performance - How to do saturating addition in C? -

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what best (cleanest, efficient) way write saturating addition in c?

the function or macro should add 2 unsigned inputs (need both 16- , 32-bit versions) , return all-bits-one (0xffff or 0xffffffff) if sum overflows.

target x86 , arm using gcc (4.1.2) , visual studio (for simulation only, fallback implementation ok there).

you want portable c code here, compiler turn proper arm assembly. arm has conditional moves, , these can conditional on overflow. algorithm becomes add, , conditionally set destination unsigned(-1) if overflow detected.

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

note differs other algorithms in corrects overflow, instead of relying on calculation detect overflow.

x86-64 clang 3.7 -o3 output adds32: better other answer:

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

armv7: gcc 4.8 -o3 -mcpu=cortex-a15 -fverbose-asm output adds32:

    adds    r0, r0, r1      @ c, a, b          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,     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  @

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