Why does this code break when -O2 or higher is enabled?

It was a bug in the compiler.

I posted the question in the manufacturer's forum. Other people have indeed reproduced the issue, which happens when compiling for certain parts. Other parts are unaffected.

As a workaround, I changed the macros into real functions, and split the operation in two lines:

uint32_t ROL(uint32_t x, uint8_t r) {
    uint32_t intermedio;
    intermedio = x << r;
    intermedio |= x >> (32 - r);
    return intermedio;
}

This gives the correct result.

Posting compilable test code as a reference.

#include <stdint.h>
#include <string.h>
//#include "speck.h"

#define ROR(x, r) ((x >> r) | (x << (32 - r)))
#define ROL(x, r) ((x << r) | (x >> (32 - r)))
#define R(x, y, k) (x = ROR(x, 8), x += y, x ^= k, y = ROL(y, 3), y ^= x)
#define ROUNDS 27

void encrypt_block(uint32_t ct[2], uint32_t const pt[2], uint32_t const K[4]) {
  uint32_t x = pt[0], y = pt[1];
  uint32_t a = K[0], b = K[1], c = K[2], d = K[3];

  R(y, x, a);
  // for (int i = 0; i < ROUNDS - 3; i += 3) {
  for (uint32_t i = 0; i < ROUNDS - 3; i += 3) {
    R(b, a, i);
    R(y, x, a);
    R(c, a, i + 1);
    R(y, x, a);
    R(d, a, i + 2);
    R(y, x, a);
  }
  R(b, a, ROUNDS - 3);
  R(y, x, a);
  R(c, a, ROUNDS - 2);
  R(y, x, a);

  ct[0] = x;
  ct[1] = y;
}

int main(void) {
  uint32_t out[2];
  uint32_t in[] = {0x7475432d, 0x3b726574};
  uint32_t key[] = {0x03020100, 0x0b0a0908, 0x13121110, 0x1b1a1918};
  encrypt_block(out, in, key);

  printf("%8lx %8lx\n", (unsigned long) out[0], 0x454e028bLU);
  printf("%8lx %8lx\n", (unsigned long) out[1], 0x8c6fa548LU);
}

Output

454e028b 454e028b
8c6fa548 8c6fa548

Unexpected output

0x8FA3FED7
0x53D8CEA8