[PATCH 1/2] gcc_qsort: source code changes

[Alexander Monakov <amonakov at ispras dot ru>]

	* sort.cc: New file.
        * system.h [!CHECKING_P] (qsort): Redirect to gcc_qsort.
        * vec.c (qsort_chk): Use gcc_qsort.

---
 gcc/sort.cc  | 232 +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 gcc/system.h |   7 +-
 gcc/vec.c    |   2 +-
 3 files changed, 238 insertions(+), 3 deletions(-)
 create mode 100644 gcc/sort.cc

diff --git a/gcc/sort.cc b/gcc/sort.cc
new file mode 100644
index 00000000000..4faf6d45dc6
--- /dev/null
+++ b/gcc/sort.cc
@@ -0,0 +1,232 @@
+/* Platform-independent deterministic sort function.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   Contributed by Alexander Monakov.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation; either version 3, or (at your option) any
+later version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+/* This implements a sort function suitable for GCC use cases:
+   - signature-compatible to C qsort, but relaxed contract:
+     - may apply the comparator to elements in a temporary buffer
+     - may abort on allocation failure
+   - deterministic (but not necessarily stable)
+   - fast, especially for common cases (0-5 elements of size 8 or 4)
+
+   The implementation uses a network sort for up to 5 elements and
+   a merge sort on top of that.  Neither stage has branches depending on
+   comparator result, trading extra arithmetic for branch mispredictions.  */
+
+#ifdef GENERATOR_FILE
+#include "bconfig.h"
+#else
+#include "config.h"
+#endif
+
+#include "system.h"
+
+#define likely(cond) __builtin_expect ((cond), 1)
+
+#ifdef __GNUC__
+#define noinline __attribute__ ((__noinline__))
+#else
+#define noinline
+#endif
+
+/* C-style qsort comparator function type.  */
+typedef int cmp_fn (const void *, const void *);
+
+/* Structure holding read-mostly (read-only in netsort) context. */
+struct sort_ctx
+{
+  cmp_fn *cmp; // pointer to comparator
+  char   *out; // output buffer
+  size_t n;    // number of elements
+  size_t size; // element size
+};
+
+/* Helper for netsort. Permute, possibly in-place, 2 or 3 elements,
+   placing E0 to C->OUT, E1 to C->OUT + C->SIZE, and so on. */
+static void
+reorder23 (sort_ctx *c, char *e0, char *e1, char *e2)
+{
+#define REORDER_23(SIZE, STRIDE, OFFSET)        \
+do {                                            \
+  size_t t0, t1;                                \
+  memcpy (&t0, e0 + OFFSET, SIZE);              \
+  memcpy (&t1, e1 + OFFSET, SIZE);              \
+  char *out = c->out + OFFSET;                  \
+  if (likely (c->n == 3))                       \
+    memcpy (out + 2*STRIDE, e2 + OFFSET, SIZE); \
+  memcpy (out, &t0, SIZE); out += STRIDE;       \
+  memcpy (out, &t1, SIZE);                      \
+} while (0)
+
+  if (sizeof (size_t) == 8 && likely (c->size == 8))
+    REORDER_23 (8, 8, 0);
+  else if (likely (c->size == 4))
+    REORDER_23 (4, 4, 0);
+  else
+    {
+      size_t offset = 0, step = sizeof (size_t);
+      for (; offset + step <= c->size; offset += step)
+	REORDER_23 (step, c->size, offset);
+      for (; offset < c->size; offset++)
+	REORDER_23 (1, c->size, offset);
+    }
+}
+
+/* Like reorder23, but permute 4 or 5 elements. */
+static void
+reorder45 (sort_ctx *c, char *e0, char *e1, char *e2, char *e3, char *e4)
+{
+#define REORDER_45(SIZE, STRIDE, OFFSET)        \
+do {                                            \
+  size_t t0, t1, t2, t3;                        \
+  memcpy (&t0, e0 + OFFSET, SIZE);              \
+  memcpy (&t1, e1 + OFFSET, SIZE);              \
+  memcpy (&t2, e2 + OFFSET, SIZE);              \
+  memcpy (&t3, e3 + OFFSET, SIZE);              \
+  char *out = c->out + OFFSET;                  \
+  if (likely (c->n == 5))                       \
+    memcpy (out + 4*STRIDE, e4 + OFFSET, SIZE); \
+  memcpy (out, &t0, SIZE); out += STRIDE;       \
+  memcpy (out, &t1, SIZE); out += STRIDE;       \
+  memcpy (out, &t2, SIZE); out += STRIDE;       \
+  memcpy (out, &t3, SIZE);                      \
+} while (0)
+
+  if (sizeof (size_t) == 8 && likely (c->size == 8))
+    REORDER_45 (8, 8, 0);
+  else if (likely(c->size == 4))
+    REORDER_45 (4, 4, 0);
+  else
+    {
+      size_t offset = 0, step = sizeof (size_t);
+      for (; offset + step <= c->size; offset += step)
+	REORDER_45 (step, c->size, offset);
+      for (; offset < c->size; offset++)
+	REORDER_45 (1, c->size, offset);
+    }
+}
+
+/* Helper for netsort. Invoke comparator CMP on E0 and E1.
+   Return E0^E1 if E0 compares less than E1, zero otherwise.
+   This is noinline to avoid code growth and confine invocation
+   to a single call site, assisting indirect branch prediction. */
+noinline static intptr_t
+cmp1 (char *e0, char *e1, cmp_fn *cmp)
+{
+  intptr_t x = (intptr_t)e0 ^ (intptr_t)e1;
+  return x & (cmp (e0, e1) >> 31);
+}
+
+/* Execute network sort on 2 to 5 elements from IN, placing them into C->OUT.
+   IN may be equal to C->OUT, in which case elements are sorted in place.  */
+static void
+netsort (char *in, sort_ctx *c)
+{
+#define CMP(e0, e1)                   \
+do {                                  \
+  intptr_t x = cmp1 (e1, e0, c->cmp); \
+  e0 = (char *)((intptr_t)e0 ^ x);    \
+  e1 = (char *)((intptr_t)e1 ^ x);    \
+} while (0)
+
+  char *e0 = in, *e1 = e0 + c->size, *e2 = e1 + c->size;
+  CMP (e0, e1);
+  if (likely (c->n == 3))
+    {
+      CMP (e1, e2);
+      CMP (e0, e1);
+    }
+  if (c->n <= 3)
+    return reorder23 (c, e0, e1, e2);
+  char *e3 = e2 + c->size, *e4 = e3 + c->size;
+  if (likely (c->n == 5))
+    {
+      CMP (e3, e4);
+      CMP (e2, e4);
+    }
+  CMP (e2, e3);
+  if (likely (c->n == 5))
+    {
+      CMP (e0, e3);
+      CMP (e1, e4);
+    }
+  CMP (e0, e2);
+  CMP (e1, e3);
+  CMP (e1, e2);
+  reorder45 (c, e0, e1, e2, e3, e4);
+}
+
+/* Execute merge sort on N elements from IN, placing them into OUT,
+   using TMP as temporary storage if IN is equal to OUT.
+   This is a stable sort if netsort is used only for 2 or 3 elements. */
+static void
+mergesort (char *in, sort_ctx *c, size_t n, char *out, char *tmp)
+{
+  if (likely (n <= 5))
+    {
+      c->out = out;
+      c->n = n;
+      return netsort (in, c);
+    }
+  size_t nl = n / 2, nr = n - nl, sz = nl * c->size;
+  char *mid = in + sz, *r = out + sz, *l = in == out ? tmp : in;
+  /* Sort the right half, outputting to right half of OUT. */
+  mergesort (mid, c, nr, r, tmp);
+  /* Sort the left half, leaving left half of OUT free.  */
+  mergesort (in, c, nl, l, mid);
+  /* Merge sorted halves given by L, R to [OUT, END). */
+#define MERGE_ELTSIZE(SIZE)                     \
+do {                                            \
+  intptr_t mr = c->cmp (r, l) >> 31;            \
+  intptr_t lr = (intptr_t)l ^ (intptr_t)r;      \
+  lr = (intptr_t)l ^ (lr & mr);                 \
+  out = (char *)memcpy (out, (char *)lr, SIZE); \
+  out += SIZE;                                  \
+  r += mr & SIZE;                               \
+  if (r == out) return;                         \
+  l += ~mr & SIZE;                              \
+} while (r != end)
+
+  if (likely (c->cmp(r, l + (r - out) - c->size) < 0))
+    {
+      char *end = out + n * c->size;
+      if (sizeof (size_t) == 8 && likely (c->size == 8))
+	MERGE_ELTSIZE (8);
+      else if (likely (c->size == 4))
+	MERGE_ELTSIZE (4);
+      else
+	MERGE_ELTSIZE (c->size);
+    }
+  memcpy (out, l, r - out);
+}
+
+void
+gcc_qsort (void *vbase, size_t n, size_t size, cmp_fn *cmp)
+{
+  if (n < 2)
+    return;
+  char *base = (char *)vbase;
+  sort_ctx c = {cmp, base, n, size};
+  long long scratch[32];
+  size_t bufsz = (n / 2) * size;
+  void *buf = bufsz <= sizeof scratch ? scratch : xmalloc (bufsz);
+  mergesort (base, &c, n, base, (char *)buf);
+  if (buf != scratch)
+    free (buf);
+}
diff --git a/gcc/system.h b/gcc/system.h
index 4abc321c71d..88dffccb8ab 100644
--- a/gcc/system.h
+++ b/gcc/system.h
@@ -1202,11 +1202,14 @@ helper_const_non_const_cast (const char *p)
 /* qsort comparator consistency checking: except in release-checking compilers,
    redirect 4-argument qsort calls to qsort_chk; keep 1-argument invocations
    corresponding to vec::qsort (cmp): they use C qsort internally anyway.  */
-#if CHECKING_P
+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 *));
 #define PP_5th(a1, a2, a3, a4, a5, ...) a5
 #undef qsort
+#if CHECKING_P
 #define qsort(...) PP_5th (__VA_ARGS__, qsort_chk, 3, 2, qsort, 0) (__VA_ARGS__)
-void qsort_chk (void *, size_t, size_t, int (*)(const void *, const void *));
+#else
+#define qsort(...) PP_5th (__VA_ARGS__, gcc_qsort, 3, 2, qsort, 0) (__VA_ARGS__)
 #endif
 
 #endif /* ! GCC_SYSTEM_H */
diff --git a/gcc/vec.c b/gcc/vec.c
index 11924a80a2d..2941715a34a 100644
--- a/gcc/vec.c
+++ b/gcc/vec.c
@@ -215,7 +215,7 @@ void
 qsort_chk (void *base, size_t n, size_t size,
 	   int (*cmp)(const void *, const void *))
 {
-  (qsort) (base, n, size, cmp);
+  gcc_qsort (base, n, size, cmp);
 #if 0
 #define LIM(n) (n)
 #else
-- 
2.13.3