问题描述
地板事业部是当结果总是难倒向下（朝-∞），不计入0：
Floored division is when the result is always floored down (towards −∞), not towards 0:
是否有可能有效地实现用C或地板欧几里德整数除法/ C ++？
Is it possible to efficiently implement floored or euclidean integer division in C/C++?
（显而易见的解决办法是检查分红的符号）
(the obvious solution is to check the dividend's sign)
推荐答案
我在五年后重新审视这个问题，因为这是有关对我来说太。我做了两个纯-C版本和两个内联汇编版本的x86-64，结果一些性能测量可能是有趣的。
I'm revisiting this question five years later, as this is relevant for me too. I did some performance measurements on two pure-C versions and two inline-assembly versions for x86-64, and the results may be interesting.
地板事业部的测试变体是：
The tested variants of floored division are:
- 我一直在使用了一段时间的实施情况;
- 在高于该psented了$ P $的轻微变异只使用一个师;
- 的previous之一，但另一方面，在实施的直列组装;和
- A
CMOV版本装配实现。
- The implementation I've been using for some time now;
- The slight variant on that presented above which only uses one division;
- The previous one, but hand-implemented in inline-assembly; and
- A
CMOVversion implemented in assembly.
以下是我的基准测试程序：
The following is my benchmark program:
#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#ifndef VARIANT
#define VARIANT 3
#endif
#if VARIANT == 0
#define floordiv(a, b) (((a) < 0)?((((a) + 1) / (b)) - 1):((a) / (b)))
#elif VARIANT == 1
#define floordiv(a, b) ((((a) < 0)?((a) - ((b) - 1)):(a)) / (b))
#elif VARIANT == 2
#define floordiv(a, b) ({ \
int result; \
asm("test %%eax, %%eax; jns 1f; sub %1, %%eax;" \
"add $1, %%eax; 1: cltd; idivl %1;" \
: "=a" (result) \
: "r" (b), \
"0" (a) \
: "rdx"); \
result;})
#elif VARIANT == 3
#define floordiv(a, b) ({ \
int result; \
asm("mov %%eax, %%edx; sub %1, %%edx; add $1, %%edx;" \
"test %%eax, %%eax; cmovs %%edx, %%eax; cltd;" \
"idivl %1;" \
: "=a" (result) \
: "r" (b), \
"0" (a) \
: "rdx"); \
result;})
#endif
double ntime(void)
{
struct timeval tv;
gettimeofday(&tv, NULL);
return(tv.tv_sec + (((double)tv.tv_usec) / 1000000.0));
}
void timediv(int n, int *p, int *q, int *r)
{
int i;
for(i = 0; i < n; i++)
r[i] = floordiv(p[i], q[i]);
}
int main(int argc, char **argv)
{
int n, i, *q, *p, *r;
double st;
n = 10000000;
p = malloc(sizeof(*p) * n);
q = malloc(sizeof(*q) * n);
r = malloc(sizeof(*r) * n);
for(i = 0; i < n; i++) {
p[i] = (rand() % 1000000) - 500000;
q[i] = (rand() % 1000000) + 1;
}
st = ntime();
for(i = 0; i < 100; i++)
timediv(n, p, q, r);
printf("%g\n", ntime() - st);
return(0);
}
我编这与 GCC -march =本地-Ofast 使用GCC 4.9.2，结果，在我的酷睿i5-2400，如下。结果是相当重复性的运行，从运行 - 他们总是降落在同一顺序，至少
I compiled this with gcc -march=native -Ofast using GCC 4.9.2, and the results, on my Core i5-2400, were as follows. The results are fairly reproducible from run to run -- they always land in the same order, at least.
- 0变7.21秒。
- 变1 7.26秒。
- 法2：6.73秒
- 变3：4.32秒
所以 CMOV 实施打击别人出来的水，至少。令我惊讶的是，变种2由一个相当大幅超出执行其纯C版本（变种1）。我还以为编译器应该能够至少有效放出code作为我的。
So the CMOV implementation blows the others out of the water, at least. What surprises me is that variant 2 out-does its pure-C version (variant 1) by a fairly wide margin. I'd have thought the compiler should be able to emit code at least as efficient as mine.
下面是一些其他的平台，以进行比较：
Here are some other platforms, for comparison:
AMD的Athlon 64 X2 4200+，GCC 4.7.2：
AMD Athlon 64 X2 4200+, GCC 4.7.2:
- 变0：26.33秒
- 变式1：25.38秒
- 法2：25.19秒
- 变3：22.39秒
至强E3-1271 V3，GCC 4.9.2：
Xeon E3-1271 v3, GCC 4.9.2:
- 变0普通：5.95秒。
- 变式1：5.62秒
- 法2 5.40秒。
- 变3：3.44秒
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