Re: [dpdk-dev] [PATCH] test-pmd: Fix pointer aliasing error

[Dayu Qiu <qdy220091330@gmail.com>]

Just re-post this mail as Thomas said it missed in mail list.

On Wed, Dec 3, 2014 at 7:42 PM, Bruce Richardson <bruce.richardson@intel.com
> wrote:

> On Wed, Dec 03, 2014 at 07:28:19PM +0800, Michael Qiu wrote:
> > app/test-pmd/csumonly.c: In function ‘get_psd_sum’:
> > build/include/rte_ip.h:161: error: dereferencing pointer ‘u16’
> >       does break strict-aliasing rules
> > build/include/rte_ip.h:157: note: initialized from here
> >       ...
> >
> > The root cause is that, compile enable strict aliasing by default,
> > while in function rte_raw_cksum() try to convert 'const char *'
> > to 'const uint16_t *'.
> >
>
> What compiler version is this with? Is there any other way to fix this
> other than disabling the compiler warnings. Turning off strict aliasing may
> affect performance as it reduces the number of optimizations that the
> compiler
> can perform.
>
>

The compile version is:

$ gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
...
gcc version 4.4.7 20120313 (Red Hat 4.4.7-4) (GCC)


The OS is centos6.5 x86_64


Actually, another possible solution is, as gcc manual shows, use union.
In function rte_raw_cksum() of lib/librte_net/rte_ip.h:

static inline uint16_t
rte_raw_cksum(const char *buf, size_t len){
    union {
        const char *ubuf;
        const uint16_t *uu16;
    } convert;

    convert.ubuf = buf;
    const uint16_t *u16 = convert.uu16;
    ...
}

This may be work, but not test yet.


Thanks,
Michael

> /Bruce
>
> > Need to add CFLAG '-Wno-strict-aliasing' to avoid this issue.
> >
> > Signed-off-by: Michael Qiu <michael.qiu@intel.com>
> > ---
> >  app/test-pmd/Makefile | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/app/test-pmd/Makefile b/app/test-pmd/Makefile
> > index 97dc2e6..995c874 100644
> > --- a/app/test-pmd/Makefile
> > +++ b/app/test-pmd/Makefile
> > @@ -38,7 +38,7 @@ ifeq ($(CONFIG_RTE_TEST_PMD),y)
> >  #
> >  APP = testpmd
> >
> > -CFLAGS += -O3
> > +CFLAGS += -O3 -Wno-strict-aliasing
> >  CFLAGS += $(WERROR_FLAGS)
> >
> >  ifeq ($(CONFIG_RTE_LIBRTE_PMD_PCAP),y)
> > --
> > 1.9.3
> >
>



-- 
Thanks & Best Regards
Mike