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assertion failure on darwin_x86_64, have to investigage

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PiperOrigin-RevId: 303428229
This commit is contained in:
Snappy Team
2020-03-27 23:17:50 +00:00
committed by Victor Costan
parent e19178748f
commit 4dfcad9f4e
3 changed files with 68 additions and 21 deletions
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61
snappy-internal.h
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@@ -89,12 +89,18 @@ char* CompressFragment(const char* input,
// Does not read *(s1 + (s2_limit - s2)) or beyond.
// Requires that s2_limit >= s2.
//
// In addition populate *data with the next 8 bytes from the end of the match.
// This is only done if 8 bytes are available (s2_limit - s2 >= 8). The point is
// that on some arch's this can be done faster in this routine than subsequent
// loading from s2 + n.
//
// Separate implementation for 64-bit, little-endian cpus.
#if !defined(SNAPPY_IS_BIG_ENDIAN) && \
(defined(ARCH_K8) || defined(ARCH_PPC) || defined(ARCH_ARM))
static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
const char* s2,
const char* s2_limit) {
const char* s2_limit,
uint64* data) {
assert(s2_limit >= s2);
size_t matched = 0;
@@ -103,12 +109,28 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
// uncommon code paths that determine, without extra effort, whether the match
// length is less than 8. In short, we are hoping to avoid a conditional
// branch, and perhaps get better code layout from the C++ compiler.
if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
if (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) {
uint64 a1 = UNALIGNED_LOAD64(s1);
uint64 a2 = UNALIGNED_LOAD64(s2);
if (a1 != a2) {
return std::pair<size_t, bool>(Bits::FindLSBSetNonZero64(a1 ^ a2) >> 3,
true);
if (SNAPPY_PREDICT_TRUE(a1 != a2)) {
uint64 xorval = a1 ^ a2;
int shift = Bits::FindLSBSetNonZero64(xorval);
size_t matched_bytes = shift >> 3;
#ifndef __x86_64__
*data = UNALIGNED_LOAD64(s2 + matched_bytes);
#else
// Unfortunately the compiler cannot find this using the obvious c++ code
// *data = shift == 0 ? a2 : (a2 >> shift) | (a3 << (64 - shift);
// the reason is that the above needs the conditional clause to guard
// against UB when shift == 0. The compiler doesn't realize the full
// expression can be lowered into a single "shrd" instruction and in
// effect the conditional can be ignored.
uint64 a3 = UNALIGNED_LOAD64(s2 + 8);
asm ("shrdq %%cl, %1, %0\n\t" : "+r"(a2) : "r"(a3), "c"(shift & -8));
*data = a2;
#endif
assert(*data == UNALIGNED_LOAD64(s2 + matched_bytes));
return std::pair<size_t, bool>(matched_bytes, true);
} else {
matched = 8;
s2 += 8;
@@ -119,14 +141,25 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
// time until we find a 64-bit block that doesn't match; then we find
// the first non-matching bit and use that to calculate the total
// length of the match.
while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 8)) {
if (UNALIGNED_LOAD64(s2) == UNALIGNED_LOAD64(s1 + matched)) {
while (SNAPPY_PREDICT_TRUE(s2 <= s2_limit - 16)) {
uint64 a1 = UNALIGNED_LOAD64(s1 + matched);
uint64 a2 = UNALIGNED_LOAD64(s2);
if (a1 == a2) {
s2 += 8;
matched += 8;
} else {
uint64 x = UNALIGNED_LOAD64(s2) ^ UNALIGNED_LOAD64(s1 + matched);
int matching_bits = Bits::FindLSBSetNonZero64(x);
matched += matching_bits >> 3;
uint64 xorval = a1 ^ a2;
int shift = Bits::FindLSBSetNonZero64(xorval);
size_t matched_bytes = shift >> 3;
#ifndef __x86_64__
*data = UNALIGNED_LOAD64(s2 + matched_bytes);
#else
uint64 a3 = UNALIGNED_LOAD64(s2 + 8);
asm("shrdq %%cl, %1, %0\n\t" : "+r"(a2) : "r"(a3), "c"(shift & -8));
*data = a2;
#endif
assert(*data == UNALIGNED_LOAD64(s2 + matched_bytes));
matched += matched_bytes;
assert(matched >= 8);
return std::pair<size_t, bool>(matched, false);
}
@@ -136,6 +169,9 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
++s2;
++matched;
} else {
if (s2 <= s2_limit - 8) {
*data = UNALIGNED_LOAD64(s2);
}
return std::pair<size_t, bool>(matched, matched < 8);
}
}
@@ -144,7 +180,8 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
#else
static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
const char* s2,
const char* s2_limit) {
const char* s2_limit,
uint64* data) {
// Implementation based on the x86-64 version, above.
assert(s2_limit >= s2);
int matched = 0;
@@ -158,12 +195,14 @@ static inline std::pair<size_t, bool> FindMatchLength(const char* s1,
uint32 x = UNALIGNED_LOAD32(s2) ^ UNALIGNED_LOAD32(s1 + matched);
int matching_bits = Bits::FindLSBSetNonZero(x);
matched += matching_bits >> 3;
s2 += matching_bits >> 3;
} else {
while ((s2 < s2_limit) && (s1[matched] == *s2)) {
++s2;
++matched;
}
}
if (s2 <= s2_limit - 8) *data = LittleEndian::Load64(s2);
return std::pair<size_t, bool>(matched, matched < 8);
}
#endif