MATLAB是否执行公共子表达式消除? [英] Does MATLAB perform common subexpression elimination?

问题描述

MATLAB的优化器是否对MATLAB代码执行通用的子表达式消除?例如:

Does MATLAB's optimizer perform common subexpression elimination on MATLAB code? For example:

if max(val) > minVal && max(val) < maxVal
    maxVal = max(val)
end

max(val)在那里多久被评估一次?将中间值存储在一个临时变量中是否有意义(假设重复计算很昂贵)，或者MATLAB是否会自动处理呢?

How often is max(val) evaluated there? Does it make sense to store the intermediate value in a temporary variable (assuming the repeated calculation is expensive) or does MATLAB handle this automatically?

推荐答案

很肯定，对于子表达式消除，答案是否"，对于存储中间值，答案是是".示例:

Pretty sure the answer is no to subexpression elimination and yes to storing intermediate values. Example:

>> x = rand(10000, 1);
>> tic;
for i = 1:100000
  y = max(x) + max(x);
end
toc;
Elapsed time is 4.297135 seconds.
>> tic;
for i = 1:100000
  m = max(x);
  y = m + m;
end
toc;
Elapsed time is 1.074672 seconds.

即使是+之类的内置操作，似乎也没有得到优化.类似的测试表明

Even built-in operations like + don't seem to be optimised; a similar test showed that

z = (x + x);
y = z + z + z;

比以下速度快:

y = (x + x) + (x + x) + (x + x);

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