at 13:18

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As of PR #16435 --enable-debug is actually setting "-O0", so there should be no optimization at all anymore.

answered Sep 24 '19 at 13:18

fjahr

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Debugging Bitcoin Core with gdb - how to turn off compiler optimizations

2 Answers2