selftests/bpf: Support building selftests in optimized -O2 mode

Add support for building selftests with -O2 level of optimization, which
allows more compiler warnings detection (like lots of potentially
uninitialized usage), but also is useful to have a faster-running test
for some CPU-intensive tests.

One can build optimized versions of libbpf and selftests by running:

  $ make RELEASE=1

There is a measurable speed up of about 10 seconds for me locally,
though it's mostly capped by non-parallelized serial tests. User CPU
time goes down by total 40 seconds, from 1m10s to 0m28s.

Unoptimized build (-O0)
=======================
Summary: 430/3544 PASSED, 25 SKIPPED, 4 FAILED

real    1m59.937s
user    1m10.877s
sys     3m14.880s

Optimized build (-O2)
=====================
Summary: 425/3543 PASSED, 25 SKIPPED, 9 FAILED

real    1m50.540s
user    0m28.406s
sys     3m13.198s
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20231006175744.3136675-2-andrii@kernel.org

tools/testing/selftests/bpf/Makefile

@@ -27,7 +27,9 @@ endif
 BPF_GCC		?= $(shell command -v bpf-gcc;)
 SAN_CFLAGS	?=
 SAN_LDFLAGS	?= $(SAN_CFLAGS)
-CFLAGS += -g -O0 -rdynamic						\
+RELEASE		?=
+OPT_FLAGS	?= $(if $(RELEASE),-O2,-O0)
+CFLAGS += -g $(OPT_FLAGS) -rdynamic					\
 	  -Wall -Werror 						\
 	  $(GENFLAGS) $(SAN_CFLAGS)					\
 	  -I$(CURDIR) -I$(INCLUDE_DIR) -I$(GENDIR) -I$(LIBDIR)		\
@@ -243,7 +245,7 @@ $(OUTPUT)/runqslower: $(BPFOBJ) | $(DEFAULT_BPFTOOL) $(RUNQSLOWER_OUTPUT)
 		    BPFTOOL_OUTPUT=$(HOST_BUILD_DIR)/bpftool/		       \
 		    BPFOBJ_OUTPUT=$(BUILD_DIR)/libbpf			       \
 		    BPFOBJ=$(BPFOBJ) BPF_INCLUDE=$(INCLUDE_DIR)		       \
-		    EXTRA_CFLAGS='-g -O0 $(SAN_CFLAGS)'			       \
+		    EXTRA_CFLAGS='-g $(OPT_FLAGS) $(SAN_CFLAGS)'	       \
 		    EXTRA_LDFLAGS='$(SAN_LDFLAGS)' &&			       \
 		    cp $(RUNQSLOWER_OUTPUT)runqslower $@
 
@@ -281,7 +283,7 @@ $(DEFAULT_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)    \
 		    $(HOST_BPFOBJ) | $(HOST_BUILD_DIR)/bpftool
 	$(Q)$(MAKE) $(submake_extras)  -C $(BPFTOOLDIR)			       \
 		    ARCH= CROSS_COMPILE= CC="$(HOSTCC)" LD="$(HOSTLD)" 	       \
-		    EXTRA_CFLAGS='-g -O0'				       \
+		    EXTRA_CFLAGS='-g $(OPT_FLAGS)'			       \
 		    OUTPUT=$(HOST_BUILD_DIR)/bpftool/			       \
 		    LIBBPF_OUTPUT=$(HOST_BUILD_DIR)/libbpf/		       \
 		    LIBBPF_DESTDIR=$(HOST_SCRATCH_DIR)/			       \
@@ -292,7 +294,7 @@ $(CROSS_BPFTOOL): $(wildcard $(BPFTOOLDIR)/*.[ch] $(BPFTOOLDIR)/Makefile)	\
 		    $(BPFOBJ) | $(BUILD_DIR)/bpftool
 	$(Q)$(MAKE) $(submake_extras)  -C $(BPFTOOLDIR)				\
 		    ARCH=$(ARCH) CROSS_COMPILE=$(CROSS_COMPILE)			\
-		    EXTRA_CFLAGS='-g -O0'					\
+		    EXTRA_CFLAGS='-g $(OPT_FLAGS)'				\
 		    OUTPUT=$(BUILD_DIR)/bpftool/				\
 		    LIBBPF_OUTPUT=$(BUILD_DIR)/libbpf/				\
 		    LIBBPF_DESTDIR=$(SCRATCH_DIR)/				\
@@ -315,7 +317,7 @@ $(BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
 	   $(APIDIR)/linux/bpf.h					       \
 	   | $(BUILD_DIR)/libbpf
 	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR) OUTPUT=$(BUILD_DIR)/libbpf/ \
-		    EXTRA_CFLAGS='-g -O0 $(SAN_CFLAGS)'			       \
+		    EXTRA_CFLAGS='-g $(OPT_FLAGS) $(SAN_CFLAGS)'	       \
 		    EXTRA_LDFLAGS='$(SAN_LDFLAGS)'			       \
 		    DESTDIR=$(SCRATCH_DIR) prefix= all install_headers
 
@@ -324,7 +326,7 @@ $(HOST_BPFOBJ): $(wildcard $(BPFDIR)/*.[ch] $(BPFDIR)/Makefile)		       \
 		$(APIDIR)/linux/bpf.h					       \
 		| $(HOST_BUILD_DIR)/libbpf
 	$(Q)$(MAKE) $(submake_extras) -C $(BPFDIR)                             \
-		    EXTRA_CFLAGS='-g -O0' ARCH= CROSS_COMPILE=		       \
+		    EXTRA_CFLAGS='-g $(OPT_FLAGS)' ARCH= CROSS_COMPILE=	       \
 		    OUTPUT=$(HOST_BUILD_DIR)/libbpf/			       \
 		    CC="$(HOSTCC)" LD="$(HOSTLD)"			       \
 		    DESTDIR=$(HOST_SCRATCH_DIR)/ prefix= all install_headers