How to do unsigned saturating addition in C?

You probably want portable C code here, which your compiler will turn into proper ARM assembly. ARM has conditional moves, and these can be conditional on overflow. The algorithm then becomes: add and conditionally set the destination to unsigned(-1), if overflow was detected.

uint16_t add16(uint16_t a, uint16_t b)
{
  uint16_t c = a + b;
  if (c < a)  /* Can only happen due to overflow */
    c = -1;
  return c;
}

Note that this differs from the other algorithms in that it corrects overflow, instead of relying on another calculation to detect overflow.

x86-64 clang 3.7 -O3 output for adds32: significantly better than any other answer:

add     edi, esi
mov     eax, -1
cmovae  eax, edi
ret

ARMv7: gcc 4.8 -O3 -mcpu=cortex-a15 -fverbose-asm output for adds32:

adds    r0, r0, r1      @ c, a, b
it      cs
movcs   r0, #-1         @ conditional-move
bx      lr

16bit: still doesn’t use ARM’s unsigned-saturating add instruction (UADD16)

add     r1, r1, r0        @ tmp114, a
movw    r3, #65535      @ tmp116,
uxth    r1, r1  @ c, tmp114
cmp     r0, r1    @ a, c
ite     ls        @
movls   r0, r1        @,, c
movhi   r0, r3        @,, tmp116
bx      lr  @