CACHE XSORT AND QSORT SORT.CC CODE
This hot loop corresponds directly to this code from glibc (from the file msort.c 5) : There are also two checks for termination ( test r12,r12 and test rbp,rbp). bench qsort to capture profiling data, and perf report -stdio to print a summary 3:ĥ.24 : 39224: mov rdx,QWORD PTR ģ2.69 : 39236: mov rax,QWORD PTR ĭepending on your level of assembly reading skill, it may not be obvious, but this is basically a classic merge routine: it is merging two lists by comparing the top elements of each list (pointed to by r13 and r15), and then storing the smaller element (the line QWORD PTR ,rax) and loading the next element from that list. Benchmarking Qsortįirst, let’s take a look at what qsort is doing, to see if there is any delicous low-hanging performance fruit.
This naturally, this raises the question: how fast is qsort when it comes to sorting integers and can we do better?Īll of the code for this post is available on GitHub, so if you’d like to follow along with the code open in an editor, go right ahead (warning: there are obviously some spoilers if you dig through the code first). bench & perf report) shows that more than 90% of the time spent in this approach is in the sorting routine, qsort - so we are right to focus on this step, rather than say the de-duplication step or the initial random number generation. While Daniel suggests a clever method of avoiding a sort entirely 2, I’m also interested in they why for the underlying performace of the sort method: it takes more than 100 ns per element, which means 100s of CPU clock cycles and usually even more instructions than that (on a superscalar processor)! As a sanity check, a quick benchmark ( perf record. In the case we want sorted output, an obvious solution presents itself: sorting randomly chosen values and de-duplicating the list, which is easy since identical values are now adjacent. This page is part of release 4.04 of the Linux man-pages project.Recently, Daniel Lemire tackled the topic of selecting N distinct numbers at random. SEE ALSO sort(1), alphasort(3), strcmp(3), versionsort(3) Qsort(&argv, argc - 1, sizeof(char *), cmpstringp) Return strcmp(* (char * const *) p1, * (char * const *) p2)
CACHE XSORT AND QSORT SORT.CC PLUS
To char", hence the following cast plus dereference */ Pointers to char", but strcmp(3) arguments are "pointers * The actual arguments to this function are "pointers to To compare C strings, the comparison function can call strcmp(3), as shown in the exampleįor one example of use, see the example under bsearch(3).Īnother example is the following program, which sorts the strings given in its command-Ĭmpstringp(const void *p1, const void *p2) │ qsort(), qsort_r() │ Thread safety │ MT-Safe │ VERSIONS qsort_r() was added to glibc in version 2.8.įor an explanation of the terms used in this section, see attributes(7).
The qsort() and qsort_r() functions return no value. Way, the comparison function does not need to use global variables to pass throughĪrbitrary arguments, and is therefore reentrant and safe to use in threads. A pointer is passed to the comparison function via arg. The qsort_r() function is identical to qsort() except that the comparison function compar
If two members compare as equal, their order in the sorted array is If the first argument is considered to be respectively less than, equal to, or greater The comparison function must return an integer less than, equal to, or greater than zero Pointed to by compar, which is called with two arguments that point to the objects being The contents of the array are sorted in ascending order according to a comparison function The qsort() function sorts an array with nmemb elements of size size. SYNOPSIS #include void qsort(void * base, size_t nmemb, size_t size, int (* compar )(const void *, const void *)) void qsort_r(void * base, size_t nmemb, size_t size, int (* compar )(const void *, const void *, void *), void * arg ) įeature Test Macro Requirements for glibc (see feature_test_macros(7)):
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