RetroArch/rewind.c

/*  RetroArch - A frontend for libretro.
 *  Copyright (C) 2010-2014 - Hans-Kristian Arntzen
 *  Copyright (C) 2014 - Alfred Agrell
 *
 *  RetroArch 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 Found-
 *  ation, either version 3 of the License, or (at your option) any later version.
 *
 *  RetroArch 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 RetroArch.
 *  If not, see <http://www.gnu.org/licenses/>.
 */

#define __STDC_LIMIT_MACROS
#include "rewind.h"
#include "performance.h"
#include <stdlib.h>
#include <stdint.h>
#include <string.h>

#ifndef UINT16_MAX
#define UINT16_MAX 0xffff
#endif
#ifndef UINT32_MAX
#define UINT32_MAX 0xffffffffu
#endif

#undef CPU_X86
#if defined(__x86_64__) || defined(__i386__) || defined(__i486__) || defined(__i686__)
#define CPU_X86
#endif

// Other arches SIGBUS (usually) on unaligned accesses.
#ifndef CPU_X86
#define NO_UNALIGNED_MEM
#endif

// Format per frame:
// size nextstart;
// repeat {
//   uint16 numchanged; // everything is counted in units of uint16
//   if (numchanged) {
//     uint16 numunchanged; // skip these before handling numchanged
//     uint16[numchanged] changeddata;
//   }
//   else
//   {
//     uint32 numunchanged;
//     if (!numunchanged) break;
//   }
// }
// size thisstart;
//
// The start offsets point to 'nextstart' of any given compressed frame.
// Each uint16 is stored native endian; anything that claims any other endianness refers to the endianness of this specific item.
// The uint32 is stored little endian.
// Each size value is stored native endian if alignment is not enforced; if it is, they're little endian.
// The start of the buffer contains a size pointing to the end of the buffer; the end points to its start.
// Wrapping is handled by returning to the start of the buffer if the compressed data could potentially hit the edge;
// if the compressed data could potentially overwrite the tail pointer, the tail retreats until it can no longer collide.
// This means that on average, ~2 * maxcompsize is unused at any given moment.

// These are called very few constant times per frame, keep it as simple as possible.
static inline void write_size_t(void *ptr, size_t val)
{
   memcpy(ptr, &val, sizeof(val));
}
static inline size_t read_size_t(const void *ptr)
{
   size_t ret;
   memcpy(&ret, ptr, sizeof(ret));
   return ret;
}

struct state_manager
{
   uint8_t *data;
   size_t capacity;
   uint8_t *head; // Reading and writing is done here.
   uint8_t *tail; // If head comes close to this, discard a frame.

   uint8_t *thisblock;
   uint8_t *nextblock;

   size_t blocksize; // This one is runded up from reset::blocksize.
   size_t maxcompsize; // size_t + (blocksize + 131071) / 131072 * (blocksize + u16 + u16) + u16 + u32 + size_t (yes, the math is a bit ugly).

   unsigned entries;
   bool thisblock_valid;
};

state_manager_t *state_manager_new(size_t state_size, size_t buffer_size)
{
   state_manager_t *state = (state_manager_t*)calloc(1, sizeof(*state));
   if (!state)
      return NULL;

   size_t newblocksize = ((state_size - 1) | (sizeof(uint16_t) - 1)) + 1;
   state->blocksize = newblocksize;

   const int maxcblkcover = UINT16_MAX * sizeof(uint16_t);
   const int maxcblks = (state->blocksize + maxcblkcover - 1) / maxcblkcover;
   state->maxcompsize = state->blocksize + maxcblks * sizeof(uint16_t) * 2 + sizeof(uint16_t) + sizeof(uint32_t) + sizeof(size_t) * 2;

   state->data = (uint8_t*)malloc(buffer_size);

   state->thisblock = (uint8_t*)calloc(state->blocksize + sizeof(uint16_t) * 4 + 16, 1);
   state->nextblock = (uint8_t*)calloc(state->blocksize + sizeof(uint16_t) * 4 + 16, 1);
   if (!state->data || !state->thisblock || !state->nextblock)
      goto error;

   // Force in a different byte at the end, so we don't need to check bounds in the innermost loop (it's expensive).
   // There is also a large amount of data that's the same, to stop the other scan
   // There is also some padding at the end. This is so we don't read outside the buffer end if we're reading in large blocks;
   // it doesn't make any difference to us, but sacrificing 16 bytes to get Valgrind happy is worth it.
   *(uint16_t*)(state->thisblock + state->blocksize + sizeof(uint16_t) * 3) = 0xFFFF;
   *(uint16_t*)(state->nextblock + state->blocksize + sizeof(uint16_t) * 3) = 0x0000;

   state->capacity = buffer_size;

   state->head = state->data + sizeof(size_t);
   state->tail = state->data + sizeof(size_t);

   return state;

error:
   state_manager_free(state);
   return NULL;
}

void state_manager_free(state_manager_t *state)
{
   free(state->data);
   free(state->thisblock);
   free(state->nextblock);
   free(state);
}

bool state_manager_pop(state_manager_t *state, const void **data)
{
   *data = NULL;

   if (state->thisblock_valid)
   {
      state->thisblock_valid = false;
      state->entries--;
      *data = state->thisblock;
      return true;
   }

   if (state->head == state->tail)
      return false;

   size_t start = read_size_t(state->head - sizeof(size_t));
   state->head = state->data + start;

   const uint8_t *compressed = state->data + start + sizeof(size_t);
   uint8_t *out = state->thisblock;

   // Begin decompression code
   // out is the last pushed (or returned) state
   const uint16_t *compressed16 = (const uint16_t*)compressed;
   uint16_t *out16 = (uint16_t*)out;

   for (;;)
   {
      uint16_t i;
      uint16_t numchanged = *(compressed16++);
      if (numchanged)
      {
         out16 += *compressed16++;
         // We could do memcpy, but it seems that memcpy has a constant-per-call overhead that actually shows up.
         // Our average size in here seems to be 8 or something.
         // Therefore, we do something with lower overhead.
         for (i = 0; i < numchanged; i++)
            out16[i] = compressed16[i];

         compressed16 += numchanged;
         out16 += numchanged;
      }
      else
      {
         uint32_t numunchanged = compressed16[0] | (compressed16[1] << 16);
         if (!numunchanged)
            break;
         compressed16 += 2;
         out16 += numunchanged;
      }
   }
   // End decompression code

   state->entries--;
   *data = state->thisblock;
   return true;
}

void state_manager_push_where(state_manager_t *state, void **data)
{
   // We need to ensure we have an uncompressed copy of the last pushed state, or we could
   // end up applying a 'patch' to wrong savestate, and that'd blow up rather quickly.
   if (!state->thisblock_valid) 
   {
      const void *ignored;
      if (state_manager_pop(state, &ignored))
      {
         state->thisblock_valid = true;
         state->entries++;
      }
   }
   
   *data = state->nextblock;
}

#if __SSE2__
#if defined(__GNUC__)
static inline int compat_ctz(unsigned x)
{
   return __builtin_ctz(x);
}
#else
// Only checks at nibble granularity, because that's what we need.
static inline int compat_ctz(unsigned x)
{
   if (x & 0x000f)
      return 0;
   if (x & 0x00f0)
      return 4;
   if (x & 0x0f00)
      return 8;
   if (x & 0xf000)
      return 12;
   return 16;
}
#endif

#include <emmintrin.h>
// There's no equivalent in libc, you'd think so ... std::mismatch exists, but it's not optimized at all. :(
static inline size_t find_change(const uint16_t *a, const uint16_t *b)
{
	const __m128i *a128 = (const __m128i*)a;
	const __m128i *b128 = (const __m128i*)b;
	
   for (;;)
	{
		__m128i v0 = _mm_loadu_si128(a128);
		__m128i v1 = _mm_loadu_si128(b128);
		__m128i c = _mm_cmpeq_epi32(v0, v1);

		uint32_t mask = _mm_movemask_epi8(c);
		if (mask != 0xffff) // Something has changed, figure out where.
		{
			size_t ret = (((uint8_t*)a128 - (uint8_t*)a) | (compat_ctz(~mask))) >> 1;
			return ret | (a[ret] == b[ret]);
		}

		a128++;
		b128++;
	}
}
#else
static inline size_t find_change(const uint16_t *a, const uint16_t *b)
{
	const uint16_t *a_org = a;
#ifdef NO_UNALIGNED_MEM
	while (((uintptr_t)a & (sizeof(size_t) - 1)) && *a == *b)
	{
		a++;
		b++;
	}
	if (*a == *b)
#endif
	{
		const size_t *a_big = (const size_t*)a;
		const size_t *b_big = (const size_t*)b;
		
		while (*a_big == *b_big)
		{
			a_big++;
			b_big++;
		}
		a = (const uint16_t*)a_big;
		b = (const uint16_t*)b_big;
		
		while (*a == *b)
		{
			a++;
			b++;
		}
	}
	return a - a_org;
}
#endif

static inline size_t find_same(const uint16_t *a, const uint16_t *b)
{
	const uint16_t *a_org = a;
#ifdef NO_UNALIGNED_MEM
	if (((uintptr_t)a & (sizeof(uint32_t) - 1)) && *a != *b)
	{
		a++;
		b++;
	}
	if (*a != *b)
#endif
	{
		// With this, it's random whether two consecutive identical words are caught.
		// Luckily, compression rate is the same for both cases, and three is always caught.
		// (We prefer to miss two-word blocks, anyways; fewer iterations of the outer loop, as well as in the decompressor.)
		const uint32_t *a_big = (const uint32_t*)a;
		const uint32_t *b_big = (const uint32_t*)b;
		
		while (*a_big != *b_big)
		{
			a_big++;
			b_big++;
		}
		a = (const uint16_t*)a_big;
		b = (const uint16_t*)b_big;
		
		if (a != a_org && a[-1] == b[-1])
		{
			a--;
			b--;
		}
	}
	return a - a_org;
}

void state_manager_push_do(state_manager_t *state)
{
   if (state->thisblock_valid)
   {
      if (state->capacity < sizeof(size_t) + state->maxcompsize)
         return;

recheckcapacity:;

      size_t headpos = state->head - state->data;
      size_t tailpos = state->tail - state->data;
      size_t remaining = (tailpos + state->capacity - sizeof(size_t) - headpos - 1) % state->capacity + 1;
      if (remaining <= state->maxcompsize)
      {
         state->tail = state->data + read_size_t(state->tail);
         state->entries--;
         goto recheckcapacity;
      }

      RARCH_PERFORMANCE_INIT(gen_deltas);
      RARCH_PERFORMANCE_START(gen_deltas);

      const uint8_t *oldb = state->thisblock;
      const uint8_t *newb = state->nextblock;
      uint8_t *compressed = state->head + sizeof(size_t);

      // Begin compression code; 'compressed' will point to the end of the compressed data (excluding the prev pointer).
      const uint16_t *old16 = (const uint16_t*)oldb;
      const uint16_t *new16 = (const uint16_t*)newb;
      uint16_t *compressed16 = (uint16_t*)compressed;
      size_t num16s = state->blocksize / sizeof(uint16_t);

      while (num16s)
      {
         size_t i;
         size_t skip = find_change(old16, new16);

         if (skip >= num16s)
            break;

         old16 += skip;
         new16 += skip;
         num16s -= skip;

         if (skip > UINT16_MAX)
         {
            if (skip > UINT32_MAX)
            {
               // This will make it scan the entire thing again, but it only hits on 8GB unchanged
               // data anyways, and if you're doing that, you've got bigger problems.
               skip = UINT32_MAX;
            }
            *compressed16++ = 0;
            *compressed16++ = skip;
            *compressed16++ = skip >> 16;
            skip = 0;
            continue;
         }

         size_t changed = find_same(old16, new16);
         if (changed > UINT16_MAX)
            changed = UINT16_MAX;

         *compressed16++ = changed;
         *compressed16++ = skip;

         for (i = 0; i < changed; i++)
            compressed16[i] = old16[i];

         old16 += changed;
         new16 += changed;
         num16s -= changed;
         compressed16 += changed;
      }

      compressed16[0] = 0;
      compressed16[1] = 0;
      compressed16[2] = 0;
      compressed = (uint8_t*)(compressed16 + 3);
      // End compression code.

      if (compressed - state->data + state->maxcompsize > state->capacity)
      {
         compressed = state->data;
         if (state->tail == state->data + sizeof(size_t))
            state->tail = state->data + read_size_t(state->tail);
      }
      write_size_t(compressed, state->head-state->data);
      compressed += sizeof(size_t);
      write_size_t(state->head, compressed-state->data);
      state->head = compressed;

      RARCH_PERFORMANCE_STOP(gen_deltas);
   }
   else
      state->thisblock_valid = true;

   uint8_t *swap = state->thisblock;
   state->thisblock = state->nextblock;
   state->nextblock = swap;

   state->entries++;
   return;
}

void state_manager_capacity(state_manager_t *state, unsigned *entries, size_t *bytes, bool *full)
{
   size_t headpos = state->head - state->data;
   size_t tailpos = state->tail - state->data;
   size_t remaining = (tailpos + state->capacity - sizeof(size_t) - headpos - 1) % state->capacity + 1;

   if (entries)
      *entries = state->entries;
   if (bytes)
      *bytes = state->capacity-remaining;
   if (full)
      *full = remaining <= state->maxcompsize * 2;
}