Question

What is the best (cleanest, most efficient) way to write saturating addition in C?

The function or macro should add two unsigned inputs (need both 16- and 32-bit versions) and return all-bits-one (0xFFFF or 0xFFFFFFFF) if the sum overflows.

Target is x86 and ARM using gcc (4.1.2) and Visual Studio (for simulation only, so a fallback implementation is OK there).

Answer 1

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  @