我们如何安全是可以到＆QUOT;签署＆QUOT;到＆QUOT;无符号＆QUOT;招保存在这个情况下，一个比较呢？ [英] Can we and how safe is to "signed" to "unsigned" trick to save one comparison in this case?

问题描述

例如

 布尔CheckWithinBoundary（INT X，INT B）{
  则返回（x＆GT = 0）及＆放大器; （X LT; = b）的;
}
布尔CheckWithinBoundary2（INT X，INT B）{
  返回的static_cast＆LT;＆UINT32 GT;（x）的＆LT; =的static_cast＆LT;＆UINT32 GT;（二）;
}
 

CheckWithinBoundary2还能再救一个比较。

我的问题是：

1. ，今天的编译器优化code。使用呢？或者，我该怎么做
   编译器做这种优化的？




2. 有没有危险，用这一招？

解决方案

这个问题的答案2，对了，还有，这两个是不一样的。看来你是默默的假设 B＆GT; = 0 ，太。考虑如 X == 1 和 B == -1 ，这将给假对于第一种情况和真正第二。

（我切换到C符号，这很容易给我，因为你似乎也有兴趣吧）

因此​​，我们必须在事实上

  static_assert（INT_MAX＆LT; UINT_MAX）;布尔CheckWithinBoundary（INT X，INT B）{
  返回（B个= 0）及＆放大器; （X GT = 0）及＆放大器; （X LT; = b）的;
}
布尔CheckWithinBoundary2（无符号X，无符号B）{
  返回（B个= 0）及＆放大器; （X LT; = b）的;
}
 

如果汇编，是等价的在所有的架构，其中 INT_MAX＆LT; UINT_MAX ，然后隐式转换 INT - ＆GT;无符号做正确的事。

但要小心，你注意，我用无符号，而不是 uint32_t的，因为你必须要确保使用无符号类型有相同的宽度。我不知道是否有与64位架构 INT ，但你的方法会失败。

For example

bool CheckWithinBoundary(int x, int b) {
  return (x>=0) && (x <= b);
}
bool CheckWithinBoundary2(int x, int b) { 
  return static_cast<uint32>(x) <= static_cast<uint32>(b);
}

CheckWithinBoundary2 can save one comparison.

My question is:

1. Can today's compiler optimize code using this? Or how can I make the compiler do this kind of optimization?

2. Is there any danger to use this trick?

解决方案

The answer to 2 is, yes, there is, these two are not the same. It seems that you are silently assuming that b >= 0, too. Consider e.g x == 1 and b == -1, this would give false for the first case and true for the second.

(I switch to C notation, this is easier to me, and since you also seem to be interested in it)

So we have that in fact

static_assert(INT_MAX < UINT_MAX);

bool CheckWithinBoundary(int x, int b) {
  return (b >=0) && (x>=0) && (x <= b);
}
bool CheckWithinBoundary2(unsigned x, unsigned b) { 
  return (b >=0) && (x <= b);
}

if it compiles, are equivalent on all architectures where INT_MAX < UINT_MAX, and then the implicit conversion int --> unsigned does the right thing.

But be careful, you note that I use unsigned and not uint32_t, because you have to be sure to use an unsigned type with the same width. I don't know if there are architectures with 64 bit int, but there your method would fail.

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