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All the FreeBSD build slaves seem to be experiencing the same symptom: ./configure --with-pydebug eventually results in this:
 gcc -pthread -c -fno-strict-aliasing -g -O0 -Wall -Wstrict-prototypes -O2 -pipe -fno-strict-aliasing -Wall -march=native -I. -I./Include -DPy_BUILD_CORE -o Modules/python.o ./Modules/python.c
What I find odd:
 % grep -e '-O2' Makefile | wc -l
 0
There are references to -O2 elsewhere, like in configure.ac|configure, but, the Makefile doesn't source any of those. (Right?)
This is occurring on all of my slaves as well as others, so I'm pretty sure it's not just specific to my environment.

Ah!
% gmake
gcc -pthread -c -fno-strict-aliasing -g -O0 -Wall -Wstrict-prototypes -I. -I./Include -DPy_BUILD_CORE -o Modules/python.o ./Modules/python.c
gcc -pthread -c -fno-strict-aliasing -g -O0 -Wall -Wstrict-prototypes -I. -I./Include -DPy_BUILD_CORE -o Parser/acceler.o Parser/acceler.c
^C
% make
% make
gcc -pthread -c -fno-strict-aliasing -g -O0 -Wall -Wstrict-prototypes -O2 -pipe -I. -I./Include -DPy_BUILD_CORE -o Modules/python.o ./Modules/python.c
gcc -pthread -c -fno-strict-aliasing -g -O0 -Wall -Wstrict-prototypes -O2 -pipe -I. -I./Include -DPy_BUILD_CORE -o Parser/acceler.o Parser/acceler.c
gcc -pthread -c -fno-strict-aliasing -g -O0 -Wall -Wstrict-prototypes -O2 -pipe -I. -I./Include -DPy_BUILD_CORE -o Parser/grammar1.o Parser/grammar1.c
^C
So, the default BSD make likes to add in -O2 -pipe. I'll do a bit more digging.

So, looks like FreeBSD's /usr/share/mk/sys.mk is to blame here. It unconditionally sets CFLAGS to `-O2 -pipe`.
[trent@hydrogen/ttypts/1(~s/cpython)%] uname -a
FreeBSD hydrogen.snakebite.net 9.1-PRERELEASE FreeBSD 9.1-PRERELEASE #0 r0: Mon Jul 16 06:28:19 UTC 2012 root@hydrogen.snakebite.net:/usr/obj/src/freebsd/9/r238513m/sys/AMD64 amd64
[trent@hydrogen/ttypts/1(~s/cpython)%] grep -A3 'bsd_make_test' Makefile
bsd_make_test:
	@echo "CFLAGS: $(CFLAGS)"
	@echo "EXTRA_CFLAGS: $(EXTRA_CFLAGS)"
[trent@hydrogen/ttypts/1(~s/cpython)%] make bsd_make_test 
CFLAGS: -O2 -pipe 
EXTRA_CFLAGS: 
[trent@hydrogen/ttypts/1(~s/cpython)%] make CFLAGS= bsd_make_test
CFLAGS: 
EXTRA_CFLAGS: 
I can think of a few ways to work around this... some better than others.
1. Change the FreeBSD buildbots to always invoke make via `make CFLAGS=`.
2. Hack configure.* to automatically invoke make via `make CFLAGS=` when `-O2 -pipe` crops up in a `--with-pydebug` build.
Some of the less desirable ones:
3. Switch FreeBSD to gmake.
4. Patch FreeBSD make so that it is 'debug aware' and stops appending `-O2` everywhere.

> So, looks like FreeBSD's /usr/share/mk/sys.mk is to blame here.
> It unconditionally sets CFLAGS to `-O2 -pipe`.
I've been debugging this once, too. My conclusion was that if the OS is set up
that way, we shouldn't do anything about it in the Python source tree.
All important features of --with-pydebug should also work if -O0 is overridden
by -O2.
So I think that FreeBSD users who really want -O0 should change sys.mk
or set CFLAGS manually. For the buildbots it should not matter, except
that compile times are slower.

Trent, do you want to keep this open? I think sys.mk is behaving exactly
as intended.