在C中将整个文件转换为小写的最佳方法

即如果您使用SSE2,您可以将toupper_parallel编码为(伪代码)：

for (cur_parallel_word = begin_of_block;
     cur_parallel_word < end_of_block;
     cur_parallel_word += parallel_word_width) {
    /*
     * in SSE2,parallel compares are either about 'greater' or 'equal'
     * so '>=' and '<=' have to be constructed. This would use 'PCMPGTB'.
     * The 'ALL' macro is supposed to replicate into all parallel bytes.
     */
    mask1 = parallel_compare_greater_than(*cur_parallel_word,ALL('A' - 1));
    mask2 = parallel_compare_greater_than(ALL('Z'),*cur_parallel_word);
    /*
     * vector op - and all bytes in two vectors,'PAND'
     */
    mask = mask1 & mask2;
    /*
     * vector op - add a vector of bytes. Would use 'PADDB'.
     */
    new = parallel_add(cur_parallel_word,ALL('a' - 'A'));
    /*
     * vector op - zero bytes in the original vector that will be replaced
     */
    *cur_parallel_word &= !mask;           // that'd become 'PANDN'
    /*
     * vector op - extract characters from new that replace old,then or in.
     */
    *cur_parallel_word |= (new & mask);    // PAND / POR
}

即您可以使用并行比较来检查哪些字节是大写的,然后在您或它们一起形成结果之前屏蔽原始值和“大写”版本(一个带掩码,另一个带反面).

如果你使用mmap的文件访问,这甚至可以就地执行,保存在反弹缓冲区,并节省许多功能和/或系统调用.

当你的起点是逐个字符的’fgetc’/’fputc’循环时,有很多要优化的东西;即使是shell实用程序也很可能比这更好.

但我同意,如果您的需求是非常特殊的(即像ASCII输入那样明确的东西要转换为大写),那么使用矢量指令集(如SSE内在函数/汇编或ARM NEON,如上所述)手工循环或者PPC Altivec),可能比现有的通用公用事业产生显着的加速.