我正在try 编译以下代码:
#pragma omp parallel shared (j)
{
#pragma omp for schedule(dynamic)
for(i = 0; i != j; i++)
{
// do something
}
}
但我得到了以下错误:error: invalid controlling predicate.
OpenMP standard声明对于parallel for构造函数，它"只"允许以下运算符之一:<、<=、>、>=.
我不明白为什么不允许i != j人.对于static schedule，我可以理解，因为编译器需要预先计算分配给每个线程的迭代次数.但我不明白为什么在这种情况下会有这种限制.有线索吗？
即使我赚了for(i = 0; i != 100; i++)分，我也可以写上"<；"或"<；=".
.
For signed int, the wrap around behavior is undefined. If we allow !=, programmers may get unexpected tripcount. The problem is whether the compiler can generate code to compute a trip count for the loop.
对于一个简单的循环，例如:
for( i = 0; i < n; ++i )
the compiler can determine that there are 'n' iterations, if n>=0, and zero iterations if n < 0.
对于如下循环:
for( i = 0; i != n; ++i )
again, a compiler should be able to determine that there are 'n' iterations, if n >= 0; if n < 0, we don't know how many iterations it has.
对于如下循环:
for( i = 0; i < n; i += 2 )
the compiler can generate code to compute the trip count (loop iteration count) as floor((n+1)/2) if n >= 0, and 0 if n < 0.
对于如下循环:
for( i = 0; i != n; i += 2 )
the compiler can't determine whether 'i' will ever hit 'n'. What if 'n' is an odd number?
对于如下循环:
for( i = 0; i < n; i += k )
the compiler can generate code to compute the trip count as floor((n+k-1)/k) if n >= 0, and 0 if n < 0, because the compiler knows that the loop must count up; in this case, if k < 0, it's not a legal OpenMP program.
对于如下循环:
for( i = 0; i != n; i += k )
the compiler doesn't even know if i is counting up or down. It doesn't know if 'i' will ever hit 'n'. It may be an infinite loop.
Credits:OpenMP ARB