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

[Ferruh Yigit]

On 6/10/2022 11:39 AM, Morten Brørup wrote:
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> 
>> From: Ferruh Yigit [mailto:ferruh.yigit at xilinx.com]
>> Sent: Friday, 10 June 2022 12.13
>>
>> 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 at 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.
> 
> Any such testing will be highly compiler dependent.
> 
> Do you have any specific compilers in mind, where you see a risk for lower performance?
> 

Hi Morten,

My point is possible performance impact, not about any possible risk or 
specific compiler version.
The possible performance impact part can be separated to its own patch 
and these can be discussed there, independent from gcc12 build error.