Re: [dpdk-dev] [PATCH v2] app/testpmd: fix IP checksum calculation

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 12/5/2020 5:42 AM, George Prekas wrote:
> Strict-aliasing rules are violated by cast to uint16_t* in flowgen.c
> and the calculated IP checksum is wrong on GCC 9 and GCC 10.
> 
> Signed-off-by: George Prekas <prekageo@amazon.com>
> ---
> v2:
> * Instead of a compiler barrier, use a compiler flag.
> ---
>   app/test-pmd/meson.build | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/app/test-pmd/meson.build b/app/test-pmd/meson.build
> index 7e9c7bdd6..5d24e807f 100644
> --- a/app/test-pmd/meson.build
> +++ b/app/test-pmd/meson.build
> @@ -4,6 +4,7 @@
>   # override default name to drop the hyphen
>   name = 'testpmd'
>   cflags += '-Wno-deprecated-declarations'
> +cflags += '-fno-strict-aliasing'
>   sources = files('5tswap.c',
>   	'cmdline.c',
>   	'cmdline_flow.c',
> 

Hi George,

I am trying to understand this, the relevant code is as below:
ip_hdr->hdr_checksum = ip_sum((unaligned_uint16_t *)ip_hdr, sizeof(*ip_hdr));

You are suspicious of strict aliasing rule violation, with more details:
The concern is the "struct rte_ipv4_hdr *ip_hdr;" aliased to "const 
unaligned_uint16_t *hdr", and compiler can optimize out the calculations using 
data pointed by 'hdr' pointer, since the 'hdr' pointer is not used to alter the 
data and compiler may think data is not changed at all.

1) But the pointer "hdr" is assigned in the loop, from another pointer whose 
content is changing, why this is not helping to figure out that the data 'hdr' 
pointing is changed.

2) I tried to debug this, but I am not able to reproduce the issue, 'ip_sum()' 
called each time and checksum calculated correctly. Using gcc 10.2.1-9. Can you 
able to confirm the case with debug, or from the assembly/object file?


And if the issue is strict aliasing rule violation as you said, compiler flag is 
an option but not sure how much it reduces the compiler optimization benefit, I 
guess other options also not so good, memcpy brings too much work on runtime and 
union requires bigger change and makes code complex.
I wonder if making 'ip_sum()' a non inline function can help, can you please 
give a try since you can reproduce it?