Efficiently dividing unsigned value by a power of two, rounding up - in CUDA

Question

I was just reading:

Efficiently dividing unsigned value by a power of two, rounding up

and I was wondering what was the fastest way to do this in CUDA. Of course

Answer 1

With funnel shifting available, a possible 32 bit strategy is doing a 33bit shift (essentially) preserving the carry of the addition so it be done before the shift, such as this: (not tested)

unsigned sum = dividend + mask;
unsigned result = __funnelshift_r(sum, sum < mask, log_2_of_divisor);

Edit by @einpoklum:

Tested using @RobertCrovella's program, seems to work fine. The test kernel PTX for SM_61 is:

    .reg .pred      %p<2>;
    .reg .b32       %r<12>;


    ld.param.u32    %r5, [_Z4testjj_param_0];
    ld.param.u32    %r6, [_Z4testjj_param_1];
    neg.s32         %r7, %r6;
    and.b32         %r8, %r6, %r7;
    clz.b32         %r9, %r8;
    mov.u32         %r10, 31;
    sub.s32         %r4, %r10, %r9;
    add.s32         %r11, %r6, -1;
    add.s32         %r2, %r11, %r5;
    setp.lt.u32     %p1, %r2, %r11;
    selp.u32        %r3, 1, 0, %p1;
    // inline asm
    shf.r.wrap.b32 %r1, %r2, %r3, %r4;
    // inline asm
    st.global.u32   [r], %r1;
    ret;

and the SASS is:

/*0008*/                   MOV R1, c[0x0][0x20];                 /* 0x4c98078000870001 */
/*0010*/                   MOV R0, c[0x0][0x144];                /* 0x4c98078005170000 */
/*0018*/                   IADD R2, RZ, -c[0x0][0x144];          /* 0x4c1100000517ff02 */
                                                                 /* 0x001c4c00fe4007f1 */
/*0028*/                   IADD32I R0, R0, -0x1;                 /* 0x1c0ffffffff70000 */
/*0030*/                   LOP.AND R2, R2, c[0x0][0x144];        /* 0x4c47000005170202 */
/*0038*/                   FLO.U32 R2, R2;                       /* 0x5c30000000270002 */
                                                                 /* 0x003fd800fe2007e6 */
/*0048*/                   IADD R5, R0, c[0x0][0x140];           /* 0x4c10000005070005 */
/*0050*/                   ISETP.LT.U32.AND P0, PT, R5, R0, PT;  /* 0x5b62038000070507 */
/*0058*/                   IADD32I R0, -R2, 0x1f;                /* 0x1d00000001f70200 */
                                                                 /* 0x001fc400fe2007f6 */
/*0068*/                   IADD32I R0, -R0, 0x1f;                /* 0x1d00000001f70000 */
/*0070*/                   SEL R6, RZ, 0x1, !P0;                 /* 0x38a004000017ff06 */
/*0078*/                   MOV32I R2, 0x0;                       /* 0x010000000007f002 */
                                                                 /* 0x0003c400fe4007e4 */
/*0088*/                   MOV32I R3, 0x0;                       /* 0x010000000007f003 */
/*0090*/                   SHF.R.W R0, R5, R0, R6;               /* 0x5cfc030000070500 */
/*0098*/                   STG.E [R2], R0;                       /* 0xeedc200000070200 */
                                                                 /* 0x001f8000ffe007ff */
/*00a8*/                   EXIT;                                 /* 0xe30000000007000f */
/*00b0*/                   BRA 0xb0;                             /* 0xe2400fffff87000f */
/*00b8*/                   NOP;                                  /* 0x50b0000000070f00 */