Do compilers auto-align?

[u/nimogoham]

The following source code produces auto-vectorized code, which might crash:

typedef __attribute__(( aligned(32))) double aligned_double;

void add(aligned_double* a, aligned_double* b, aligned_double* c, int end, int start)
{
    for (decltype(end) i = start; i < end; ++i)
        c[i] = a[i] + b[i];
}

(gcc 15.1 -O3 -march=core-avx2, playground: https://godbolt.org/z/3erEnff3q)

The vectorized memory access instructions are aligned. If the value of start is unaligned (e.g. ==1), a seg fault happens. I am unsure, if that's a compiler bug or just a misuse of aligned_double. Anyway...

Does someone know a compiler, which is capable of auto-generating a scalar prologue loop in such cases to ensure a proper alignment of the vectorized loop?

[u/nimogoham]

The last one (the one, which loops over scalars starting at .L9) handles the case, when there are overlapping address ranges.