#include <stdio.h>

    typedef float xform_3d[4][3];

    int main() {
        xform_3d x = {
            {1.f, 0.f, 0.f},
            {0.f, 1.f, 0.f},
            {0.f, 0.f, 1.f},
            {0.f, 0.f, 0.f} };

        for(int i=0; i<12; i++) printf("%f\n", x[0][i]);
    }

with g++ 4.9 (debian jessie -O3 -Wall):

    xf.c: In function ‘int main()’:
    xf.c:12:50: warning: iteration 3u invokes undefined
              behavior [-Waggressive-loop-optimizations]
         for(int i=0; i<12; i++) printf("%f\n", x[0][i]);
                                                      ^
    xf.c:12:5: note: containing loop
         for(int i=0; i<12; i++) printf("%f\n", x[0][i]);
         ^

(yes, this is slightly different than the code I posted first---"laundering" &x[0][0] through an intermediate pointer variable f makes the warning go away in the compiler I tested, but my suspicion is that if one is undefined behavior the other probably is too)

Edited to add: and when built and run, it printed all x[0][i] but x[0][0] as 0.0, so gcc did emit code that doesn't do what the programmer wants.