test.cc

#include <stdint.h>

/* clang version 3.5.0 (trunk 214024) */

void f(uint32_t* p) {
  while (__builtin_uadd_overflow(*p, 1, p)) {
    p++;
  }
}

/* Loop from `f` compiled with `-target armv7a-none-eabi -O3`

   Notice that: adds is not used and bool is materialized.

  .LBB0_1:                                @ %while.cond
                                          @ =>This Inner Loop Header: Depth=1
    ldr r1, [r0]
    add r2, r1, #1
    str r2, [r0], #4
    cmp r2, r1
    mov r1, #1
    movwhs  r1, #0
    cmp r1, #0
    bne .LBB0_1
*/

/* Loop from `f` compiled with `-target aarch64-none-eabi -O3`

  WAT? `adds` is used but we still materialize and dematerialize bool into `w9`.

  .LBB0_1:                                // %while.cond
                                          // =>This Inner Loop Header: Depth=1
    ldr  w8, [x0]
    adds  w8, w8, #1              // =1
    cset   w9, hs
    str w8, [x0], #4
    tbnz  w9, #0, .LBB0_1
*/


/* NOW LETS GET FUNKY */

void g(uint32_t* p) {
  if (*p) {
    while (__builtin_uadd_overflow(*p, 1, p)) {
      p++;
    }
  } else {
    *p = 1;
  }
}

/* Loop from `g` compiled with `-target armv7a-none-eabi -O3`

   Notice that: adds is not used and bool is materialized.

   Value of the *p is forwarded on the loop entry edge, hence the strange
   post/pre increments choice. 

    add r0, r0, #4
  .LBB1_2:                                @ %while.cond
                                          @ =>This Inner Loop Header: Depth=1
    add r3, r1, #1
    sub r2, r0, #4
    cmp r3, r1
    mov r1, #1
    movwhs  r1, #0
    cmp r1, #0
    str r3, [r2]
    bxeq  lr
    ldr r1, [r0], #4
    b .LBB1_2
*/

/*

  Loop from `g` compiled with `-target aarch64-none-eabi -O3`

  HOORAY b.hs is used.

  // BB#1:                                // %while.cond.preheader
    add x8, x0, #4              // =4
    b .LBB1_3
  .LBB1_2:                                // %while.cond.while.cond_crit_edge
                                          //   in Loop: Header=BB1_3 Depth=1
    ldr w9, [x8], #4
  .LBB1_3:                                // %while.cond
                                          // =>This Inner Loop Header: Depth=1
    adds  w9, w9, #1              // =1
    stur  w9, [x8, #-4]
    b.hs  .LBB1_2  

*/