[PATCH v3 3/5] include: asm-generic: don't do 64-bit soft division on 64-bit platforms
Ahmad Fatoum
a.fatoum at pengutronix.de
Thu Feb 18 04:39:00 EST 2021
barebox implements do_div() as soft division unconditionally, even on
64-bit platforms that could use hardware 64-bit division directly.
Import the whole Linux asm-generic/div64.h header to avoid this.
This also has potential positive effect on 32-bit platforms:
64-bit division with constant divisors can now be optimized into
multiplications and shifts at compile time.
Signed-off-by: Ahmad Fatoum <a.fatoum at pengutronix.de>
---
v2 -> v3:
- no change
v1 -> v2:
- no change
---
include/asm-generic/div64.h | 216 +++++++++++++++++++++++++++++++++++-
1 file changed, 215 insertions(+), 1 deletion(-)
diff --git a/include/asm-generic/div64.h b/include/asm-generic/div64.h
index 2e0ba8389e11..a3b98c86f077 100644
--- a/include/asm-generic/div64.h
+++ b/include/asm-generic/div64.h
@@ -1,9 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0 */
#ifndef _ASM_GENERIC_DIV64_H
#define _ASM_GENERIC_DIV64_H
/*
* Copyright (C) 2003 Bernardo Innocenti <bernie at 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)
@@ -18,8 +22,200 @@
*/
#include <linux/types.h>
+#include <linux/compiler.h>
+
+#if BITS_PER_LONG == 64
+
+/**
+ * do_div - returns 2 values: calculate remainder and update new dividend
+ * @n: uint64_t dividend (will be updated)
+ * @base: uint32_t divisor
+ *
+ * Summary:
+ * ``uint32_t remainder = n % base;``
+ * ``n = n / base;``
+ *
+ * Return: (uint32_t)remainder
+ *
+ * NOTE: macro parameter @n is evaluated multiple times,
+ * beware of side effects!
+ */
+# define do_div(n,base) ({ \
+ uint32_t __base = (base); \
+ uint32_t __rem; \
+ __rem = ((uint64_t)(n)) % __base; \
+ (n) = ((uint64_t)(n)) / __base; \
+ __rem; \
+ })
+
+#elif BITS_PER_LONG == 32
+
+#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; \
+ \
+ /* 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) / (1 << 64). \
+ * From now on there will be actual runtime code generated. \
+ */ \
+ ___res = __arch_xprod_64(___m, ___n, ___bias); \
+ \
+ ___res /= ___p; \
+})
+
+#ifndef __arch_xprod_64
+/*
+ * Default C implementation for __arch_xprod_64()
+ *
+ * Prototype: uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
+ * Semantic: retval = ((bias ? m : 0) + m * n) >> 64
+ *
+ * The product is a 128-bit value, scaled down to 64 bits.
+ * Assuming constant propagation to optimize away unused conditional code.
+ * Architectures may provide their own optimized assembly implementation.
+ */
+static inline uint64_t __arch_xprod_64(const uint64_t m, uint64_t n, bool bias)
+{
+ uint32_t m_lo = m;
+ uint32_t m_hi = m >> 32;
+ uint32_t n_lo = n;
+ uint32_t n_hi = n >> 32;
+ uint64_t res;
+ uint32_t res_lo, res_hi, tmp;
+
+ if (!bias) {
+ res = ((uint64_t)m_lo * n_lo) >> 32;
+ } else if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
+ /* there can't be any overflow here */
+ res = (m + (uint64_t)m_lo * n_lo) >> 32;
+ } else {
+ res = m + (uint64_t)m_lo * n_lo;
+ res_lo = res >> 32;
+ res_hi = (res_lo < m_hi);
+ res = res_lo | ((uint64_t)res_hi << 32);
+ }
+
+ if (!(m & ((1ULL << 63) | (1ULL << 31)))) {
+ /* there can't be any overflow here */
+ res += (uint64_t)m_lo * n_hi;
+ res += (uint64_t)m_hi * n_lo;
+ res >>= 32;
+ } else {
+ res += (uint64_t)m_lo * n_hi;
+ tmp = res >> 32;
+ res += (uint64_t)m_hi * n_lo;
+ res_lo = res >> 32;
+ res_hi = (res_lo < tmp);
+ res = res_lo | ((uint64_t)res_hi << 32);
+ }
+
+ res += (uint64_t)m_hi * n_hi;
+
+ return res;
+}
+#endif
+
+#ifndef __div64_32
extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
+#endif
/* The unnecessary pointer compare is there
* to check for type safety (n must be 64bit)
@@ -28,7 +224,19 @@ extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
uint32_t __base = (base); \
uint32_t __rem; \
(void)(((typeof((n)) *)0) == ((uint64_t *)0)); \
- if (((n) >> 32) == 0) { \
+ if (__builtin_constant_p(__base) && \
+ 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; \
} else \
@@ -36,4 +244,10 @@ extern uint32_t __div64_32(uint64_t *dividend, uint32_t divisor);
__rem; \
})
+#else /* BITS_PER_LONG == ?? */
+
+# error do_div() does not yet support the C64
+
+#endif /* BITS_PER_LONG */
+
#endif /* _ASM_GENERIC_DIV64_H */
--
2.29.2
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