[swift-users] Compile time exceeded. Anything wrong?

Tue Jun 6 07:52:59 CDT 2017

Interesting, I guess it’s anything with multiple possible function overloads then! It looks pretty trivial though to figure out the two components before trying to figure out which function, which should lead to the result in a reasonable time. There must be something else going on. Good work on filing the bug! Hope this gets fixed soon.

Geordie

> Am 06.06.2017 um 22:49 schrieb Jens Persson <jens at bitcycle.com>:
> 
> You're right that it's not functionally eq, sorry about that.
> I filed https://bugs.swift.org/browse/SR-5093 <https://bugs.swift.org/browse/SR-5093> containing a reduced example program to demonstrate the issue..
> Haven't tested it with anything later than the 2017-06-02 snapshot.
> But it shows that it's not just max that is the problem.
> /Jens
> 
> On Tue, Jun 6, 2017 at 4:51 AM, Geordie Jay <geojay at gmail.com <mailto:geojay at gmail.com>> wrote:
> I doubt it'll be "fixed" in the next release (which came out less than 24 hours ago), but I gather type checker speed optimisations and fixing crashes are ongoing goals of the compiler team. With no guarantees or expectations, you may find your code compiles in an acceptable time with Swift 4 (Xcode 9).
> 
> Cheers,
> Geordie
> 
> On Tue 6. Jun 2017 at 12:29, <www.hbu.cn at 163.com <mailto:www.hbu.cn at 163.com>> wrote:
> thanks,  guys!  dp array should be var, not let, but the compile run too slow. It indeed a bug.  Optimizing  the code for the compiling time is really a headache for coders.  I am using the Xcode 8.1. Hoping it can be fixed in next release.
> 
> 
> best wishes for you 
> 
> 2017-06-06 8:57 GMT+08:00 Geordie J <geojay at gmail.com <mailto:geojay at gmail.com>>:
> 
>> Am 06.06.2017 um 09:02 schrieb Jens Persson via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>>:
>> 
>> When compiling that from the command line, I get the following (after about 6 seconds):
>> 
>> test.swift:7:18: error: cannot assign through subscript: 'dp' is a 'let' constant
>>         dp[0][0] = 0
>>         ~~       ^
>> /.../
>> 
>> After fixing that (changing let dp to var dp), it will compile successfully, still taking a very long time though. This usually means that some expression(s) in the code happen to be a bit hard on the type checker (in its current state).
>> I tried the latest dev snapshot and it is a bit faster, perhaps 3 s instead of 6.
>> 
>> Anyway, here is a logically equivalent rewrite which will compile faster:
>> 
>> class Solution {
>>     func rob(nums: [Int]) -> Int {
>>         guard nums.count > 0 else { return 0 }
>>         var dp = Array.init(repeating: Array.init(repeating: 0, count: nums.count),
>>                             count: 2)
>>         dp[0][0] = 0
>>         dp[0][1] = nums[0]
>>         for i in 1 ..< nums.count {
>>             let dp_iMinus1_0 = dp[i - 1][0] 
>>             let dp_iMinus1_1 = dp[i - 1][1] 
>>             dp[i][0] = max(dp_iMinus1_0, dp_iMinus1_1)
>>             dp[i][1] = dp_iMinus1_0 + nums[i]
> 
> Just a nitpick: this isn’t functionally equivalent to the original code (dp[i][1] = dp[i][0] + nums[i]), because dp[i][0] might have actually changed on the previous line. But you can return this line to the original version without any effect on the compile time (less than 250ms on my machine).
> 
> I think the compile time issue is that max() is a generic function that takes any Comparable, so the type checker seems to go berserk trying to ensure the term is satisfiable.
> 
> If you create a non-generic replacement for max in the same file:
> 
> private func myMax(_ x1: Int, _ x2: Int) -> Int {
>     return (x1 >= x2) ? x1 : x2
> }
> 
> … and replace the assignment to max(dp[i - 1][0], ……) in the for loop with myMax(dp[i - 1][0], ….)
> 
> The compile time will be just as fast as the one that swaps out the internal elements (on my machine it’s actually about 10% faster with the non-generic max).
> 
> Regards,
> Geordie
> 
>>         }
>>         return 0
>>     }
>> }
>> 
>> 
>> 
>> 
>> 
>> On Mon, Jun 5, 2017 at 2:32 PM, Hbucius Smith via swift-users <swift-users at swift.org <mailto:swift-users at swift.org>> wrote:
>> Hi Swift-Users,
>> 
>>     when I compiled the code, Xcode cannot stop, I do not know why. It is very strange. Can anyone help ? Here is the code. I am using Xcode 8.1
>> 
>> class Solution {
>> 
>>     func rob(nums: [Int]) -> Int {
>> 
>>         guard nums.count > 0 else { return 0 }
>> 
>>         let dp = Array.init(repeating: Array.init(repeating: 0, count: nums.count),
>> 
>>                             count: 2)
>> 
>>         dp[0][0] = 0
>> 
>>         dp[0][1] = nums[0]
>> 
>>         for i in 1 ..< nums.count {
>> 
>>             dp[i][0] = max(dp[i - 1][0], dp[i - 1][1])
>> 
>>             dp[i][1] = dp[i - 1][0] + nums[i]
>> 
>>         }
>> 
>>         return 0
>> 
>>     }
>> 
>> }
>> 
>> 
>> 
>> 
>> best wishes for you 
>> 
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> 
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