我尝试了以下代码，但是代码之间的性能差异很大。我听说顶级代码不适合进行数值计算，但是性能似乎也取决于顶级变量（此处为N）是否出现在for循环范围内。避免使用此类顶级变量总是更好吗？
N = 10000000
# case1 (slow)
x = 0.0
@time for k = 1:N
x += float( k )
end
# case2 (slow)
@time let
y = 0.0
for j = 1:N
y += float( j )
end
end
# case3 (very fast)
@time let
n::Int64
n = N
z = 0.0
for m = 1:n
z += float( m )
end
end
# case 4 (slow)
function func1()
c = 0.0
for i = 1:N
c += float( i )
end
end
# case 5 (fast)
function func2( n )
c = 0.0
for i = 1:n
c += float( i )
end
end
# case 6 (fast)
function func3()
n::Int
n = N
c = 0.0
for i = 1:n
c += float( i )
end
end
# case 7 (slow)
function func4()
n = N # n = int( N ) is also slow
c = 0.0
for i = 1:n
c += float( i )
end
end
@time func1()
@time func2( N )
@time func3()
@time func4()
使用Julia 0.3.7（在Linux x86_64上）获得的结果是
elapsed time: 2.595440598 seconds (959985496 bytes allocated, 10.70% gc time)
elapsed time: 2.469471127 seconds (959983688 bytes allocated, 11.49% gc time)
elapsed time: 1.608e-6 seconds (16 bytes allocated)
elapsed time: 2.535243279 seconds (960021976 bytes allocated, 11.21% gc time)
elapsed time: 0.002601149 seconds (75592 bytes allocated)
elapsed time: 0.003471583 seconds (84456 bytes allocated)
elapsed time: 2.480343146 seconds (960020752 bytes allocated, 11.48% gc time)
答案是“避免这样的顶级变量总是更好吗？” 字面量当然是“不，取决于”，但有用的注释是将全局变量声明为常量
const N = 10000000
使情况2与情况3一样快。
编辑：我应该补充一点，情况2的问题是，即使累加器变量是本地的，顶层也N会使范围1:N和迭代器变量的类型j不稳定y。解决此问题的方法更加灵活
let
y = 0.0
for j = 1:(N::Int)
y += float( j )
end
y
end
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