Re: [RFC] eal/x86: disable array bounds checks in rte_memcpy_generic with gcc-12

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

On 6/8/2022 11:49 PM, Stephen Hemminger wrote:
> Gcc 12 adds more array bounds checking (good); but it is not smart
> enough to realize that for small fixed sizes, the bigger move options
> are not used.
> 
> An example is using rte_memcpy() on a RSS key of 40 bytes may trigger
> rte_memcpy complaints from rte_mov128 reading past end of input.
> 
> In order to keep some of the checks add special case for calls
> to rte_memcpy() with fixed size arguments to use the compiler
> builtin instead. Don't want to give all the checking for
> code that uses rte_memcpy() everywhere.
> 
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> ---
>   lib/eal/x86/include/rte_memcpy.h | 16 +++++++++++-----
>   1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/lib/eal/x86/include/rte_memcpy.h b/lib/eal/x86/include/rte_memcpy.h
> index 18aa4e43a743..b90cdd8d7326 100644
> --- a/lib/eal/x86/include/rte_memcpy.h
> +++ b/lib/eal/x86/include/rte_memcpy.h
> @@ -27,6 +27,10 @@ extern "C" {
>   #pragma GCC diagnostic ignored "-Wstringop-overflow"
>   #endif
>   
> +#if defined(RTE_TOOLCHAIN_GCC) && (GCC_VERSION >= 120000)
> +#pragma GCC diagnostic ignored "-Warray-bounds"
> +#endif
> +
>   /**
>    * Copy bytes from one location to another. The locations must not overlap.
>    *
> @@ -842,19 +846,21 @@ rte_memcpy_aligned(void *dst, const void *src, size_t n)
>   	return ret;
>   }
>   
> +#if defined(RTE_TOOLCHAIN_GCC) && (GCC_VERSION >= 100000)
> +#pragma GCC diagnostic pop
> +#endif
> +
>   static __rte_always_inline void *
>   rte_memcpy(void *dst, const void *src, size_t n)
>   {
> -	if (!(((uintptr_t)dst | (uintptr_t)src) & ALIGNMENT_MASK))
> +	if (__builtin_constant_p(n))
> +		return __builtin_memcpy(dst, src, n);
> +	else if (!(((uintptr_t)dst | (uintptr_t)src) & ALIGNMENT_MASK))

This patch does two things,

1. Disable "-Warray-bounds" with above pragma to silence compiler warnings.

2. Use compiler builtin for some cases.

Second can impact the performance and not really needed for the build 
error, what do you think to split the patch in two, since 1. is simple 
change but 2. may require more testing before accepting.