带符号的64位整数饱和加法? [英] Signed saturated add of 64-bit ints?

问题描述

我正在寻找一些带符号的饱和64位加法使用的C代码，这些代码可使用gcc优化器编译为有效的x86-64代码.可移植的代码将是理想的选择，尽管必要时可以使用asm解决方案.

I'm looking for some C code for signed saturated 64-bit addition that compiles to efficient x86-64 code with the gcc optimizer. Portable code would be ideal, although an asm solution could be used if necessary.

static const int64 kint64max = 0x7fffffffffffffffll;
static const int64 kint64min = 0x8000000000000000ll;

int64 signed_saturated_add(int64 x, int64 y) {
  bool x_is_negative = (x & kint64min) != 0;
  bool y_is_negative = (y & kint64min) != 0;
  int64 sum = x+y;
  bool sum_is_negative = (sum & kint64min) != 0;
  if (x_is_negative != y_is_negative) return sum;  // can't overflow
  if (x_is_negative && !sum_is_negative) return kint64min;
  if (!x_is_negative && sum_is_negative) return kint64max;
  return sum;
}

编写的函数产生带有多个分支的相当长的汇编输出.关于优化的任何提示吗?似乎它应该只用带有几个CMOV指令的ADD即可实现，但是我对此东西有点生锈.

The function as written produces a fairly lengthy assembly output with several branches. Any tips on optimization? Seems like it ought to be be implementable with just an ADD with a few CMOV instructions but I'm a little bit rusty with this stuff.

推荐答案

可以对此进行进一步优化，但这是一个可移植的解决方案.它不会调用未定义的行为，并且会在整数溢出发生之前对其进行检查.

This may be optimized further but here is a portable solution. It does not invoked undefined behavior and it checks for integer overflow before it could occur.

#include <stdint.h>

int64_t sadd64(int64_t a, int64_t b)
{
    if (a > 0) {
        if (b > INT64_MAX - a) {
            return INT64_MAX;
        }
    } else if (b < INT64_MIN - a) {
            return INT64_MIN;
    }

    return a + b;
}

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