Directly return 0 in memcmp/bcmp when input pointers point to same memory location

[xiongji90]

Hi, @gchatelet
This patch handles a corner case when memcmp/bcmp is trying to compare same pointers. Could you help review it?
Thanks very much!

[gchatelet]

Hi @xiongji90 !
We explicitly don't handle this use case because it is in the critical path and we care more about latency in general than rarely occurring calls.
If p1 == p2 is expected to occur it should be handled on the client side.

Thx for the patch though.

[xiongji90]

In D152507#4408010, @gchatelet wrote:

Hi @xiongji90 !
We explicitly don't handle this use case because it is in the critical path and we care more about latency in general than rarely occurring calls.
If p1 == p2 is expected to occur it should be handled on the client side.

Thx for the patch though.

Make sense, thanks for the explaination!

libc/src/string/memory_utils/bcmp_implementations.h

if (auto value = x86::avx512bw::Bcmp<64>::block(p1, p2))
       return value;
     align_to_next_boundary<64, Arg::P1>(p1, p2, count);
   }
   return x86::avx512bw::Bcmp<64>::loop_and_tail(p1, p2, count);
 }
 
 [[maybe_unused]] LIBC_INLINE BcmpReturnType inline_bcmp_x86(CPtr p1, CPtr p2,
                                                             size_t count) {
-  if (count == 0)
+  if (count == 0 || p1 == p2)
     return BcmpReturnType::ZERO();
   if (count == 1)
     return generic::Bcmp<1>::block(p1, p2);
   if (count == 2)
     return generic::Bcmp<2>::block(p1, p2);
   if (count <= 4)
     return generic::Bcmp<2>::head_tail(p1, p2, count);
   if (count <= 8)

libc/src/string/memory_utils/x86_64/memcmp_implementations.h

if (auto value = x86::avx512bw::Memcmp<64>::block(p1, p2))
       return value;
     align_to_next_boundary<64, Arg::P1>(p1, p2, count);
   }
   return x86::avx512bw::Memcmp<64>::loop_and_tail(p1, p2, count);
 }
 
 LIBC_INLINE MemcmpReturnType inline_memcmp_x86(CPtr p1, CPtr p2, size_t count) {
 
-  if (count == 0)
+  if (count == 0 || p1 == p2)
     return MemcmpReturnType::ZERO();
   if (count == 1)
     return generic::Memcmp<1>::block(p1, p2);
   if (count == 2)
     return generic::Memcmp<2>::block(p1, p2);
   if (count == 3)
     return generic::Memcmp<3>::block(p1, p2);
   if (count <= 8)