Do the two statements cost the same amount of time? - 未名空间MITBBS历史存档

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Do the two statements cost the same amount of time?

Do the two statements cost the same amount of time?# Programming - 葵花宝典

t*s2007-10-15 07:10

1 楼

1) for i =1 to floor(sqrt(n))
2) for i=1 to n
I guess that they take different amount of time.
The reason is that in the first one each check to see whether i exceeds
the limit involves a floor(sqrt(n)) operation, while the second one
does not.
Or maybe the limit 'floor(sqrt(n))' is calculated only once and therefore I
am wrong ?
Can somebody confirm please?

l*d2007-10-15 07:10

2 楼

Why not try to run a short piece of code and compare
the run time
1. x= 1e16+0.1;
for (i=1; i
2. x= 1e16+0.1;
for (i=1; i
I

【在 t**********s 的大作中提到】

: 1) for i =1 to floor(sqrt(n))
: 2) for i=1 to n
: I guess that they take different amount of time.
: The reason is that in the first one each check to see whether i exceeds
: the limit involves a floor(sqrt(n)) operation, while the second one
: does not.
: Or maybe the limit 'floor(sqrt(n))' is calculated only once and therefore I
: am wrong ?
: Can somebody confirm please?

v*u2007-10-15 07:10

3 楼

I
我相信大部分编译器会做这个优化

【在 t**********s 的大作中提到】

a*l2007-10-15 07:10

4 楼

嘿嘿,我相信编译器绝对是不会做这个优化的,因为这个优化会非常难的,if ever
possible.理由自己想.

【在 v*****u 的大作中提到】

:
: I
: 我相信大部分编译器会做这个优化

g*y2007-10-15 07:10

5 楼

If 'n' is not referenced/modified inside the loop, this should be a very
safe optimization for the compiler.

a*l2007-10-15 07:10

6 楼

well, that's a very big "if"...

【在 g*****y 的大作中提到】

: If 'n' is not referenced/modified inside the loop, this should be a very
: safe optimization for the compiler.

e*e2007-10-15 07:10

7 楼

I think in general the compiler is unlikely to optimize this and evaluate
the expression only once at the beginning of the loop. There are too
many ways in C that the value of n can be modified directly and
indirectly (pointer, global variable, etc.)
In Fortran, the final value of the do-loop variable must be evaluated
only once at the beginning of the loop, and this will give the (maximum)
number of times the loop will be executed.

【在 g*****y 的大作中提到】

: If 'n' is not referenced/modified inside the loop, this should be a very
: safe optimization for the compiler.

j*r2007-10-15 07:10

8 楼

That depends on the compiling option and your compiler.
If you use GCC, you can use option -O to specify the optimization level you
want. If you use full optimization(-O3), I think the compiler will try to
optimize it. If variable n is not modified inside the loop, it will be
optimized of course. If variable n is modified inside the loop, I don't
think the compiler will optimize it. If it is difficult for the compiler to
judge between these two situations, I guess the complier takes a
conservati

【在 t**********s 的大作中提到】