z*******n
发帖数: 1034 | | z*******n
发帖数: 1034 | 2 水平飛行最快的鳥類
wikipedia
Swifts are the most aerial of birds. Larger species are amongst the fastest
fliers in the animal kingdom, with the White-throated Needletail having been
reported flying at up to 169 km/h (105 mph).[6] Even the Common Swift can
cruise at a maximum speed of 31 metres per second (112 km/h, 70 mph). In a
single year the common swift can cover at least 200,000 km.[7]
【在 z*******n 的大作中提到】
: https://developer.apple.com/swift/
| z*******n
发帖数: 1034 | 3 https://stackoverflow.com/questions/24101718/swift-performance-sorting-
arrays#comment37183103_24102237
tl;dr Swift without aggressive compiler optimizations at this stage is very
slow; with them it is very fast. Keep it in mind.
Here is an in-place quicksort in Swift:
func quicksort_swift(inout a:CInt[], start:Int, end:Int) {
if (end - start < 2){
return
}
var p = a[start + (end - start)/2]
var l = start
var r = end - 1
while (l <= r){
if (a[l] < p){
l += 1
continue
}
if (a[r] > p){
r -= 1
continue
}
var t = a[l]
a[l] = a[r]
a[r] = t
l += 1
r -= 1
}
quicksort_swift(&a, start, r + 1)
quicksort_swift(&a, r + 1, end)
}
And the same in C:
void quicksort_c(int *a, int n) {
if (n < 2)
return;
int p = a[n / 2];
int *l = a;
int *r = a + n - 1;
while (l <= r) {
if (*l < p) {
l++;
continue;
}
if (*r > p) {
r--;
continue;
}
int t = *l;
*l++ = *r;
*r-- = t;
}
quicksort_c(a, r - a + 1);
quicksort_c(l, a + n - l);
}
Both work:
var a_swift:CInt[] = [0,5,2,8,1234,-1,2]
var a_c:CInt[] = [0,5,2,8,1234,-1,2]
quicksort_swift(&a_swift, 0, a_swift.count)
quicksort_c(&a_c, CInt(a_c.count))
// [-1, 0, 2, 2, 5, 8, 1234]
// [-1, 0, 2, 2, 5, 8, 1234]
Both are called in the same program as written.
var x_swift = CInt[](count: n, repeatedValue: 0)
var x_c = CInt[](count: n, repeatedValue: 0)
for var i = 0; i < n; ++i {
x_swift[i] = CInt(random())
x_c[i] = CInt(random())
}
let swift_start:UInt64 = mach_absolute_time();
quicksort_swift(&x_swift, 0, x_swift.count)
let swift_stop:UInt64 = mach_absolute_time();
let c_start:UInt64 = mach_absolute_time();
quicksort_c(&x_c, CInt(x_c.count))
let c_stop:UInt64 = mach_absolute_time();
Here is a summary of the compiler's optimazation levels:
[-Onone] no optimizations, the default for debug.
[-O] perform optimizations, the default for release.
[-Ofast] perform optimizations and disable runtime overflow checks and
runtime type checks.
Time in seconds with [-Onone] for n=10_000:
Swift: 0.895296452
C: 0.001223848
Here is Swift's builtin sort() for n=10_000:
Swift_builtin: 0.77865783
Here is [-O] for n=10_000:
Swift: 0.045478346
C: 0.000784666
Swift_builtin: 0.032513488
As you can see, Swift's performance improved by a factor of 20.
As per mweathers' answer, setting [-Ofast] makes the real difference,
resulting in these times for n=10_000:
Swift: 0.000706745
C: 0.000742374
Swift_builtin: 0.000603576
And for n=1_000_000:
Swift: 0.107111846
C: 0.114957179
Swift_sort: 0.092688548
For comparison, this is with [-Onone] for n=1_000_000:
Swift: 142.659763258
C: 0.162065333
Swift_sort: 114.095478272
So Swift with no optimizations was almost 1000x slower than C in this
benchmark, at this stage in its development. On the other hand with both
compilers set to [-Ofast] Swift actually performed at least as well if not
slightly better than C.
It has been pointed out that [-Ofast] changes the semantics of the language,
making it potentially unsafe. This is what Apple states in the Xcode 5.0
release notes:
A new optimization level -Ofast, available in LLVM, enables aggressive
optimizations. -Ofast relaxes some conservative restrictions, mostly for
floating-point operations, that are safe for most code. It can yield
significant high-performance wins from the compiler.
They all but advocate it. Whether that's wise or not I couldn't say, but
from what I can tell it seems reasonable enough to use [-Ofast] in a release
if you're not doing high-precision floating point arithmetic and you're
confident no integer or array overflows are possible in your program. If you
do need high performance and overflow checks / precise arithmetic then
choose another language for now.
【在 z*******n 的大作中提到】
: https://developer.apple.com/swift/
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