Hi,

I'm sorry for a long delay. Here an updated patch.
Following Jakub's suggestion from previous review, I've added a 
get_nonzero_bits stuff into handle_builtin_alloca in order to avoid 
redundant redzone size calculations in case we know that alloca has 
alignment >= ASAN_RED_ZONE_SIZE. Thus, for the following code:

struct __attribute__((aligned (N))) S { char s[N]; };

void bar (struct S *, struct S *);

void
foo (int x)
{
   struct S a;
   {
     struct S b[x];
     bar (&a, &b[0]);
   }
   {
     struct S b[x + 4];
     bar (&a, &b[0]);
   }
}

void
baz (int x)
{
   struct S a;
   struct S b[x];
   bar (&a, &b[0]);
}

compiled with -O2 -fsanitize=address -DN=64, we have expected

   _2 = (sizetype) x_1(D);
   _8 = _2 * 64;
   _14 = _8 + 96;
   _15 = __builtin_alloca_with_align (_14, 512);
   _16 = _15 + 64;
   __builtin___asan_alloca_poison (_16, _8);

instead of previous

   _1 = (sizetype) x_4(D);
   _2 = _1 * 64;
   _24 = _2 & 31;
   _19 = _2 + 128;
   _27 = _19 - _24;
   _28 = __builtin_alloca_with_align (_27, 512);
   _29 = _28 + 64;
   __builtin___asan_alloca_poison (_29, _2);


Also, I've added a simple pattern for X & C -> 0 if we know that corresponding bits are zero, but I'm not sure this pattern has a practical value.
Tested and bootstrapped on x86_64-unknown-linux-gnu. Could you take a look?

-Maxim