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/* Platform-independent deterministic sort function.
Copyright (C) 2018-2019 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
size_t nlim; // limit for network sort
};
/* 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(TYPE, STRIDE, OFFSET) \
do { \
TYPE t0, t1; \
memcpy (&t0, e0 + OFFSET, sizeof (TYPE)); \
memcpy (&t1, e1 + OFFSET, sizeof (TYPE)); \
char *out = c->out + OFFSET; \
if (likely (c->n == 3)) \
memmove (out + 2*STRIDE, e2 + OFFSET, sizeof (TYPE));\
memcpy (out, &t0, sizeof (TYPE)); out += STRIDE; \
memcpy (out, &t1, sizeof (TYPE)); \
} while (0)
if (likely (c->size == sizeof (size_t)))
REORDER_23 (size_t, sizeof (size_t), 0);
else if (likely (c->size == sizeof (int)))
REORDER_23 (int, sizeof (int), 0);
else
{
size_t offset = 0, step = sizeof (size_t);
for (; offset + step <= c->size; offset += step)
REORDER_23 (size_t, c->size, offset);
for (; offset < c->size; offset++)
REORDER_23 (char, 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(TYPE, STRIDE, OFFSET) \
do { \
TYPE t0, t1, t2, t3; \
memcpy (&t0, e0 + OFFSET, sizeof (TYPE)); \
memcpy (&t1, e1 + OFFSET, sizeof (TYPE)); \
memcpy (&t2, e2 + OFFSET, sizeof (TYPE)); \
memcpy (&t3, e3 + OFFSET, sizeof (TYPE)); \
char *out = c->out + OFFSET; \
if (likely (c->n == 5)) \
memmove (out + 4*STRIDE, e4 + OFFSET, sizeof (TYPE));\
memcpy (out, &t0, sizeof (TYPE)); out += STRIDE; \
memcpy (out, &t1, sizeof (TYPE)); out += STRIDE; \
memcpy (out, &t2, sizeof (TYPE)); out += STRIDE; \
memcpy (out, &t3, sizeof (TYPE)); \
} while (0)
if (likely (c->size == sizeof (size_t)))
REORDER_45 (size_t, sizeof (size_t), 0);
else if (likely(c->size == sizeof (int)))
REORDER_45 (int, sizeof (int), 0);
else
{
size_t offset = 0, step = sizeof (size_t);
for (; offset + step <= c->size; offset += step)
REORDER_45 (size_t, c->size, offset);
for (; offset < c->size; offset++)
REORDER_45 (char, 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 <= c->nlim))
{
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;
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, nlim};
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);
#if CHECKING_P
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);
}
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