[Mono-dev] Stack overflow in Array.Sort for large arrays

[Martin Potter]

I am currently working on testing Mono to 2.11 with the hope to ease the transition to 2.12 when it is release. When testing, I found a nasty bug when sorting a large (228,000 elements) List<int> that resulted in a stack overflow. The partial stack trace from the crash:

System.Collections.Generic.List`1<int>:Sort ()
System.Array:Sort<int> (int[],int,int)
System.Array:Sort<int> (int[],int,int,System.Collections.Generic.IComparer`1<int>)
System.Array:SortImpl<int, int> (int[],int[],int,int,System.Collections.Generic.IComparer`1<int>)
System.Array:qsort<int, int> (int[],int[],int,int)
...
System.Array:qsort<int, int> (int[],int[],int,int)

Upon pulling the related sorting code from System.Array into a separate project, I determined that the stack overflow was occurring for this particular List/Array due to the fact that over half of the list elements were same number. It appears that this occurs as a result of the change to the various qsort methods in https://github.com/mono/mono/commit/d97cdb0c124729152be551c421c4a11732e45fc9, which introduced a change in the treatment of elements with equal values. Reverting this commit fixes the stack overflow in the test app. In testing, I noticed that the old qsort code was significantly faster sorting when there were lots of duplicate values, was there are particular reason for changing the logic for dealing with equal values?

— Martin


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