util/bufferiszero.c

   1 /*
   2  * Simple C functions to supplement the C library
   3  *
   4  * Copyright (c) 2006 Fabrice Bellard
   5  *
   6  * Permission is hereby granted, free of charge, to any person obtaining a copy
   7  * of this software and associated documentation files (the "Software"), to deal
   8  * in the Software without restriction, including without limitation the rights
   9  * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  10  * copies of the Software, and to permit persons to whom the Software is
  11  * furnished to do so, subject to the following conditions:
  12  *
  13  * The above copyright notice and this permission notice shall be included in
  14  * all copies or substantial portions of the Software.
  15  *
  16  * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  17  * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  18  * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
  19  * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  20  * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  21  * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
  22  * THE SOFTWARE.
  23  */
  24 #include "qemu/osdep.h"
  25 #include "qemu/cutils.h"
  26 #include "qemu/bswap.h"
  27
  28 static bool
  29 buffer_zero_int(const void *buf, size_t len)
  30 {
  31     if (unlikely(len < 8)) {
  32         /* For a very small buffer, simply accumulate all the bytes.  */
  33         const unsigned char *p = buf;
  34         const unsigned char *e = buf + len;
  35         unsigned char t = 0;
  36
  37         do {
  38             t |= *p++;
  39         } while (p < e);
  40
  41         return t == 0;
  42     } else {
  43         /* Otherwise, use the unaligned memory access functions to
  44            handle the beginning and end of the buffer, with a couple
  45            of loops handling the middle aligned section.  */
  46         uint64_t t = ldq_he_p(buf);
  47         const uint64_t *p = (uint64_t *)(((uintptr_t)buf + 8) & -8);
  48         const uint64_t *e = (uint64_t *)(((uintptr_t)buf + len) & -8);
  49
  50         for (; p + 8 <= e; p += 8) {
  51             __builtin_prefetch(p + 8);
  52             if (t) {
  53                 return false;
  54             }
  55             t = p[0] | p[1] | p[2] | p[3] | p[4] | p[5] | p[6] | p[7];
  56         }
  57         while (p < e) {
  58             t |= *p++;
  59         }
  60         t |= ldq_he_p(buf + len - 8);
  61
  62         return t == 0;
  63     }
  64 }
  65
  66 #if defined(CONFIG_AVX2_OPT) || defined(__SSE2__)
  67 /* Do not use push_options pragmas unnecessarily, because clang
  68  * does not support them.
  69  */
  70 #ifdef CONFIG_AVX2_OPT
  71 #pragma GCC push_options
  72 #pragma GCC target("sse2")
  73 #endif
  74 #include <emmintrin.h>
  75
  76 /* Note that each of these vectorized functions require len >= 64.  */
  77
  78 static bool
  79 buffer_zero_sse2(const void *buf, size_t len)
  80 {
  81     __m128i t = _mm_loadu_si128(buf);
  82     __m128i *p = (__m128i *)(((uintptr_t)buf + 5 * 16) & -16);
  83     __m128i *e = (__m128i *)(((uintptr_t)buf + len) & -16);
  84     __m128i zero = _mm_setzero_si128();
  85
  86     /* Loop over 16-byte aligned blocks of 64.  */
  87     while (likely(p <= e)) {
  88         __builtin_prefetch(p);
  89         t = _mm_cmpeq_epi8(t, zero);
  90         if (unlikely(_mm_movemask_epi8(t) != 0xFFFF)) {
  91             return false;
  92         }
  93         t = p[-4] | p[-3] | p[-2] | p[-1];
  94         p += 4;
  95     }
  96
  97     /* Finish the aligned tail.  */
  98     t |= e[-3];
  99     t |= e[-2];
 100     t |= e[-1];
 101
 102     /* Finish the unaligned tail.  */
 103     t |= _mm_loadu_si128(buf + len - 16);
 104
 105     return _mm_movemask_epi8(_mm_cmpeq_epi8(t, zero)) == 0xFFFF;
 106 }
 107 #ifdef CONFIG_AVX2_OPT
 108 #pragma GCC pop_options
 109 #endif
 110
 111 #ifdef CONFIG_AVX2_OPT
 112 /* Note that due to restrictions/bugs wrt __builtin functions in gcc <= 4.8,
 113  * the includes have to be within the corresponding push_options region, and
 114  * therefore the regions themselves have to be ordered with increasing ISA.
 115  */
 116 #pragma GCC push_options
 117 #pragma GCC target("sse4")
 118 #include <smmintrin.h>
 119
 120 static bool
 121 buffer_zero_sse4(const void *buf, size_t len)
 122 {
 123     __m128i t = _mm_loadu_si128(buf);
 124     __m128i *p = (__m128i *)(((uintptr_t)buf + 5 * 16) & -16);
 125     __m128i *e = (__m128i *)(((uintptr_t)buf + len) & -16);
 126
 127     /* Loop over 16-byte aligned blocks of 64.  */
 128     while (likely(p <= e)) {
 129         __builtin_prefetch(p);
 130         if (unlikely(!_mm_testz_si128(t, t))) {
 131             return false;
 132         }
 133         t = p[-4] | p[-3] | p[-2] | p[-1];
 134         p += 4;
 135     }
 136
 137     /* Finish the aligned tail.  */
 138     t |= e[-3];
 139     t |= e[-2];
 140     t |= e[-1];
 141
 142     /* Finish the unaligned tail.  */
 143     t |= _mm_loadu_si128(buf + len - 16);
 144
 145     return _mm_testz_si128(t, t);
 146 }
 147
 148 #pragma GCC pop_options
 149 #pragma GCC push_options
 150 #pragma GCC target("avx2")
 151 #include <immintrin.h>
 152
 153 static bool
 154 buffer_zero_avx2(const void *buf, size_t len)
 155 {
 156     /* Begin with an unaligned head of 32 bytes.  */
 157     __m256i t = _mm256_loadu_si256(buf);
 158     __m256i *p = (__m256i *)(((uintptr_t)buf + 5 * 32) & -32);
 159     __m256i *e = (__m256i *)(((uintptr_t)buf + len) & -32);
 160
 161     if (likely(p <= e)) {
 162         /* Loop over 32-byte aligned blocks of 128.  */
 163         do {
 164             __builtin_prefetch(p);
 165             if (unlikely(!_mm256_testz_si256(t, t))) {
 166                 return false;
 167             }
 168             t = p[-4] | p[-3] | p[-2] | p[-1];
 169             p += 4;
 170         } while (p <= e);
 171     } else {
 172         t |= _mm256_loadu_si256(buf + 32);
 173         if (len <= 128) {
 174             goto last2;
 175         }
 176     }
 177
 178     /* Finish the last block of 128 unaligned.  */
 179     t |= _mm256_loadu_si256(buf + len - 4 * 32);
 180     t |= _mm256_loadu_si256(buf + len - 3 * 32);
 181  last2:
 182     t |= _mm256_loadu_si256(buf + len - 2 * 32);
 183     t |= _mm256_loadu_si256(buf + len - 1 * 32);
 184
 185     return _mm256_testz_si256(t, t);
 186 }
 187 #pragma GCC pop_options
 188 #endif /* CONFIG_AVX2_OPT */
 189
 190 /* Note that for test_buffer_is_zero_next_accel, the most preferred
 191  * ISA must have the least significant bit.
 192  */
 193 #define CACHE_AVX2    1
 194 #define CACHE_SSE4    2
 195 #define CACHE_SSE2    4
 196
 197 /* Make sure that these variables are appropriately initialized when
 198  * SSE2 is enabled on the compiler command-line, but the compiler is
 199  * too old to support CONFIG_AVX2_OPT.
 200  */
 201 #ifdef CONFIG_AVX2_OPT
 202 # define INIT_CACHE 0
 203 # define INIT_ACCEL buffer_zero_int
 204 #else
 205 # ifndef __SSE2__
 206 #  error "ISA selection confusion"
 207 # endif
 208 # define INIT_CACHE CACHE_SSE2
 209 # define INIT_ACCEL buffer_zero_sse2
 210 #endif
 211
 212 static unsigned cpuid_cache = INIT_CACHE;
 213 static bool (*buffer_accel)(const void *, size_t) = INIT_ACCEL;
 214
 215 static void init_accel(unsigned cache)
 216 {
 217     bool (*fn)(const void *, size_t) = buffer_zero_int;
 218     if (cache & CACHE_SSE2) {
 219         fn = buffer_zero_sse2;
 220     }
 221 #ifdef CONFIG_AVX2_OPT
 222     if (cache & CACHE_SSE4) {
 223         fn = buffer_zero_sse4;
 224     }
 225     if (cache & CACHE_AVX2) {
 226         fn = buffer_zero_avx2;
 227     }
 228 #endif
 229     buffer_accel = fn;
 230 }
 231
 232 #ifdef CONFIG_AVX2_OPT
 233 #include "qemu/cpuid.h"
 234
 235 static void __attribute__((constructor)) init_cpuid_cache(void)
 236 {
 237     int max = __get_cpuid_max(0, NULL);
 238     int a, b, c, d;
 239     unsigned cache = 0;
 240
 241     if (max >= 1) {
 242         __cpuid(1, a, b, c, d);
 243         if (d & bit_SSE2) {
 244             cache |= CACHE_SSE2;
 245         }
 246         if (c & bit_SSE4_1) {
 247             cache |= CACHE_SSE4;
 248         }
 249
 250         /* We must check that AVX is not just available, but usable.  */
 251         if ((c & bit_OSXSAVE) && (c & bit_AVX) && max >= 7) {
 252             int bv;
 253             __asm("xgetbv" : "=a"(bv), "=d"(d) : "c"(0));
 254             __cpuid_count(7, 0, a, b, c, d);
 255             if ((bv & 6) == 6 && (b & bit_AVX2)) {
 256                 cache |= CACHE_AVX2;
 257             }
 258         }
 259     }
 260     cpuid_cache = cache;
 261     init_accel(cache);
 262 }
 263 #endif /* CONFIG_AVX2_OPT */
 264
 265 bool test_buffer_is_zero_next_accel(void)
 266 {
 267     /* If no bits set, we just tested buffer_zero_int, and there
 268        are no more acceleration options to test.  */
 269     if (cpuid_cache == 0) {
 270         return false;
 271     }
 272     /* Disable the accelerator we used before and select a new one.  */
 273     cpuid_cache &= cpuid_cache - 1;
 274     init_accel(cpuid_cache);
 275     return true;
 276 }
 277
 278 static bool select_accel_fn(const void *buf, size_t len)
 279 {
 280     if (likely(len >= 64)) {
 281         return buffer_accel(buf, len);
 282     }
 283     return buffer_zero_int(buf, len);
 284 }
 285
 286 #else
 287 #define select_accel_fn  buffer_zero_int
 288 bool test_buffer_is_zero_next_accel(void)
 289 {
 290     return false;
 291 }
 292 #endif
 293
 294 /*
 295  * Checks if a buffer is all zeroes
 296  */
 297 bool buffer_is_zero(const void *buf, size_t len)
 298 {
 299     if (unlikely(len == 0)) {
 300         return true;
 301     }
 302
 303     /* Fetch the beginning of the buffer while we select the accelerator.  */
 304     __builtin_prefetch(buf);
 305
 306     /* Use an optimized zero check if possible.  Note that this also
 307        includes a check for an unrolled loop over 64-bit integers.  */
 308     return select_accel_fn(buf, len);
 309 }