/*
- * lzms-common.c
- *
- * Code shared between the compressor and decompressor for the LZMS compression
- * format.
+ * lzms-common.c - Common code for LZMS compression and decompression
*/
/*
pthread_once(&once, lzms_compute_slots);
}
-static s32
-lzms_maybe_do_x86_translation(u8 data[restrict], s32 i, s32 num_op_bytes,
- s32 * restrict closest_target_usage_p,
- s32 last_target_usages[restrict],
- s32 max_trans_offset, bool undo)
-{
- u16 pos;
-
- if (undo) {
- if (i - *closest_target_usage_p <= max_trans_offset) {
- LZMS_DEBUG("Undid x86 translation at position %d "
- "(opcode 0x%02x)", i, data[i]);
- void *p32 = &data[i + num_op_bytes];
- u32 n = get_unaligned_u32_le(p32);
- put_unaligned_u32_le(n - i, p32);
- }
- pos = i + get_unaligned_u16_le(&data[i + num_op_bytes]);
- } else {
- pos = i + get_unaligned_u16_le(&data[i + num_op_bytes]);
-
- if (i - *closest_target_usage_p <= max_trans_offset) {
- LZMS_DEBUG("Did x86 translation at position %d "
- "(opcode 0x%02x)", i, data[i]);
- void *p32 = &data[i + num_op_bytes];
- u32 n = get_unaligned_u32_le(p32);
- put_unaligned_u32_le(n + i, p32);
- }
- }
-
- i += num_op_bytes + sizeof(le32) - 1;
-
- if (i - last_target_usages[pos] <= LZMS_X86_MAX_GOOD_TARGET_OFFSET)
- *closest_target_usage_p = i;
-
- last_target_usages[pos] = i;
-
- return i + 1;
-}
-
-static inline s32
-lzms_may_x86_translate(const u8 p[restrict], s32 *restrict max_offset_ret)
-{
- /* Switch on first byte of the opcode, assuming it is really an x86
- * instruction. */
- *max_offset_ret = LZMS_X86_MAX_TRANSLATION_OFFSET;
- switch (p[0]) {
- case 0x48:
- if (p[1] == 0x8b) {
- if (p[2] == 0x5 || p[2] == 0xd) {
- /* Load relative (x86_64) */
- return 3;
- }
- } else if (p[1] == 0x8d) {
- if ((p[2] & 0x7) == 0x5) {
- /* Load effective address relative (x86_64) */
- return 3;
- }
- }
- break;
-
- case 0x4c:
- if (p[1] == 0x8d) {
- if ((p[2] & 0x7) == 0x5) {
- /* Load effective address relative (x86_64) */
- return 3;
- }
- }
- break;
-
- case 0xe8:
- /* Call relative */
- *max_offset_ret = LZMS_X86_MAX_TRANSLATION_OFFSET / 2;
- return 1;
-
- case 0xe9:
- /* Jump relative */
- *max_offset_ret = 0;
- return 5;
-
- case 0xf0:
- if (p[1] == 0x83 && p[2] == 0x05) {
- /* Lock add relative */
- return 3;
- }
- break;
-
- case 0xff:
- if (p[1] == 0x15) {
- /* Call indirect */
- return 2;
- }
- break;
- }
- *max_offset_ret = 0;
- return 1;
-}
-
/*
* Translate relative addresses embedded in x86 instructions into absolute
* addresses (@undo == %false), or undo this translation (@undo == %true).
* Note: this filter runs unconditionally and uses a custom algorithm to
* detect data regions that probably contain x86 code.
*
- * 'closest_target_usage' tracks the most recent position that has a
- * good chance of being an x86 instruction. When the filter detects a
+ * 'last_x86_pos' tracks the most recent position that has a good chance
+ * of being the start of an x86 instruction. When the filter detects a
* likely x86 instruction, it updates this variable and considers the
- * next 1023 bytes of data as valid for x86 translations.
+ * next LZMS_X86_MAX_TRANSLATION_OFFSET bytes of data as valid for x86
+ * translations.
*
* If part of the data does not, in fact, contain x86 machine code, then
- * 'closest_target_usage' will, very likely, eventually fall more than
- * 1023 bytes behind the current position. This results in x86
- * translations being disabled until the next likely x86 instruction is
- * detected.
- *
- * Translations on relative call (e8 opcode) instructions are slightly
- * more restricted. They require that the most recent likely x86
- * instruction was in the last 511 bytes, rather than the last 1023
- * bytes.
+ * 'last_x86_pos' will, very likely, eventually fall more than
+ * LZMS_X86_MAX_TRANSLATION_OFFSET bytes behind the current position.
+ * This results in x86 translations being disabled until the next likely
+ * x86 instruction is detected.
*
* To identify "likely x86 instructions", the algorithm attempts to
* track the position of the most recent potential relative-addressing
* instruction that referenced each possible memory address. If it
- * finds two references to the same memory address within a 65535 byte
- * window, the second reference is flagged as a likely x86 instruction.
- * Since the instructions considered for translation necessarily use
- * relative addressing, the algorithm does a tentative translation into
- * absolute addresses. In addition, so that memory addresses can be
- * looked up in an array of reasonable size (in this code,
- * 'last_target_usages'), only the low-order 2 bytes of each address are
- * considered significant.
+ * finds two references to the same memory address within an
+ * LZMS_X86_ID_WINDOW_SIZE-byte sized window, then the second reference
+ * is flagged as a likely x86 instruction. Since the instructions
+ * considered for translation necessarily use relative addressing, the
+ * algorithm does a tentative translation into absolute addresses. In
+ * addition, so that memory addresses can be looked up in an array of
+ * reasonable size (in this code, 'last_target_usages'), only the
+ * low-order 2 bytes of each address are considered significant.
*/
- s32 closest_target_usage = -LZMS_X86_MAX_TRANSLATION_OFFSET - 1;
+ s32 i;
+ s32 tail_idx;
+ u8 saved_byte;
+ s32 last_x86_pos;
+
+ if (size <= 17)
+ return;
- for (s32 i = 0; i < 65536; i++)
- last_target_usages[i] = -LZMS_X86_MAX_GOOD_TARGET_OFFSET - 1;
+ for (i = 0; i < 65536; i++)
+ last_target_usages[i] = -(s32)LZMS_X86_ID_WINDOW_SIZE - 1;
- for (s32 i = 1; i < size - 16; ) {
+ /*
+ * Optimization: only check for end-of-buffer when we already have a
+ * byte that is a potential opcode for x86 translation. To do this,
+ * overwrite one of the bytes near the end of the buffer, and restore it
+ * later. The correctness of this optimization relies on two
+ * characteristics of compressed format:
+ *
+ * 1. No translation can follow an opcode beginning in the last 16
+ * bytes.
+ * 2. A translation following an opcode starting at the last possible
+ * position (17 bytes from the end) never extends more than 7 bytes.
+ * Consequently, we can overwrite any of the bytes starting at
+ * data[(size - 16) + 7] and have no effect on the result, as long
+ * as we restore those bytes later.
+ */
+ tail_idx = size - 16;
+ saved_byte = data[tail_idx + 8];
+ data[tail_idx + 8] = 0xE8;
+ last_x86_pos = -LZMS_X86_MAX_TRANSLATION_OFFSET - 1;
+
+ /* Note: the very first byte must be ignored completely! */
+ i = 0;
+ for (;;) {
s32 max_trans_offset;
- s32 n;
+ s32 opcode_nbytes;
+ u16 target16;
+
+ /*
+ * The following table is used to accelerate the common case
+ * where the byte has nothing to do with x86 translation and
+ * must simply be skipped. This is the fastest (at least on
+ * x86_64) of the implementations I tested. The other
+ * implementations I tested were:
+ * - Jump table with 256 entries
+ * - Switch statement with default
+ */
+ static const u8 is_potential_opcode[256] = {
+ [0x48] = 1, [0x4C] = 1, [0xE8] = 1,
+ [0xE9] = 1, [0xF0] = 1, [0xFF] = 1,
+ };
+
+ for (;;) {
+ if (is_potential_opcode[data[++i]])
+ break;
+ if (is_potential_opcode[data[++i]])
+ break;
+ if (is_potential_opcode[data[++i]])
+ break;
+ if (is_potential_opcode[data[++i]])
+ break;
+ }
+
+ if (i >= tail_idx)
+ break;
+
+ max_trans_offset = LZMS_X86_MAX_TRANSLATION_OFFSET;
+ switch (data[i]) {
+ case 0x48:
+ if (data[i + 1] == 0x8B) {
+ if (data[i + 2] == 0x5 || data[i + 2] == 0xD) {
+ /* Load relative (x86_64) */
+ opcode_nbytes = 3;
+ goto have_opcode;
+ }
+ } else if (data[i + 1] == 0x8D) {
+ if ((data[i + 2] & 0x7) == 0x5) {
+ /* Load effective address relative (x86_64) */
+ opcode_nbytes = 3;
+ goto have_opcode;
+ }
+ }
+ break;
+ case 0x4C:
+ if (data[i + 1] == 0x8D) {
+ if ((data[i + 2] & 0x7) == 0x5) {
+ /* Load effective address relative (x86_64) */
+ opcode_nbytes = 3;
+ goto have_opcode;
+ }
+ }
+ break;
+ case 0xE8:
+ /* Call relative. Note: 'max_trans_offset' must be
+ * halved for this instruction. This means that we must
+ * be more confident that we are in a region of x86
+ * machine code before we will do a translation for this
+ * particular instruction. */
+ opcode_nbytes = 1;
+ max_trans_offset /= 2;
+ goto have_opcode;
+ case 0xE9:
+ /* Jump relative */
+ i += 4;
+ break;
+ case 0xF0:
+ if (data[i + 1] == 0x83 && data[i + 2] == 0x05) {
+ /* Lock add relative */
+ opcode_nbytes = 3;
+ goto have_opcode;
+ }
+ break;
+ case 0xFF:
+ if (data[i + 1] == 0x15) {
+ /* Call indirect */
+ opcode_nbytes = 2;
+ goto have_opcode;
+ }
+ break;
+ }
- n = lzms_may_x86_translate(data + i, &max_trans_offset);
+ continue;
- if (max_trans_offset) {
- /* Recognized opcode. */
- i = lzms_maybe_do_x86_translation(data, i, n,
- &closest_target_usage,
- last_target_usages,
- max_trans_offset,
- undo);
+ have_opcode:
+ if (undo) {
+ if (i - last_x86_pos <= max_trans_offset) {
+ LZMS_DEBUG("Undid x86 translation at position %d "
+ "(opcode 0x%02x)", i, data[i]);
+ void *p32 = &data[i + opcode_nbytes];
+ u32 n = get_unaligned_u32_le(p32);
+ put_unaligned_u32_le(n - i, p32);
+ }
+ target16 = i + get_unaligned_u16_le(&data[i + opcode_nbytes]);
} else {
- /* Not a recognized opcode. */
- i += n;
+ target16 = i + get_unaligned_u16_le(&data[i + opcode_nbytes]);
+ if (i - last_x86_pos <= max_trans_offset) {
+ LZMS_DEBUG("Did x86 translation at position %d "
+ "(opcode 0x%02x)", i, data[i]);
+ void *p32 = &data[i + opcode_nbytes];
+ u32 n = get_unaligned_u32_le(p32);
+ put_unaligned_u32_le(n + i, p32);
+ }
}
+
+ i += opcode_nbytes + sizeof(le32) - 1;
+
+ if (i - last_target_usages[target16] <= LZMS_X86_ID_WINDOW_SIZE)
+ last_x86_pos = i;
+
+ last_target_usages[target16] = i;
+
+ continue;
}
+
+ data[tail_idx + 8] = saved_byte;
}
void
git://wimlib.net / wimlib / blobdiff