diff options
Diffstat (limited to 'include/asm-generic/div64.h')
| -rw-r--r-- | include/asm-generic/div64.h | 147 |
1 files changed, 147 insertions, 0 deletions
diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h index 5d974683193a..5a1bf1aff502 100644 --- a/include/asm-generic/div64.h +++ b/include/asm-generic/div64.h @@ -4,6 +4,9 @@ * Copyright (C) 2003 Bernardo Innocenti <bernie@develer.com> * Based on former asm-ppc/div64.h and asm-m68knommu/div64.h * + * Optimization for constant divisors on 32-bit machines: + * Copyright (C) 2006-2015 Nicolas Pitre + * * The semantics of do_div() are: * * uint32_t do_div(uint64_t *n, uint32_t base) @@ -34,6 +37,142 @@ #include <linux/log2.h> +/* + * If the divisor happens to be constant, we determine the appropriate + * inverse at compile time to turn the division into a few inline + * multiplications which ought to be much faster. And yet only if compiling + * with a sufficiently recent gcc version to perform proper 64-bit constant + * propagation. + * + * (It is unfortunate that gcc doesn't perform all this internally.) + */ + +#ifndef __div64_const32_is_OK +#define __div64_const32_is_OK (__GNUC__ >= 4) +#endif + +#define __div64_const32(n, ___b) \ +({ \ + /* \ + * Multiplication by reciprocal of b: n / b = n * (p / b) / p \ + * \ + * We rely on the fact that most of this code gets optimized \ + * away at compile time due to constant propagation and only \ + * a few multiplication instructions should remain. \ + * Hence this monstrous macro (static inline doesn't always \ + * do the trick here). \ + */ \ + uint64_t ___res, ___x, ___t, ___m, ___n = (n); \ + uint32_t ___p, ___bias, ___m_lo, ___m_hi, ___n_lo, ___n_hi; \ + \ + /* determine MSB of b */ \ + ___p = 1 << ilog2(___b); \ + \ + /* compute m = ((p << 64) + b - 1) / b */ \ + ___m = (~0ULL / ___b) * ___p; \ + ___m += (((~0ULL % ___b + 1) * ___p) + ___b - 1) / ___b; \ + \ + /* one less than the dividend with highest result */ \ + ___x = ~0ULL / ___b * ___b - 1; \ + \ + /* test our ___m with res = m * x / (p << 64) */ \ + ___res = ((___m & 0xffffffff) * (___x & 0xffffffff)) >> 32; \ + ___t = ___res += (___m & 0xffffffff) * (___x >> 32); \ + ___res += (___x & 0xffffffff) * (___m >> 32); \ + ___t = (___res < ___t) ? (1ULL << 32) : 0; \ + ___res = (___res >> 32) + ___t; \ + ___res += (___m >> 32) * (___x >> 32); \ + ___res /= ___p; \ + \ + /* Now sanitize and optimize what we've got. */ \ + if (~0ULL % (___b / (___b & -___b)) == 0) { \ + /* special case, can be simplified to ... */ \ + ___n /= (___b & -___b); \ + ___m = ~0ULL / (___b / (___b & -___b)); \ + ___p = 1; \ + ___bias = 1; \ + } else if (___res != ___x / ___b) { \ + /* \ + * We can't get away without a bias to compensate \ + * for bit truncation errors. To avoid it we'd need an \ + * additional bit to represent m which would overflow \ + * a 64-bit variable. \ + * \ + * Instead we do m = p / b and n / b = (n * m + m) / p. \ + */ \ + ___bias = 1; \ + /* Compute m = (p << 64) / b */ \ + ___m = (~0ULL / ___b) * ___p; \ + ___m += ((~0ULL % ___b + 1) * ___p) / ___b; \ + } else { \ + /* \ + * Reduce m / p, and try to clear bit 31 of m when \ + * possible, otherwise that'll need extra overflow \ + * handling later. \ + */ \ + uint32_t ___bits = -(___m & -___m); \ + ___bits |= ___m >> 32; \ + ___bits = (~___bits) << 1; \ + /* \ + * If ___bits == 0 then setting bit 31 is unavoidable. \ + * Simply apply the maximum possible reduction in that \ + * case. Otherwise the MSB of ___bits indicates the \ + * best reduction we should apply. \ + */ \ + if (!___bits) { \ + ___p /= (___m & -___m); \ + ___m /= (___m & -___m); \ + } else { \ + ___p >>= ilog2(___bits); \ + ___m >>= ilog2(___bits); \ + } \ + /* No bias needed. */ \ + ___bias = 0; \ + } \ + \ + /* \ + * Now we have a combination of 2 conditions: \ + * \ + * 1) whether or not we need to apply a bias, and \ + * \ + * 2) whether or not there might be an overflow in the cross \ + * product determined by (___m & ((1 << 63) | (1 << 31))). \ + * \ + * Select the best way to do (m_bias + m * n) / (p << 64). \ + * From now on there will be actual runtime code generated. \ + */ \ + \ + ___m_lo = ___m; \ + ___m_hi = ___m >> 32; \ + ___n_lo = ___n; \ + ___n_hi = ___n >> 32; \ + \ + if (!___bias) { \ + ___res = ((uint64_t)___m_lo * ___n_lo) >> 32; \ + } else if (!(___m & ((1ULL << 63) | (1ULL << 31)))) { \ + ___res = (___m + (uint64_t)___m_lo * ___n_lo) >> 32; \ + } else { \ + ___res = ___m + (uint64_t)___m_lo * ___n_lo; \ + ___t = (___res < ___m) ? (1ULL << 32) : 0; \ + ___res = (___res >> 32) + ___t; \ + } \ + \ + if (!(___m & ((1ULL << 63) | (1ULL << 31)))) { \ + ___res += (uint64_t)___m_lo * ___n_hi; \ + ___res += (uint64_t)___m_hi * ___n_lo; \ + ___res >>= 32; \ + } else { \ + ___t = ___res += (uint64_t)___m_lo * ___n_hi; \ + ___res += (uint64_t)___m_hi * ___n_lo; \ + ___t = (___res < ___t) ? (1ULL << 32) : 0; \ + ___res = (___res >> 32) + ___t; \ + } \ + \ + ___res += (uint64_t)___m_hi * ___n_hi; \ + \ + ___res /= ___p; \ +}) + extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor); /* The unnecessary pointer compare is there @@ -47,6 +186,14 @@ extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor); is_power_of_2(__base)) { \ __rem = (n) & (__base - 1); \ (n) >>= ilog2(__base); \ + } else if (__div64_const32_is_OK && \ + __builtin_constant_p(__base) && \ + __base != 0) { \ + uint32_t __res_lo, __n_lo = (n); \ + (n) = __div64_const32(n, __base); \ + /* the remainder can be computed with 32-bit regs */ \ + __res_lo = (n); \ + __rem = __n_lo - __res_lo * __base; \ } else if (likely(((n) >> 32) == 0)) { \ __rem = (uint32_t)(n) % __base; \ (n) = (uint32_t)(n) / __base; \ |