我不时地读到Fortran在繁重的计算中比C更快。这是真的吗?我必须承认我几乎不懂Fortran,但是到目前为止我看到的Fortran代码并没有显示出该语言具有C语言所不具备的特性。
如果是真的,请告诉我原因。请不要告诉我什么语言或库适合处理数字,我不打算写一个应用程序或库来做这个,我只是好奇。
我不时地读到Fortran在繁重的计算中比C更快。这是真的吗?我必须承认我几乎不懂Fortran,但是到目前为止我看到的Fortran代码并没有显示出该语言具有C语言所不具备的特性。
如果是真的,请告诉我原因。请不要告诉我什么语言或库适合处理数字,我不打算写一个应用程序或库来做这个,我只是好奇。
当前回答
This is more than somewhat subjective, because it gets into the quality of compilers and such more than anything else. However, to more directly answer your question, speaking from a language/compiler standpoint there is nothing about Fortran over C that is going to make it inherently faster or better than C. If you are doing heavy math operations, it will come down to the quality of the compiler, the skill of the programmer in each language and the intrinsic math support libraries that support those operations to ultimately determine which is going to be faster for a given implementation.
编辑:@Nils等人提出了一个很好的观点,即C语言中指针使用的差异,以及可能存在的别名,这可能会使C语言中最简单的实现变慢。然而,在C99中有一些方法可以解决这个问题,比如通过编译器优化标志和/或C语言的实际编写方式。这在@Nils的回答和随后的评论中有很好的介绍。
其他回答
I think the key point in favor of Fortran is that it is a language slightly more suited for expressing vector- and array-based math. The pointer analysis issue pointed out above is real in practice, since portable code cannot really assume that you can tell a compiler something. There is ALWAYS an advantage to expression computaitons in a manner closer to how the domain looks. C does not really have arrays at all, if you look closely, just something that kind of behaves like it. Fortran has real arrawys. Which makes it easier to compile for certain types of algorithms especially for parallel machines.
在运行时系统和调用约定等方面,C语言和现代的Fortran非常相似,很难看出有什么不同。注意,这里的C实际上是基础C: c++是一个完全不同的问题,具有非常不同的性能特征。
我还没有听说过Fortan比C快得多,但是可以想象在某些情况下它会更快。关键不在于语言特征的存在,而在于那些(通常)不存在的特征。
一个例子是C指针。C指针几乎到处都在使用,但指针的问题是编译器通常无法判断它们是否指向同一个数组的不同部分。
例如,如果你写了一个strcpy例程,看起来像这样:
strcpy(char *d, const char* s)
{
while(*d++ = *s++);
}
编译器必须在d和s可能是重叠数组的假设下工作。所以当数组重叠时,它不能执行会产生不同结果的优化。正如您所期望的,这在很大程度上限制了可以执行的优化类型。
[我应该注意到,C99有一个“restrict”关键字,显式地告诉编译器指针不重叠。还要注意,Fortran也有指针,语义不同于C语言,但指针不像C语言那样无处不在。
但是回到C与Fortran的问题上,可以想象,Fortran编译器能够执行一些对于(直接编写的)C程序可能无法实现的优化。所以我不会对这种说法感到太惊讶。不过,我确实希望性能差异不会太大。(~ 5 - 10%)
There is another item where Fortran is different than C - and potentially faster. Fortran has better optimization rules than C. In Fortran, the evaluation order of an expressions is not defined, which allows the compiler to optimize it - if one wants to force a certain order, one has to use parentheses. In C the order is much stricter, but with "-fast" options, they are more relaxed and "(...)" are also ignored. I think Fortran has a way which lies nicely in the middle. (Well, IEEE makes the live more difficult as certain evaluation-order changes require that no overflows occur, which either has to be ignored or hampers the evaluation).
另一个更聪明的规则领域是复数。这不仅是因为直到c99才有了它们,而且Fortran中管理它们的规则更好;由于gfortran的Fortran库部分是用C编写的,但实现了Fortran语义,GCC获得了这个选项(也可以用于“普通”C程序):
-fcx-fortran-rules 复杂的乘法和除法遵循Fortran规则。范围缩减是作为复杂除法的一部分进行的,但是没有检查复杂乘法或除法的结果是否是“NaN + I*NaN”,试图在这种情况下挽救这种情况。
The alias rules mentioned above is another bonus and also - at least in principle - the whole-array operations, which if taken properly into account by the optimizer of the compiler, can lead faster code. On the contra side are that certain operation take more time, e.g. if one does an assignment to an allocatable array, there are lots of checks necessary (reallocate? [Fortran 2003 feature], has the array strides, etc.), which make the simple operation more complex behind the scenes - and thus slower, but makes the language more powerful. On the other hand, the array operations with flexible bounds and strides makes it easier to write code - and the compiler is usually better optimizing code than a user.
总的来说,我认为C和Fortran的速度差不多;选择应该更多的是你更喜欢哪种语言,或者是使用Fortran的全数组操作及其更好的可移植性更有用,还是使用C中更好的系统接口和图形用户界面库。
Fortran traditionally doesn't set options such as -fp:strict (which ifort requires to enable some of the features in USE IEEE_arithmetic, a part of f2003 standard). Intel C++ also doesn't set -fp:strict as a default, but that is required for ERRNO handling, for example, and other C++ compilers don't make it convenient to turn off ERRNO or gain optimizations such as simd reduction. gcc and g++ have required me to set up Makefile to avoid using the dangerous combination -O3 -ffast-math -fopenmp -march=native. Other than these issues, this question about relative performance gets more nit-picky and dependent on local rules about choice of compilers and options.
一般来说,FORTRAN比C慢。C可以使用硬件级指针,允许程序员手动优化。FORTRAN(在大多数情况下)不能访问硬件内存寻址黑客。(VAX FORTRAN是另一回事。)我从70年代开始断断续续地使用FORTRAN。(真的)。
然而,从90年代开始,FORTRAN已经发展到包括特定的语言结构,可以优化成内在的并行算法,真正可以在多核处理器上运行。例如,自动矢量化允许多个处理器同时处理数据向量中的每个元素。16个处理器——16个元素向量——处理需要1/16的时间。
在C语言中,您必须管理自己的线程并为多处理仔细设计算法,然后使用一堆API调用来确保并行性正确发生。
在FORTRAN中,您只需要为多处理仔细设计算法。编译器和运行时可以为您处理其余的工作。
您可以阅读一些关于高性能Fortran的内容,但是您会发现许多死链接。你最好阅读并行编程(比如OpenMP.org)以及FORTRAN如何支持并行编程。