User provided compile flags overridden by configure

[evancox10]

I'm trying to debug spike and riscv-fesvr with GDB. spike by default is compiled with debug symbols (-g), which is good, but riscv-fesvr is not. On both, I also want to turn down optimization (-O0)to make tracing the code simpler. So I'm trying to figure out how to add these flags, but whatever I do is getting over-written down the line by the autoconf scripts.

I'm no expert in autotools, but I believe I should be able to do this to compile with debug symbols:

cd riscv-fesvr/build
../configure CXXFLAGS="-O0 -g"
etc...

You should also be able to pass them in via environment variables, or directly to make when you do the build (make CXXFLAGS="-O0 -g" build).

However, no matter what I do the Makefile ends up with this:

CFLAGS        += -Wall -O2
CXXFLAGS      += -Wall -O2 -std=c++11 -DPREFIX=\"$(prefix)\"

So this is going to override the -O0 level, since gcc takes the last -O it finds. (-g should stick around I guess?)

My guess is that this part of the Makefile comes from this in the configure.ac file:

#-------------------------------------------------------------------------
# Default compiler flags
#-------------------------------------------------------------------------

AC_SUBST([CFLAGS],  ["-Wall -O2"])
AC_SUBST([CXXFLAGS],["-Wall -O2 -std=c++11"])

Or possibly from the Makefile.am template. I'm having a hard time figuring out which file controls the ultimate output here.

Is there another way that autoconf/automake can be used to allow the user to supply these? The automake docs has this to say:

CPPFLAGS is the user variable (see User Variables), AM_CPPFLAGS is the Automake variable
...
Automake always uses two of these variables when compiling C sources files. When compiling an object file for the mumble target, the first variable will be mumble_CPPFLAGS if it is defined, or AM_CPPFLAGS otherwise. The second variable is always CPPFLAGS.
CPPFLAGS is a user variable, i.e., a variable that users are entitled to modify in order to compile the package. This variable, like many others, is documented at the end of the output of ‘configure --help’.
...
The reason ‘$(CPPFLAGS)’ appears after ‘$(AM_CPPFLAGS)’ or ‘$(mumble_CPPFLAGS)’ in the compile command is that users should always have the last say.

(emphasis mine)