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Date: Wed, 3 Dec 2014 14:51:48 +0000
From: Bruce Richardson <bruce.richardson@intel.com>
To: "Qiu, Michael" <michael.qiu@intel.com>
Message-ID: <20141203145148.GB2396@bricha3-MOBL3>
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Cc: "dev@dpdk.org" <dev@dpdk.org>, Michael Qiu <qdy220091330@gmail.com>
Subject: Re: [dpdk-dev] [PATCH] test-pmd: Fix pointer aliasing error
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On Wed, Dec 03, 2014 at 01:59:58PM +0000, Qiu, Michael wrote:
> On 2014/12/3 19:43, Richardson, Bruce 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.
> 
> Any comments about this solution?

what happens if you make rte_raw_cksum take a void * (or const void *) parameter
instead of "const char *"?

/Bruce