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Re: [PATCH 1/2] gcc_qsort: source code changes


On May 10, 2018 5:56:40 PM GMT+02:00, Alexander Monakov <amonakov@ispras.ru> wrote:
>	* sort.cc: New file.
>        * system.h [!CHECKING_P] (qsort): Redirect to gcc_qsort.
>        * vec.c (qsort_chk): Use gcc_qsort.

Just a quick first remark - how about putting this into libiberty?  And then name it xqsort? 

Richard. 

>---
>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


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