src/stdlib/SDL_qsort.c
author Ryan C. Gordon <icculus@icculus.org>
Mon, 05 Jan 2015 01:41:42 -0500
changeset 9306 817656bd36ec
parent 8093 b43765095a6f
child 10068 19998f9082dc
permissions -rw-r--r--
Clang static analysis builds should use C runtime directly.

This is a little macro magic to use malloc() directly instead of SDL_malloc(),
etc, so static analysis tests that know about the C runtime can function
properly, and understand that we are dealing with heap allocations, etc.

This changed our static analysis report from 5 outstanding bugs to 30.

5x as many bugs were hidden by SDL_malloc() not being recognized as malloc()
by the static analyzer!
     1 /* qsort.c
     2  * (c) 1998 Gareth McCaughan
     3  *
     4  * This is a drop-in replacement for the C library's |qsort()| routine.
     5  *
     6  * Features:
     7  *   - Median-of-three pivoting (and more)
     8  *   - Truncation and final polishing by a single insertion sort
     9  *   - Early truncation when no swaps needed in pivoting step
    10  *   - Explicit recursion, guaranteed not to overflow
    11  *   - A few little wrinkles stolen from the GNU |qsort()|.
    12  *   - separate code for non-aligned / aligned / word-size objects
    13  *
    14  * This code may be reproduced freely provided
    15  *   - this file is retained unaltered apart from minor
    16  *     changes for portability and efficiency
    17  *   - no changes are made to this comment
    18  *   - any changes that *are* made are clearly flagged
    19  *   - the _ID string below is altered by inserting, after
    20  *     the date, the string " altered" followed at your option
    21  *     by other material. (Exceptions: you may change the name
    22  *     of the exported routine without changing the ID string.
    23  *     You may change the values of the macros TRUNC_* and
    24  *     PIVOT_THRESHOLD without changing the ID string, provided
    25  *     they remain constants with TRUNC_nonaligned, TRUNC_aligned
    26  *     and TRUNC_words/WORD_BYTES between 8 and 24, and
    27  *     PIVOT_THRESHOLD between 32 and 200.)
    28  *
    29  * You may use it in anything you like; you may make money
    30  * out of it; you may distribute it in object form or as
    31  * part of an executable without including source code;
    32  * you don't have to credit me. (But it would be nice if
    33  * you did.)
    34  *
    35  * If you find problems with this code, or find ways of
    36  * making it significantly faster, please let me know!
    37  * My e-mail address, valid as of early 1998 and certainly
    38  * OK for at least the next 18 months, is
    39  *    gjm11@dpmms.cam.ac.uk
    40  * Thanks!
    41  *
    42  * Gareth McCaughan   Peterhouse   Cambridge   1998
    43  */
    44 
    45 #if defined(__clang_analyzer__) && !defined(SDL_DISABLE_ANALYZE_MACROS)
    46 #define SDL_DISABLE_ANALYZE_MACROS 1
    47 #endif
    48 
    49 #include "../SDL_internal.h"
    50 
    51 /*
    52 #include <assert.h>
    53 #include <stdlib.h>
    54 #include <string.h>
    55 */
    56 #include "SDL_stdinc.h"
    57 #include "SDL_assert.h"
    58 
    59 #if defined(HAVE_QSORT)
    60 void
    61 SDL_qsort(void *base, size_t nmemb, size_t size, int (*compare) (const void *, const void *))
    62 {
    63     qsort(base, nmemb, size, compare);
    64 }
    65 #else
    66 
    67 #ifdef assert
    68 #undef assert
    69 #endif
    70 #define assert(X) SDL_assert(X)
    71 #ifdef malloc
    72 #undef malloc
    73 #endif
    74 #define malloc	SDL_malloc
    75 #ifdef free
    76 #undef free
    77 #endif
    78 #define free	SDL_free
    79 #ifdef memcpy
    80 #undef memcpy
    81 #endif
    82 #define memcpy	SDL_memcpy
    83 #ifdef memmove
    84 #undef memmove
    85 #endif
    86 #define memmove	SDL_memmove
    87 #ifdef qsort
    88 #undef qsort
    89 #endif
    90 #define qsort	SDL_qsort
    91 
    92 static const char _ID[] = "<qsort.c gjm 1.12 1998-03-19>";
    93 
    94 /* How many bytes are there per word? (Must be a power of 2,
    95  * and must in fact equal sizeof(int).)
    96  */
    97 #define WORD_BYTES sizeof(int)
    98 
    99 /* How big does our stack need to be? Answer: one entry per
   100  * bit in a |size_t|.
   101  */
   102 #define STACK_SIZE (8*sizeof(size_t))
   103 
   104 /* Different situations have slightly different requirements,
   105  * and we make life epsilon easier by using different truncation
   106  * points for the three different cases.
   107  * So far, I have tuned TRUNC_words and guessed that the same
   108  * value might work well for the other two cases. Of course
   109  * what works well on my machine might work badly on yours.
   110  */
   111 #define TRUNC_nonaligned	12
   112 #define TRUNC_aligned		12
   113 #define TRUNC_words		12*WORD_BYTES   /* nb different meaning */
   114 
   115 /* We use a simple pivoting algorithm for shortish sub-arrays
   116  * and a more complicated one for larger ones. The threshold
   117  * is PIVOT_THRESHOLD.
   118  */
   119 #define PIVOT_THRESHOLD 40
   120 
   121 typedef struct
   122 {
   123     char *first;
   124     char *last;
   125 } stack_entry;
   126 #define pushLeft {stack[stacktop].first=ffirst;stack[stacktop++].last=last;}
   127 #define pushRight {stack[stacktop].first=first;stack[stacktop++].last=llast;}
   128 #define doLeft {first=ffirst;llast=last;continue;}
   129 #define doRight {ffirst=first;last=llast;continue;}
   130 #define pop {if (--stacktop<0) break;\
   131   first=ffirst=stack[stacktop].first;\
   132   last=llast=stack[stacktop].last;\
   133   continue;}
   134 
   135 /* Some comments on the implementation.
   136  * 1. When we finish partitioning the array into "low"
   137  *    and "high", we forget entirely about short subarrays,
   138  *    because they'll be done later by insertion sort.
   139  *    Doing lots of little insertion sorts might be a win
   140  *    on large datasets for locality-of-reference reasons,
   141  *    but it makes the code much nastier and increases
   142  *    bookkeeping overhead.
   143  * 2. We always save the shorter and get to work on the
   144  *    longer. This guarantees that every time we push
   145  *    an item onto the stack its size is <= 1/2 of that
   146  *    of its parent; so the stack can't need more than
   147  *    log_2(max-array-size) entries.
   148  * 3. We choose a pivot by looking at the first, last
   149  *    and middle elements. We arrange them into order
   150  *    because it's easy to do that in conjunction with
   151  *    choosing the pivot, and it makes things a little
   152  *    easier in the partitioning step. Anyway, the pivot
   153  *    is the middle of these three. It's still possible
   154  *    to construct datasets where the algorithm takes
   155  *    time of order n^2, but it simply never happens in
   156  *    practice.
   157  * 3' Newsflash: On further investigation I find that
   158  *    it's easy to construct datasets where median-of-3
   159  *    simply isn't good enough. So on large-ish subarrays
   160  *    we do a more sophisticated pivoting: we take three
   161  *    sets of 3 elements, find their medians, and then
   162  *    take the median of those.
   163  * 4. We copy the pivot element to a separate place
   164  *    because that way we can always do our comparisons
   165  *    directly against a pointer to that separate place,
   166  *    and don't have to wonder "did we move the pivot
   167  *    element?". This makes the inner loop better.
   168  * 5. It's possible to make the pivoting even more
   169  *    reliable by looking at more candidates when n
   170  *    is larger. (Taking this to its logical conclusion
   171  *    results in a variant of quicksort that doesn't
   172  *    have that n^2 worst case.) However, the overhead
   173  *    from the extra bookkeeping means that it's just
   174  *    not worth while.
   175  * 6. This is pretty clean and portable code. Here are
   176  *    all the potential portability pitfalls and problems
   177  *    I know of:
   178  *      - In one place (the insertion sort) I construct
   179  *        a pointer that points just past the end of the
   180  *        supplied array, and assume that (a) it won't
   181  *        compare equal to any pointer within the array,
   182  *        and (b) it will compare equal to a pointer
   183  *        obtained by stepping off the end of the array.
   184  *        These might fail on some segmented architectures.
   185  *      - I assume that there are 8 bits in a |char| when
   186  *        computing the size of stack needed. This would
   187  *        fail on machines with 9-bit or 16-bit bytes.
   188  *      - I assume that if |((int)base&(sizeof(int)-1))==0|
   189  *        and |(size&(sizeof(int)-1))==0| then it's safe to
   190  *        get at array elements via |int*|s, and that if
   191  *        actually |size==sizeof(int)| as well then it's
   192  *        safe to treat the elements as |int|s. This might
   193  *        fail on systems that convert pointers to integers
   194  *        in non-standard ways.
   195  *      - I assume that |8*sizeof(size_t)<=INT_MAX|. This
   196  *        would be false on a machine with 8-bit |char|s,
   197  *        16-bit |int|s and 4096-bit |size_t|s. :-)
   198  */
   199 
   200 /* The recursion logic is the same in each case: */
   201 #define Recurse(Trunc)				\
   202       { size_t l=last-ffirst,r=llast-first;	\
   203         if (l<Trunc) {				\
   204           if (r>=Trunc) doRight			\
   205           else pop				\
   206         }					\
   207         else if (l<=r) { pushLeft; doRight }	\
   208         else if (r>=Trunc) { pushRight; doLeft }\
   209         else doLeft				\
   210       }
   211 
   212 /* and so is the pivoting logic: */
   213 #define Pivot(swapper,sz)			\
   214   if ((size_t)(last-first)>PIVOT_THRESHOLD*sz) mid=pivot_big(first,mid,last,sz,compare);\
   215   else {	\
   216     if (compare(first,mid)<0) {			\
   217       if (compare(mid,last)>0) {		\
   218         swapper(mid,last);			\
   219         if (compare(first,mid)>0) swapper(first,mid);\
   220       }						\
   221     }						\
   222     else {					\
   223       if (compare(mid,last)>0) swapper(first,last)\
   224       else {					\
   225         swapper(first,mid);			\
   226         if (compare(mid,last)>0) swapper(mid,last);\
   227       }						\
   228     }						\
   229     first+=sz; last-=sz;			\
   230   }
   231 
   232 #ifdef DEBUG_QSORT
   233 #include <stdio.h>
   234 #endif
   235 
   236 /* and so is the partitioning logic: */
   237 #define Partition(swapper,sz) {			\
   238   int swapped=0;				\
   239   do {						\
   240     while (compare(first,pivot)<0) first+=sz;	\
   241     while (compare(pivot,last)<0) last-=sz;	\
   242     if (first<last) {				\
   243       swapper(first,last); swapped=1;		\
   244       first+=sz; last-=sz; }			\
   245     else if (first==last) { first+=sz; last-=sz; break; }\
   246   } while (first<=last);			\
   247   if (!swapped) pop				\
   248 }
   249 
   250 /* and so is the pre-insertion-sort operation of putting
   251  * the smallest element into place as a sentinel.
   252  * Doing this makes the inner loop nicer. I got this
   253  * idea from the GNU implementation of qsort().
   254  */
   255 #define PreInsertion(swapper,limit,sz)		\
   256   first=base;					\
   257   last=first + (nmemb>limit ? limit : nmemb-1)*sz;\
   258   while (last!=base) {				\
   259     if (compare(first,last)>0) first=last;	\
   260     last-=sz; }					\
   261   if (first!=base) swapper(first,(char*)base);
   262 
   263 /* and so is the insertion sort, in the first two cases: */
   264 #define Insertion(swapper)			\
   265   last=((char*)base)+nmemb*size;		\
   266   for (first=((char*)base)+size;first!=last;first+=size) {	\
   267     char *test;					\
   268     /* Find the right place for |first|.	\
   269      * My apologies for var reuse. */		\
   270     for (test=first-size;compare(test,first)>0;test-=size) ;	\
   271     test+=size;					\
   272     if (test!=first) {				\
   273       /* Shift everything in [test,first)	\
   274        * up by one, and place |first|		\
   275        * where |test| is. */			\
   276       memcpy(pivot,first,size);			\
   277       memmove(test+size,test,first-test);	\
   278       memcpy(test,pivot,size);			\
   279     }						\
   280   }
   281 
   282 #define SWAP_nonaligned(a,b) { \
   283   register char *aa=(a),*bb=(b); \
   284   register size_t sz=size; \
   285   do { register char t=*aa; *aa++=*bb; *bb++=t; } while (--sz); }
   286 
   287 #define SWAP_aligned(a,b) { \
   288   register int *aa=(int*)(a),*bb=(int*)(b); \
   289   register size_t sz=size; \
   290   do { register int t=*aa;*aa++=*bb; *bb++=t; } while (sz-=WORD_BYTES); }
   291 
   292 #define SWAP_words(a,b) { \
   293   register int t=*((int*)a); *((int*)a)=*((int*)b); *((int*)b)=t; }
   294 
   295 /* ---------------------------------------------------------------------- */
   296 
   297 static char *
   298 pivot_big(char *first, char *mid, char *last, size_t size,
   299           int compare(const void *, const void *))
   300 {
   301     size_t d = (((last - first) / size) >> 3) * size;
   302     char *m1, *m2, *m3;
   303     {
   304         char *a = first, *b = first + d, *c = first + 2 * d;
   305 #ifdef DEBUG_QSORT
   306         fprintf(stderr, "< %d %d %d\n", *(int *) a, *(int *) b, *(int *) c);
   307 #endif
   308         m1 = compare(a, b) < 0 ?
   309             (compare(b, c) < 0 ? b : (compare(a, c) < 0 ? c : a))
   310             : (compare(a, c) < 0 ? a : (compare(b, c) < 0 ? c : b));
   311     }
   312     {
   313         char *a = mid - d, *b = mid, *c = mid + d;
   314 #ifdef DEBUG_QSORT
   315         fprintf(stderr, ". %d %d %d\n", *(int *) a, *(int *) b, *(int *) c);
   316 #endif
   317         m2 = compare(a, b) < 0 ?
   318             (compare(b, c) < 0 ? b : (compare(a, c) < 0 ? c : a))
   319             : (compare(a, c) < 0 ? a : (compare(b, c) < 0 ? c : b));
   320     }
   321     {
   322         char *a = last - 2 * d, *b = last - d, *c = last;
   323 #ifdef DEBUG_QSORT
   324         fprintf(stderr, "> %d %d %d\n", *(int *) a, *(int *) b, *(int *) c);
   325 #endif
   326         m3 = compare(a, b) < 0 ?
   327             (compare(b, c) < 0 ? b : (compare(a, c) < 0 ? c : a))
   328             : (compare(a, c) < 0 ? a : (compare(b, c) < 0 ? c : b));
   329     }
   330 #ifdef DEBUG_QSORT
   331     fprintf(stderr, "-> %d %d %d\n", *(int *) m1, *(int *) m2, *(int *) m3);
   332 #endif
   333     return compare(m1, m2) < 0 ?
   334         (compare(m2, m3) < 0 ? m2 : (compare(m1, m3) < 0 ? m3 : m1))
   335         : (compare(m1, m3) < 0 ? m1 : (compare(m2, m3) < 0 ? m3 : m2));
   336 }
   337 
   338 /* ---------------------------------------------------------------------- */
   339 
   340 static void
   341 qsort_nonaligned(void *base, size_t nmemb, size_t size,
   342                  int (*compare) (const void *, const void *))
   343 {
   344 
   345     stack_entry stack[STACK_SIZE];
   346     int stacktop = 0;
   347     char *first, *last;
   348     char *pivot = malloc(size);
   349     size_t trunc = TRUNC_nonaligned * size;
   350     assert(pivot != 0);
   351 
   352     first = (char *) base;
   353     last = first + (nmemb - 1) * size;
   354 
   355     if ((size_t) (last - first) > trunc) {
   356         char *ffirst = first, *llast = last;
   357         while (1) {
   358             /* Select pivot */
   359             {
   360                 char *mid = first + size * ((last - first) / size >> 1);
   361                 Pivot(SWAP_nonaligned, size);
   362                 memcpy(pivot, mid, size);
   363             }
   364             /* Partition. */
   365             Partition(SWAP_nonaligned, size);
   366             /* Prepare to recurse/iterate. */
   367         Recurse(trunc)}
   368     }
   369     PreInsertion(SWAP_nonaligned, TRUNC_nonaligned, size);
   370     Insertion(SWAP_nonaligned);
   371     free(pivot);
   372 }
   373 
   374 static void
   375 qsort_aligned(void *base, size_t nmemb, size_t size,
   376               int (*compare) (const void *, const void *))
   377 {
   378 
   379     stack_entry stack[STACK_SIZE];
   380     int stacktop = 0;
   381     char *first, *last;
   382     char *pivot = malloc(size);
   383     size_t trunc = TRUNC_aligned * size;
   384     assert(pivot != 0);
   385 
   386     first = (char *) base;
   387     last = first + (nmemb - 1) * size;
   388 
   389     if ((size_t) (last - first) > trunc) {
   390         char *ffirst = first, *llast = last;
   391         while (1) {
   392             /* Select pivot */
   393             {
   394                 char *mid = first + size * ((last - first) / size >> 1);
   395                 Pivot(SWAP_aligned, size);
   396                 memcpy(pivot, mid, size);
   397             }
   398             /* Partition. */
   399             Partition(SWAP_aligned, size);
   400             /* Prepare to recurse/iterate. */
   401         Recurse(trunc)}
   402     }
   403     PreInsertion(SWAP_aligned, TRUNC_aligned, size);
   404     Insertion(SWAP_aligned);
   405     free(pivot);
   406 }
   407 
   408 static void
   409 qsort_words(void *base, size_t nmemb,
   410             int (*compare) (const void *, const void *))
   411 {
   412 
   413     stack_entry stack[STACK_SIZE];
   414     int stacktop = 0;
   415     char *first, *last;
   416     char *pivot = malloc(WORD_BYTES);
   417     assert(pivot != 0);
   418 
   419     first = (char *) base;
   420     last = first + (nmemb - 1) * WORD_BYTES;
   421 
   422     if (last - first > TRUNC_words) {
   423         char *ffirst = first, *llast = last;
   424         while (1) {
   425 #ifdef DEBUG_QSORT
   426             fprintf(stderr, "Doing %d:%d: ",
   427                     (first - (char *) base) / WORD_BYTES,
   428                     (last - (char *) base) / WORD_BYTES);
   429 #endif
   430             /* Select pivot */
   431             {
   432                 char *mid =
   433                     first + WORD_BYTES * ((last - first) / (2 * WORD_BYTES));
   434                 Pivot(SWAP_words, WORD_BYTES);
   435                 *(int *) pivot = *(int *) mid;
   436             }
   437 #ifdef DEBUG_QSORT
   438             fprintf(stderr, "pivot=%d\n", *(int *) pivot);
   439 #endif
   440             /* Partition. */
   441             Partition(SWAP_words, WORD_BYTES);
   442             /* Prepare to recurse/iterate. */
   443         Recurse(TRUNC_words)}
   444     }
   445     PreInsertion(SWAP_words, (TRUNC_words / WORD_BYTES), WORD_BYTES);
   446     /* Now do insertion sort. */
   447     last = ((char *) base) + nmemb * WORD_BYTES;
   448     for (first = ((char *) base) + WORD_BYTES; first != last;
   449          first += WORD_BYTES) {
   450         /* Find the right place for |first|. My apologies for var reuse */
   451         int *pl = (int *) (first - WORD_BYTES), *pr = (int *) first;
   452         *(int *) pivot = *(int *) first;
   453         for (; compare(pl, pivot) > 0; pr = pl, --pl) {
   454             *pr = *pl;
   455         }
   456         if (pr != (int *) first)
   457             *pr = *(int *) pivot;
   458     }
   459     free(pivot);
   460 }
   461 
   462 /* ---------------------------------------------------------------------- */
   463 
   464 void
   465 qsort(void *base, size_t nmemb, size_t size,
   466       int (*compare) (const void *, const void *))
   467 {
   468 
   469     if (nmemb <= 1)
   470         return;
   471     if (((uintptr_t) base | size) & (WORD_BYTES - 1))
   472         qsort_nonaligned(base, nmemb, size, compare);
   473     else if (size != WORD_BYTES)
   474         qsort_aligned(base, nmemb, size, compare);
   475     else
   476         qsort_words(base, nmemb, compare);
   477 }
   478 
   479 #endif /* !SDL_qsort */
   480 
   481 /* vi: set ts=4 sw=4 expandtab: */