Re: [PATCH 2/4] introduce gcc_stablesort

[Alexander Monakov <amonakov at ispras dot ru>]

This adds a stable sort to sort.cc: mergesort implementation is stable, so
we just need network sort limited to sizes 2-3 to get the complete sort stable.

As I don't want to duplicate code for this, I've chosen to have gcc_qsort
accept bit-inverted 'size' parameter to request stable sorting.

	* gcc/sort.cc (struct sort_ctx): New field 'nlim'. Use it...
	(mergesort): ... here as maximum count for using netsort.
	(gcc_qsort): Set nlim to 3 if stable sort is requested.
	(gcc_stablesort): New.
	* gcc/system.h (gcc_stablesort): Declare.

diff --git a/gcc/sort.cc b/gcc/sort.cc
index 9f8ee12e13b..b3be1eac72b 100644
--- a/gcc/sort.cc
+++ b/gcc/sort.cc
@@ -55,6 +55,7 @@ struct sort_ctx
   char   *out; // output buffer
   size_t n;    // number of elements
   size_t size; // element size
+  size_t nlim; // limit for network sort
 };
 
 /* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
@@ -178,7 +179,7 @@ do {                                  \
 static void
 mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
 {
-  if (likely (n <= 5))
+  if (likely (n <= c->nlim))
     {
       c->out = out;
       c->n = n;
@@ -221,8 +222,12 @@ gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
 {
   if (n < 2)
     return;
+  size_t nlim = 5;
+  bool stable = (ssize_t) size < 0;
+  if (stable)
+    nlim = 3, size = ~size;
   char *base = (char *)vbase;
-  sort_ctx c = {cmp, base, n, size};
+  sort_ctx c = {cmp, base, n, size, nlim};
   long long scratch[32];
   size_t bufsz = (n / 2) * size;
   void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
@@ -233,3 +238,9 @@ gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
   qsort_chk (vbase, n, size, cmp);
 #endif
 }
+
+void
+gcc_stablesort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
+{
+  gcc_qsort (vbase, n, ~size, cmp);
+}
diff --git a/gcc/system.h b/gcc/system.h
index 203c6a4f0cf..100feb567c9 100644
--- a/gcc/system.h
+++ b/gcc/system.h
@@ -1202,6 +1202,8 @@ helper_const_non_const_cast (const char *p)
    corresponding to vec::qsort (cmp): they use C qsort internally anyway.  */
 void qsort_chk (void *, size_t, size_t, int (*)(const void *, const void *));
 void gcc_qsort (void *, size_t, size_t, int (*)(const void *, const void *));
+void gcc_stablesort (void *, size_t, size_t,
+		     int (*)(const void *, const void *));
 #define PP_5th(a1, a2, a3, a4, a5, ...) a5
 #undef qsort
 #define qsort(...) PP_5th (__VA_ARGS__, gcc_qsort, 3, 2, qsort, 0) (__VA_ARGS__)
-- 
2.13.3