📄 csdn_文档中心_新的c语言：一切都源于fortran.htm
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<P class=MsoNormal style="TEXT-INDENT: 21pt"><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">这两种不同的参数传递机制是任何程序设计语言定义者所必须面对的不一致性的代表。语言需要特殊的实现实现技术吗？也许这将会以付出效率为代价？语言的特性是否应该为了避免争议而改变？</SPAN><SPAN
lang=EN-US>FORTRAN</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">的定义者因为效率而允许同时存在两种参数传递机制。而一旦这样的决定做了出来，某种类型的程序就变的不一致了，并将导致无法定义的结果（</SPAN><SPAN
lang=EN-US>outlawed</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">）。</SPAN></P>
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lang=EN-US> <o:p></o:p></SPAN></P>
<P class=MsoNormal style="TEXT-INDENT: 21pt"><SPAN
lang=EN-US>FORTRAN 66 </SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">标准包含了一系列可能会误导程序员的规则。在函数参数列表中，对于任何变量你都只能传递一次。如果你传递了一个变量作为函数参数，那么这个函数就不能再在全局上引用这个变量</SPAN><SPAN
lang=EN-US>(FORTRAN <B>COMMON</B>)</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">。如果你传递给一个变量给函数，你就不能再传递任何东西，并且这个函数也不能再引用任何东西，</SPAN><SPAN
lang=EN-US>that overlaps it in storage (FORTRAN
<B>EQUIVALENCE</B>)</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">。在这样的规则下，没有什么程序可以确定应该采用何种参数传递机制。</SPAN></P>
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lang=EN-US> <o:p></o:p></SPAN></P>
<P class=MsoNormal style="TEXT-INDENT: 21pt"><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">大约十年以后</SPAN><SPAN
lang=EN-US>[</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">译注：意指</SPAN><SPAN
lang=EN-US>1970s]</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">，为了实现超级计算机</SPAN><SPAN
lang=EN-US>Cray 1</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">的高性能，超级计算机需要高优化的编译器来使传统的程序能够使用机器的向量寄存器（</SPAN><SPAN
lang=EN-US>vector registers</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">）。考虑</SPAN><SPAN
lang=EN-US>Example 5</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">中的程序。其中对于函数来说最有效率的代码就是先后把数组指针</SPAN><SPAN
lang=EN-US>x</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">，</SPAN><SPAN
lang=EN-US>y</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">载入到向量寄存器中然后执行向量加指令来把两个向量寄存器中的变量加在一起。如果编译器以产生向量指令的方式来取代传统的使用循环来访问数组中的每一个元素的方式，那么代码的运行效率将得到巨大的提升。</SPAN></P>
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style="TEXT-INDENT: 31.5pt; mso-char-indent-count: 3.0; mso-char-indent-size: 10.5pt"><I><SPAN
lang=EN-US>Example 5:</SPAN></I><SPAN lang=EN-US
style="mso-font-kerning: 0pt"><o:p></o:p></SPAN></P><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US>void</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US>vector_add(float *x, float *y,</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US><SPAN style="mso-spacerun: yes"> </SPAN>float *result)</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US><SPAN style="mso-spacerun: yes"> </SPAN>{</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US><SPAN style="mso-spacerun: yes"> </SPAN>int i;</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US><SPAN style="mso-spacerun: yes"> </SPAN>for (i = 0; i < 64; ++i)</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US><SPAN style="mso-spacerun: yes"> </SPAN>result[i] = x[i] + y[i];</SPAN></PRE><PRE style="MARGIN: 0cm 36pt 0pt"><SPAN lang=EN-US><SPAN style="mso-spacerun: yes"> </SPAN>}</SPAN></PRE>
<P class=MsoNormal style="TEXT-INDENT: 21pt"><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">编译器中的优化器肯定会把循环转化成一系列的向量指令，但是问题在于那些向量指令是否真的</SPAN><SPAN
lang=EN-US>whether the sequence of vector instructions is really
equivalent to the original loop</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">。你能在处理</SPAN><SPAN
lang=EN-US>result</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">数组的存储工作之前就把</SPAN><SPAN
lang=EN-US>x,y</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">数组载入到向量寄存器中，只因为你清楚</SPAN><SPAN
lang=EN-US>result</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">数组和</SPAN><SPAN
lang=EN-US>x,y</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">数组都是不同的个体。考虑如果</SPAN><SPAN
lang=EN-US>result</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">指向</SPAN><SPAN
lang=EN-US>x[1]</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">，将会发生什么？在这种情况下</SPAN><SPAN
lang=EN-US>result[0]</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">其实就是</SPAN><SPAN
lang=EN-US>x[1]</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">，同样</SPAN><SPAN
lang=EN-US>result[I]</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">其实就是</SPAN><SPAN
lang=EN-US>x[I+1],</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">每一次循环迭代过程中都会存储下一次的迭代中会被引用的变量。如果在做</SPAN><SPAN
lang=EN-US>result</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">的存储工作之前就把</SPAN><SPAN
lang=EN-US>x</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">载入到向量寄存器中去，变量值将会改变</SPAN><SPAN
lang=EN-US>calculated change</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">。正是在这一点上，</SPAN><SPAN
lang=EN-US>FORTRAN</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">的定义就带来了冲突。为了避免在传递机制中需要引入一个特殊的参数，</SPAN><SPAN
lang=EN-US>FORTRAN</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">标准定义了一系列精确的规则用来允许向量化编译器（</SPAN><SPAN
lang=EN-US>vectorizing compiler</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">）假设</SPAN><SPAN
lang=EN-US>x,y</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">和</SPAN><SPAN
lang=EN-US>result</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">都是互不相关的，</SPAN><SPAN
lang=EN-US>non-overlapping arrays</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">。就这样偶然的，</SPAN><SPAN
lang=EN-US>FORTRAN</SPAN><SPAN
style="FONT-FAMILY: 宋体; mso-hansi-font-family: 'Times New Roman'; mso-ascii-font-family: 'Times New Roman'">在向量机上就有了巨大的性能优势。</SPAN></P>
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style="FONT-SIZE: 14pt; mso-bidi-font-size: 10.5pt; mso-hansi-font-family: 宋体; mso-bidi-font-family: 宋体"><o:p></o:p></SPAN> </P>
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style="FONT-SIZE: 9pt; mso-bidi-font-size: 10.5pt; mso-hansi-font-family: 宋体; mso-bidi-font-family: 宋体"> <o:p></o:p></SPAN></P><BR></TD></TR></TBODY></TABLE></TD></TR></TBODY></TABLE><BR>
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color=#ffffff>对该文的评论</FONT></TD>
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</SPAN> husz2001 <I>(2003-11-7 15:31:07)</I>
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<TD bgColor=#ffffff colSpan=3 width=532><BR>老实说,看的不太懂.不知道,有什么意义.
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</SPAN> whyNotHere <I>(2003-10-8 11:16:51)</I>
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<TD bgColor=#ffffff colSpan=3 width=532><BR>c语言是用得最爽的语言。 简单易用，又功能强大。
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</SPAN> MatrixCpp <I>(2003-10-7 18:10:57)</I>
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<TD bgColor=#ffffff colSpan=3 width=532><BR>楼下的这样的行为好象不是很妥啊。
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</SPAN> codecopier <I>(2003-10-6 22:54:45)</I>
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<TD bgColor=#ffffff colSpan=3 width=532><BR>This performance advantage
also carried over into parallel processor machines. The goal for an
optimizing compiler for a parallel processor is to divide a loop into
several ranges of iterations that can be done by separate processors
working independently of each other. Thus, the loop in vector_add might be
divided into two ranges: one processor might handle iterations 0 to 31
while another processor simultaneously does iterations 32 to 63. Work can
be divided up among different processors only if the compiler knows that
the results of the iterations done by one processor are not needed by a
different processor working simultaneously. Proving this usually means the
compiler needs to determine that different arrays are distinct,
non-overlapping objects. Again, the rules in FORTRAN are exactly what a
compiler needs in order to prove it can automatically parallelize a
program. While the FORTRAN performance advantage started out on
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