L*******e
发帖数: 114 | 1 I know for y=x++, definitely a temp variable holds the old value.
How about this: for(int i=0;i<100;i++) or even simpler i++?
From the claim from http://www.gotw.ca/gotw/002.htm, but I do not agree this claim, why a temp var is created but not used at all? see the following sample program and generated assembly.
Any thoughts?
int i = 0;
i++;
++i;
int k=i++;
assembly generated by g++:
movl $0, -12(%ebp) #, i
addl $1, -12(%ebp) #, i
addl $1, -12(%ebp) | z***9
发帖数: 696 | 2 depends on if you use -O2 or O3 with g++ | r**u
发帖数: 1567 | 3 i++，先要用额外的地方存原始值，返回这个值，然后＋1。
++i，就直接在原来的地址＋1，然后返回。
理论上++i，快一点。不过好像compiler已经都optimize了，
i++也不会要额外存原始值。
有人说如果i是iterator，difference会比较大。
【在 L*******e 的大作中提到】
: I know for y=x++, definitely a temp variable holds the old value.
: How about this: for(int i=0;i<100;i++) or even simpler i++?
: From the claim from http://www.gotw.ca/gotw/002.htm, but I do not agree this claim, why a temp var is created but not used at all? see the following sample program and generated assembly.
: Any thoughts?
: int i = 0;
: i++;
: ++i;
: int k=i++;
: assembly generated by g++:
: movl $0, -12(%ebp) #, i
| h**k
发帖数: 3368 | 4 如果用在for循环语句里，i++和++i是一样的，被编译器直接优化了，都不生成临时变
量。
【在 r**u 的大作中提到】
: i++，先要用额外的地方存原始值，返回这个值，然后＋1。
: ++i，就直接在原来的地址＋1，然后返回。
: 理论上++i，快一点。不过好像compiler已经都optimize了，
: i++也不会要额外存原始值。
: 有人说如果i是iterator，difference会比较大。
| L*******e
发帖数: 114 | 5 I was asked this question during on-site. My answer was pre-increment
because of temp var used in post-increment. However, rethink the question,
there is no definite yes or no answer.
Another similar question is "does inline make program faster?" I was asked
too (just different interviewer)
【在 h**k 的大作中提到】
: 如果用在for循环语句里，i++和++i是一样的，被编译器直接优化了，都不生成临时变
: 量。
| h**k
发帖数: 3368 | 6 在老编译器上，可以同时使用++i和i++的情况下，毫无疑问的使用++i更好，原因就是i
++需要生成临时变量。最新的编译器已经优化了，没有区别。
使用inline函数的缺点是执行代码变大了，这需要更大的内存，同时也增加了page
fault的可能，这往往会减慢程序的运行。
【在 L*******e 的大作中提到】
: I was asked this question during on-site. My answer was pre-increment
: because of temp var used in post-increment. However, rethink the question,
: there is no definite yes or no answer.
: Another similar question is "does inline make program faster?" I was asked
: too (just different interviewer)
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