mirror of https://git.tukaani.org/xz.git
tuklib_integer: Revise unaligned reads and writes on strict-align archs.
In XZ Utils context this doesn't matter much because unaligned reads and writes aren't used in hot code when TUKLIB_FAST_UNALIGNED_ACCESS isn't #defined.
This commit is contained in:
Lasse Collin
Jia Tan
parent
9e14743ee5
commit
5056bc5107
|
|
@ -195,6 +195,9 @@
|
|||
// Unaligned reads and writes //
|
||||
////////////////////////////////
|
||||
|
||||
// No-strict-align archs like x86-64
|
||||
// ---------------------------------
|
||||
//
|
||||
// The traditional way of casting e.g. *(const uint16_t *)uint8_pointer
|
||||
// is bad even if the uint8_pointer is properly aligned because this kind
|
||||
// of casts break strict aliasing rules and result in undefined behavior.
|
||||
|
|
@ -209,12 +212,115 @@
|
|||
// build time. A third method, casting to a packed struct, would also be
|
||||
// an option but isn't provided to keep things simpler (it's already a mess).
|
||||
// Hopefully this is flexible enough in practice.
|
||||
//
|
||||
// Some compilers on x86-64 like Clang >= 10 and GCC >= 5.1 detect that
|
||||
//
|
||||
// buf[0] | (buf[1] << 8)
|
||||
//
|
||||
// reads a 16-bit value and can emit a single 16-bit load and produce
|
||||
// identical code than with the memcpy() method. In other cases Clang and GCC
|
||||
// produce either the same or better code with memcpy(). For example, Clang 9
|
||||
// on x86-64 can detect 32-bit load but not 16-bit load.
|
||||
//
|
||||
// MSVC uses unaligned access with the memcpy() method but emits byte-by-byte
|
||||
// code for "buf[0] | (buf[1] << 8)".
|
||||
//
|
||||
// Conclusion: The memcpy() method is the best choice when unaligned access
|
||||
// is supported.
|
||||
//
|
||||
// Strict-align archs like SPARC
|
||||
// -----------------------------
|
||||
//
|
||||
// GCC versions from around 4.x to to at least 13.2.0 produce worse code
|
||||
// from the memcpy() method than from simple byte-by-byte shift-or code
|
||||
// when reading a 32-bit integer:
|
||||
//
|
||||
// (1) It may be constructed on stack using using four 8-bit loads,
|
||||
// four 8-bit stores to stack, and finally one 32-bit load from stack.
|
||||
//
|
||||
// (2) Especially with -Os, an actual memcpy() call may be emitted.
|
||||
//
|
||||
// This is true on at least on ARM, ARM64, SPARC, SPARC64, MIPS64EL, and
|
||||
// RISC-V. Of these, ARM, ARM64, and RISC-V support unaligned access in
|
||||
// some processors but not all so this is relevant only in the case when
|
||||
// GCC assumes that unaligned is not supported or -mstrict-align or
|
||||
// -mno-unaligned-access is used.
|
||||
//
|
||||
// For Clang it makes little difference. ARM64 with -O2 -mstrict-align
|
||||
// was one the very few with a minor difference: the memcpy() version
|
||||
// was one instruction longer.
|
||||
//
|
||||
// Conclusion: At least in case of GCC and Clang, byte-by-byte code is
|
||||
// the best choise for strict-align archs to do unaligned access.
|
||||
//
|
||||
// See also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111502
|
||||
//
|
||||
// Thanks to <https://godbolt.org/> it was easy to test different compilers.
|
||||
// The following is for little endian targets:
|
||||
/*
|
||||
#include <stdint.h>
|
||||
#include <string.h>
|
||||
|
||||
uint32_t bytes16(const uint8_t *b)
|
||||
{
|
||||
return (uint32_t)b[0]
|
||||
| ((uint32_t)b[1] << 8);
|
||||
}
|
||||
|
||||
uint32_t copy16(const uint8_t *b)
|
||||
{
|
||||
uint16_t v;
|
||||
memcpy(&v, b, sizeof(v));
|
||||
return v;
|
||||
}
|
||||
|
||||
uint32_t bytes32(const uint8_t *b)
|
||||
{
|
||||
return (uint32_t)b[0]
|
||||
| ((uint32_t)b[1] << 8)
|
||||
| ((uint32_t)b[2] << 16)
|
||||
| ((uint32_t)b[3] << 24);
|
||||
}
|
||||
|
||||
uint32_t copy32(const uint8_t *b)
|
||||
{
|
||||
uint32_t v;
|
||||
memcpy(&v, b, sizeof(v));
|
||||
return v;
|
||||
}
|
||||
|
||||
void wbytes16(uint8_t *b, uint16_t v)
|
||||
{
|
||||
b[0] = (uint8_t)v;
|
||||
b[1] = (uint8_t)(v >> 8);
|
||||
}
|
||||
|
||||
void wcopy16(uint8_t *b, uint16_t v)
|
||||
{
|
||||
memcpy(b, &v, sizeof(v));
|
||||
}
|
||||
|
||||
void wbytes32(uint8_t *b, uint32_t v)
|
||||
{
|
||||
b[0] = (uint8_t)v;
|
||||
b[1] = (uint8_t)(v >> 8);
|
||||
b[2] = (uint8_t)(v >> 16);
|
||||
b[3] = (uint8_t)(v >> 24);
|
||||
}
|
||||
|
||||
void wcopy32(uint8_t *b, uint32_t v)
|
||||
{
|
||||
memcpy(b, &v, sizeof(v));
|
||||
}
|
||||
*/
|
||||
|
||||
|
||||
#ifdef TUKLIB_FAST_UNALIGNED_ACCESS
|
||||
|
||||
static inline uint16_t
|
||||
read16ne(const uint8_t *buf)
|
||||
{
|
||||
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
|
||||
&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
|
||||
#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
|
||||
return *(const uint16_t *)buf;
|
||||
#else
|
||||
uint16_t num;
|
||||
|
|
@ -227,8 +333,7 @@ read16ne(const uint8_t *buf)
|
|||
static inline uint32_t
|
||||
read32ne(const uint8_t *buf)
|
||||
{
|
||||
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
|
||||
&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
|
||||
#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
|
||||
return *(const uint32_t *)buf;
|
||||
#else
|
||||
uint32_t num;
|
||||
|
|
@ -241,8 +346,7 @@ read32ne(const uint8_t *buf)
|
|||
static inline uint64_t
|
||||
read64ne(const uint8_t *buf)
|
||||
{
|
||||
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
|
||||
&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
|
||||
#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
|
||||
return *(const uint64_t *)buf;
|
||||
#else
|
||||
uint64_t num;
|
||||
|
|
@ -255,8 +359,7 @@ read64ne(const uint8_t *buf)
|
|||
static inline void
|
||||
write16ne(uint8_t *buf, uint16_t num)
|
||||
{
|
||||
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
|
||||
&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
|
||||
#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
|
||||
*(uint16_t *)buf = num;
|
||||
#else
|
||||
memcpy(buf, &num, sizeof(num));
|
||||
|
|
@ -268,8 +371,7 @@ write16ne(uint8_t *buf, uint16_t num)
|
|||
static inline void
|
||||
write32ne(uint8_t *buf, uint32_t num)
|
||||
{
|
||||
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
|
||||
&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
|
||||
#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
|
||||
*(uint32_t *)buf = num;
|
||||
#else
|
||||
memcpy(buf, &num, sizeof(num));
|
||||
|
|
@ -281,8 +383,7 @@ write32ne(uint8_t *buf, uint32_t num)
|
|||
static inline void
|
||||
write64ne(uint8_t *buf, uint64_t num)
|
||||
{
|
||||
#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
|
||||
&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
|
||||
#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
|
||||
*(uint64_t *)buf = num;
|
||||
#else
|
||||
memcpy(buf, &num, sizeof(num));
|
||||
|
|
@ -294,68 +395,122 @@ write64ne(uint8_t *buf, uint64_t num)
|
|||
static inline uint16_t
|
||||
read16be(const uint8_t *buf)
|
||||
{
|
||||
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
uint16_t num = read16ne(buf);
|
||||
return conv16be(num);
|
||||
#else
|
||||
uint16_t num = ((uint16_t)buf[0] << 8) | (uint16_t)buf[1];
|
||||
return num;
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
static inline uint16_t
|
||||
read16le(const uint8_t *buf)
|
||||
{
|
||||
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
uint16_t num = read16ne(buf);
|
||||
return conv16le(num);
|
||||
#else
|
||||
uint16_t num = ((uint16_t)buf[0]) | ((uint16_t)buf[1] << 8);
|
||||
return num;
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
static inline uint32_t
|
||||
read32be(const uint8_t *buf)
|
||||
{
|
||||
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
uint32_t num = read32ne(buf);
|
||||
return conv32be(num);
|
||||
#else
|
||||
uint32_t num = (uint32_t)buf[0] << 24;
|
||||
num |= (uint32_t)buf[1] << 16;
|
||||
num |= (uint32_t)buf[2] << 8;
|
||||
num |= (uint32_t)buf[3];
|
||||
return num;
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
static inline uint32_t
|
||||
read32le(const uint8_t *buf)
|
||||
{
|
||||
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
uint32_t num = read32ne(buf);
|
||||
return conv32le(num);
|
||||
#else
|
||||
uint32_t num = (uint32_t)buf[0];
|
||||
num |= (uint32_t)buf[1] << 8;
|
||||
num |= (uint32_t)buf[2] << 16;
|
||||
num |= (uint32_t)buf[3] << 24;
|
||||
return num;
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
static inline uint64_t
|
||||
read64be(const uint8_t *buf)
|
||||
{
|
||||
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
uint64_t num = read64ne(buf);
|
||||
return conv64be(num);
|
||||
}
|
||||
|
||||
|
||||
static inline uint64_t
|
||||
read64le(const uint8_t *buf)
|
||||
{
|
||||
uint64_t num = read64ne(buf);
|
||||
return conv64le(num);
|
||||
}
|
||||
|
||||
|
||||
// NOTE: Possible byte swapping must be done in a macro to allow the compiler
|
||||
// to optimize byte swapping of constants when using glibc's or *BSD's
|
||||
// byte swapping macros. The actual write is done in an inline function
|
||||
// to make type checking of the buf pointer possible.
|
||||
#define write16be(buf, num) write16ne(buf, conv16be(num))
|
||||
#define write32be(buf, num) write32ne(buf, conv32be(num))
|
||||
#define write64be(buf, num) write64ne(buf, conv64be(num))
|
||||
#define write16le(buf, num) write16ne(buf, conv16le(num))
|
||||
#define write32le(buf, num) write32ne(buf, conv32le(num))
|
||||
#define write64le(buf, num) write64ne(buf, conv64le(num))
|
||||
|
||||
#else
|
||||
|
||||
#ifdef WORDS_BIGENDIAN
|
||||
# define read16ne read16be
|
||||
# define read32ne read32be
|
||||
# define read64ne read64be
|
||||
# define write16ne write16be
|
||||
# define write32ne write32be
|
||||
# define write64ne write64be
|
||||
#else
|
||||
# define read16ne read16le
|
||||
# define read32ne read32le
|
||||
# define read64ne read64le
|
||||
# define write16ne write16le
|
||||
# define write32ne write32le
|
||||
# define write64ne write64le
|
||||
#endif
|
||||
|
||||
|
||||
static inline uint16_t
|
||||
read16be(const uint8_t *buf)
|
||||
{
|
||||
uint16_t num = ((uint16_t)buf[0] << 8) | (uint16_t)buf[1];
|
||||
return num;
|
||||
}
|
||||
|
||||
|
||||
static inline uint16_t
|
||||
read16le(const uint8_t *buf)
|
||||
{
|
||||
uint16_t num = ((uint16_t)buf[0]) | ((uint16_t)buf[1] << 8);
|
||||
return num;
|
||||
}
|
||||
|
||||
|
||||
static inline uint32_t
|
||||
read32be(const uint8_t *buf)
|
||||
{
|
||||
uint32_t num = (uint32_t)buf[0] << 24;
|
||||
num |= (uint32_t)buf[1] << 16;
|
||||
num |= (uint32_t)buf[2] << 8;
|
||||
num |= (uint32_t)buf[3];
|
||||
return num;
|
||||
}
|
||||
|
||||
|
||||
static inline uint32_t
|
||||
read32le(const uint8_t *buf)
|
||||
{
|
||||
uint32_t num = (uint32_t)buf[0];
|
||||
num |= (uint32_t)buf[1] << 8;
|
||||
num |= (uint32_t)buf[2] << 16;
|
||||
num |= (uint32_t)buf[3] << 24;
|
||||
return num;
|
||||
}
|
||||
|
||||
|
||||
static inline uint64_t
|
||||
read64be(const uint8_t *buf)
|
||||
{
|
||||
uint64_t num = (uint64_t)buf[0] << 56;
|
||||
num |= (uint64_t)buf[1] << 48;
|
||||
num |= (uint64_t)buf[2] << 40;
|
||||
|
|
@ -365,17 +520,12 @@ read64be(const uint8_t *buf)
|
|||
num |= (uint64_t)buf[6] << 8;
|
||||
num |= (uint64_t)buf[7];
|
||||
return num;
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
static inline uint64_t
|
||||
read64le(const uint8_t *buf)
|
||||
{
|
||||
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
uint64_t num = read64ne(buf);
|
||||
return conv64le(num);
|
||||
#else
|
||||
uint64_t num = (uint64_t)buf[0];
|
||||
num |= (uint64_t)buf[1] << 8;
|
||||
num |= (uint64_t)buf[2] << 16;
|
||||
|
|
@ -385,28 +535,9 @@ read64le(const uint8_t *buf)
|
|||
num |= (uint64_t)buf[6] << 48;
|
||||
num |= (uint64_t)buf[7] << 56;
|
||||
return num;
|
||||
#endif
|
||||
}
|
||||
|
||||
|
||||
// NOTE: Possible byte swapping must be done in a macro to allow the compiler
|
||||
// to optimize byte swapping of constants when using glibc's or *BSD's
|
||||
// byte swapping macros. The actual write is done in an inline function
|
||||
// to make type checking of the buf pointer possible.
|
||||
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
# define write16be(buf, num) write16ne(buf, conv16be(num))
|
||||
# define write32be(buf, num) write32ne(buf, conv32be(num))
|
||||
# define write64be(buf, num) write64ne(buf, conv64be(num))
|
||||
#endif
|
||||
|
||||
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
|
||||
# define write16le(buf, num) write16ne(buf, conv16le(num))
|
||||
# define write32le(buf, num) write32ne(buf, conv32le(num))
|
||||
# define write64le(buf, num) write64ne(buf, conv64le(num))
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef write16be
|
||||
static inline void
|
||||
write16be(uint8_t *buf, uint16_t num)
|
||||
{
|
||||
|
|
@ -414,10 +545,8 @@ write16be(uint8_t *buf, uint16_t num)
|
|||
buf[1] = (uint8_t)num;
|
||||
return;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef write16le
|
||||
static inline void
|
||||
write16le(uint8_t *buf, uint16_t num)
|
||||
{
|
||||
|
|
@ -425,10 +554,8 @@ write16le(uint8_t *buf, uint16_t num)
|
|||
buf[1] = (uint8_t)(num >> 8);
|
||||
return;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef write32be
|
||||
static inline void
|
||||
write32be(uint8_t *buf, uint32_t num)
|
||||
{
|
||||
|
|
@ -438,10 +565,8 @@ write32be(uint8_t *buf, uint32_t num)
|
|||
buf[3] = (uint8_t)num;
|
||||
return;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef write32le
|
||||
static inline void
|
||||
write32le(uint8_t *buf, uint32_t num)
|
||||
{
|
||||
|
|
@ -451,10 +576,8 @@ write32le(uint8_t *buf, uint32_t num)
|
|||
buf[3] = (uint8_t)(num >> 24);
|
||||
return;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef write64be
|
||||
static inline void
|
||||
write64be(uint8_t *buf, uint64_t num)
|
||||
{
|
||||
|
|
@ -468,10 +591,8 @@ write64be(uint8_t *buf, uint64_t num)
|
|||
buf[7] = (uint8_t)num;
|
||||
return;
|
||||
}
|
||||
#endif
|
||||
|
||||
|
||||
#ifndef write64le
|
||||
static inline void
|
||||
write64le(uint8_t *buf, uint64_t num)
|
||||
{
|
||||
|
|
@ -485,6 +606,7 @@ write64le(uint8_t *buf, uint64_t num)
|
|||
buf[7] = (uint8_t)(num >> 56);
|
||||
return;
|
||||
}
|
||||
|
||||
#endif
|
||||
|
||||
|
||||
|
|
|
|||
Loading…
Reference in New Issue