Re: [dpdk-dev] [PATCH] mk: add support for gdb debug info generation

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
> Sent: Tuesday, March 03, 2015 1:03 PM
> To: Marc Sune
> Cc: dev@dpdk.org
> Subject: Re: [dpdk-dev] [PATCH] mk: add support for gdb debug info generation
> 
> On Tue, Mar 03, 2015 at 01:56:19PM +0100, Marc Sune wrote:
> >
> > On 03/03/15 13:40, Panu Matilainen wrote:
> > >On 03/03/2015 02:19 PM, Marc Sune wrote:
> > >>
> > >>On 03/03/15 10:33, Bruce Richardson wrote:
> > >>>On Mon, Mar 02, 2015 at 06:32:13PM +0100, Marc Sune wrote:
> > >>>>On 22/02/15 12:51, Marc Sune wrote:
> > >>>>>I don't like the proposed patch, but I am recovering this old thread
> > >>>>>because I agree on the problem statement.
> > >>>>>
> > >>>>>On 04/04/14 11:57, Ananyev, Konstantin wrote:
> > >>>>>>Hi Cyril,
> > >>>>>>We already do have 'EXTRA_CFLAGS' and 'EXTRA_LDFLAGS' that you
> > >>>>>>can use
> > >>>>>>to enable debug, or any other compiler/linker options you need.
> > >>>>>>Wonder, why that is not enough?
> > >>>>>EXTRA_FLAGS var affects all the DPDK libraries. I was wondering why
> > >>>>>setting individually:
> > >>>>>
> > >>>>>diff --git a/config/common_linuxapp b/config/common_linuxapp
> > >>>>>index 2f9643b..04adc0d 100644
> > >>>>>--- a/config/common_linuxapp
> > >>>>>+++ b/config/common_linuxapp
> > >>>>>@@ -127,7 +127,7 @@ CONFIG_RTE_LIBRTE_KVARGS=y
> > >>>>># Compile generic ethernet library
> > >>>>>#
> > >>>>>CONFIG_RTE_LIBRTE_ETHER=y
> > >>>>>-CONFIG_RTE_LIBRTE_ETHDEV_DEBUG=n
> > >>>>>+CONFIG_RTE_LIBRTE_ETHDEV_DEBUG=y
> > >>>>>
> > >>>>>
> > >>>>>to put an example, does not set -g and -O0 in that particular module
> > >>>>>only.
> > >>>>>No one would ever use something compiled in DEBUG in production
> > >>>>>anyway.
> > >>>>>
> > >>>>>I always end up modifying manually Makefiles in the lib library that
> > >>>>>I am
> > >>>>>interested in having insides, overriding CFLAGS=-O3, which is not
> > >>>>>that
> > >>>>>nice.
> > >>>>I would like some feedback on this idea. If the community sees
> > >>>>benefit, I
> > >>>>will work on a patch for this.
> > >>>>
> > >>>>Marc
> > >>>>
> > >>>So, your proposal is to patch things so that any time one sets DEBUG=y
> > >>>in the
> > >>>build-time config for a library, we change the '-O3' to '-O0' and set
> > >>>-g also.
> > >>>Correct?
> > >>
> > >>I am not sure what you mean by 'patch things'. I would simply enable the
> > >>build system to override the default compilation flags (now DPDK-wide,
> > >>or specifically librte_ wide) when _DEBUG=y for a library, changing
> > >>compilation flags from -O3 to -O0 -g and possibly also -fno-inline. I
> > >>have to check if -O0 already implicitly means -fno-inline (even for
> > >>__attribute__((always_inline)) ).
> > >>
> > >>I did a quick test. I chose KNI because it didn't have a DEBUG flag for
> > >>the user-space library. For other libraries, the existing _DEBUG setting
> > >>would be enough:
> > >>
> > >>marc@dpdk:~/dpdk$ git diff HEAD
> > >>diff --git a/config/common_linuxapp b/config/common_linuxapp
> > >>index 97f1c9e..8a3cef8 100644
> > >>--- a/config/common_linuxapp
> > >>+++ b/config/common_linuxapp
> > >>@@ -405,6 +405,7 @@ CONFIG_RTE_LIBRTE_PIPELINE=y
> > >>  #
> > >>  CONFIG_RTE_LIBRTE_KNI=y
> > >>  CONFIG_RTE_KNI_PREEMPT_DEFAULT=y
> > >>+CONFIG_RTE_LIBRTE_KNI_DEBUG=y
> > >>  CONFIG_RTE_KNI_KO_DEBUG=n
> > >>  CONFIG_RTE_KNI_VHOST=n
> > >>  CONFIG_RTE_KNI_VHOST_MAX_CACHE_SIZE=1024
> > >>diff --git a/lib/librte_kni/Makefile b/lib/librte_kni/Makefile
> > >>index 7107832..895f64e 100644
> > >>--- a/lib/librte_kni/Makefile
> > >>+++ b/lib/librte_kni/Makefile
> > >>@@ -34,7 +34,7 @@ include $(RTE_SDK)/mk/rte.vars.mk
> > >>  # library name
> > >>  LIB = librte_kni.a
> > >>
> > >>-CFLAGS += $(WERROR_FLAGS) -I$(SRCDIR) -O3 -fno-strict-aliasing
> > >>+CFLAGS += $(WERROR_FLAGS) -I$(SRCDIR) $(CONFIG_RTE_LIBRTE_KNI_CFLAGS)
> > >>
> > >>  EXPORT_MAP := rte_kni_version.map
> > >>
> > >>diff --git a/mk/rte.app.mk b/mk/rte.app.mk
> > >>index 63a41e2..eee477d 100644
> > >>--- a/mk/rte.app.mk
> > >>+++ b/mk/rte.app.mk
> > >>@@ -78,6 +78,13 @@ endif
> > >>  ifeq ($(CONFIG_RTE_LIBRTE_KNI),y)
> > >>  ifeq ($(CONFIG_RTE_EXEC_ENV_LINUXAPP),y)
> > >>  LDLIBS += -lrte_kni
> > >>+
> > >>+ifeq ($(CONFIG_RTE_LIBRTE_KNI_DEBUG),y)
> > >>+CONFIG_RTE_LIBRTE_KNI_CFLAGS = -O0 -g -fno-strict-aliasing -fno-inline
> > >>+else
> > >>+CONFIG_RTE_LIBRTE_KNI_CFLAGS = -O3 -fno-strict-aliasing
> > >>+endif
> > >>+
> > >>  endif
> > >>  endif
> > >>
> > >>
> > >>Thoughts?
> > >
> > >My 5c is that if anything, DPDK needs *less* places that muck around with
> > >compiler flags, not more. If you something like this for all the libraries
> > >in DPDK the number doesn't just increase a bit, it explodes.
> >
> > If you check the part below this one in my original email, that you stripped
> > out (without notice), the suggestion was also to add a global _DEBUG
> > parameter for the entire DPDK set of libraries, to change all the CFLAGS at
> > once (not in the attached PATCH).
> >
> > >
> > >I dont see that much point in this thing, but I'd approach it by defining
> > >the debug flags someplace central, say DEBUG_FLAGS, and append that to the
> > >common cflags when *_DEBUG config is enabled. At least with gcc the last
> > >option wins so if you just append -O0 when debugging then that's what
> > >wins, the earlier -O3 does not matter.
> >
> > The original problem is the one you expose; libraries hardcode the CFLAGS,
> > ignoring user-flags. There is no way to change this unless you change the
> > Makefiles directly.
> >
> > But right now, each library does hardcode its *own* flags (check Makefiles
> > for the libraries), so there is already not a unified approach here. I see
> > for instance KNI having -fno-strict-aliasing while other libraries don't.
> >
> > Having said that, there are moments, specially with -O3, in which to be able
> > to reproduce a bug, you need to compile certain parts of code with -O3 and
> > the rest with -O0 -g (the ones to be debugged). The approach proposed (both
> > a global *and* a lib specific) allows that.
> >
> > Marc
> >
> 
> I believe that the global option of overriding the CFLAGS is already sufficiently
> covered - including being documented in programmers guide - by EXTRA_CFLAGS. The
> ability to turn off optimization support for a single library is not covered
> anywhere, and that suggestion seems reasonable to me. For each library, we can
> just append '-O0 -g' to the CFLAGS in that libraries makefile if the debug option
> is set. I don't see that as significantly complicating things [though I wouldn't
> make any changes to the rte.app.mk to allow this, just have it per lib in the
> lib's makefile]
> 

Well, but then people would say 'why only -g -O0' for _DEBUG config option?
And would ask for ability to add/change other compile options to add/change for particular library.
If we need that, probably we can introduce RTE_LIBRTE_<LIBNAME>_EXTRA_CFLAGS config macros for each library?
So people can set them in their config files to whatever they like? 

Konstantin