strict aliasing and memory alignment

Question

I have performance critical code and there is a huge function that allocates like 40 arrays of different size on the stack at the beginning of the function. Most of these arrays

Answer 1

32 byte alignment sounds as if you are pushing the button too far. No CPU instruction should require an alignement as large as that. Basically an alignement as wide as the largest data type of your architecture should suffice.

C11 has the concept fo maxalign_t, which is a dummy type of maximum alignment for the architecture. If your compiler doesn't have it, yet, you can easily simulate it by something like

union maxalign0 {
  long double a;
  long long b;
  ... perhaps a 128 integer type here ...
};

typedef union maxalign1 maxalign1;
union maxalign1 {
  unsigned char bytes[sizeof(union maxalign0)];
  union maxalign0;
}

Now you have a data type that has the maximal alignment of your platform and that is default initialized with all bytes set to 0.

maxalign1 history_[someSize];
short * history = history_.bytes;

This avoids the awful address computations that you do currently, you'd only have to do some adoption of someSize to take into account that you always allocate multiples of sizeof(maxalign1).

Also be asured that this has no aliasing problems. First of all unions in C made for this, and then character pointers (of any version) are always allowed to alias any other pointer.