decodeLogicalImmediate, using division instead of a table

decode-logical-immediate-division.c

#include <stdint.h>
#include <stdbool.h>

#define DECODE_FAILURE 0 // not a valid logical immediate

static inline int nonzeroCountLeadingZeros32(uint32_t n) {
    return __builtin_clz(n);
}

static inline uint64_t rotateRight64(uint64_t v, int n) {
    return (v >> (n & 63)) | (v << (-n & 63));
}

// Division expression by @zwegner:
// https://twitter.com/zwegner/status/1454543857023062016
uint64_t decodeLogicalImmediate64(int N, int immr, int imms) {
    unsigned pattern = (N << 6) | (~imms & 0x3F);
    if (!(pattern & (pattern - 1))) return DECODE_FAILURE;
    unsigned size = 0x80000000 >> nonzeroCountLeadingZeros32(pattern);
    uint64_t mask = ~0ull / ((1ull << (size & 63)) | 1);
    unsigned ones = (imms + 1) & (size - 1);
    return rotateRight64(mask ^ (mask << ones), immr);
}

uint32_t decodeLogicalImmediate32(int N, int immr, int imms) {
    if (N) return DECODE_FAILURE;
    return (uint32_t)decodeLogicalImmediate64(N, immr, imms);
}