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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Ferruh Yigit [mailto:ferruh.yigit@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@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?