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
parent
6828242735
commit
c8f715f1bc
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@ -195,6 +195,9 @@
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// Unaligned reads and writes //
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////////////////////////////////
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// No-strict-align archs like x86-64
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// ---------------------------------
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//
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// The traditional way of casting e.g. *(const uint16_t *)uint8_pointer
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// is bad even if the uint8_pointer is properly aligned because this kind
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// of casts break strict aliasing rules and result in undefined behavior.
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@ -209,12 +212,115 @@
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// build time. A third method, casting to a packed struct, would also be
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// an option but isn't provided to keep things simpler (it's already a mess).
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// Hopefully this is flexible enough in practice.
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//
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// Some compilers on x86-64 like Clang >= 10 and GCC >= 5.1 detect that
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//
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// buf[0] | (buf[1] << 8)
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//
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// reads a 16-bit value and can emit a single 16-bit load and produce
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// identical code than with the memcpy() method. In other cases Clang and GCC
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// produce either the same or better code with memcpy(). For example, Clang 9
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// on x86-64 can detect 32-bit load but not 16-bit load.
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//
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// MSVC uses unaligned access with the memcpy() method but emits byte-by-byte
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// code for "buf[0] | (buf[1] << 8)".
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//
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// Conclusion: The memcpy() method is the best choice when unaligned access
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// is supported.
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//
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// Strict-align archs like SPARC
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// -----------------------------
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//
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// GCC versions from around 4.x to to at least 13.2.0 produce worse code
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// from the memcpy() method than from simple byte-by-byte shift-or code
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// when reading a 32-bit integer:
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//
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// (1) It may be constructed on stack using using four 8-bit loads,
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// four 8-bit stores to stack, and finally one 32-bit load from stack.
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//
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// (2) Especially with -Os, an actual memcpy() call may be emitted.
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//
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// This is true on at least on ARM, ARM64, SPARC, SPARC64, MIPS64EL, and
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// RISC-V. Of these, ARM, ARM64, and RISC-V support unaligned access in
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// some processors but not all so this is relevant only in the case when
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// GCC assumes that unaligned is not supported or -mstrict-align or
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// -mno-unaligned-access is used.
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//
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// For Clang it makes little difference. ARM64 with -O2 -mstrict-align
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// was one the very few with a minor difference: the memcpy() version
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// was one instruction longer.
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//
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// Conclusion: At least in case of GCC and Clang, byte-by-byte code is
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// the best choise for strict-align archs to do unaligned access.
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//
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// See also: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111502
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//
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// Thanks to <https://godbolt.org/> it was easy to test different compilers.
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// The following is for little endian targets:
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/*
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#include <stdint.h>
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#include <string.h>
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uint32_t bytes16(const uint8_t *b)
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{
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return (uint32_t)b[0]
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| ((uint32_t)b[1] << 8);
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}
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uint32_t copy16(const uint8_t *b)
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{
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uint16_t v;
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memcpy(&v, b, sizeof(v));
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return v;
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}
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uint32_t bytes32(const uint8_t *b)
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{
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return (uint32_t)b[0]
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| ((uint32_t)b[1] << 8)
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| ((uint32_t)b[2] << 16)
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| ((uint32_t)b[3] << 24);
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}
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uint32_t copy32(const uint8_t *b)
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{
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uint32_t v;
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memcpy(&v, b, sizeof(v));
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return v;
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}
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void wbytes16(uint8_t *b, uint16_t v)
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{
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b[0] = (uint8_t)v;
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b[1] = (uint8_t)(v >> 8);
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}
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void wcopy16(uint8_t *b, uint16_t v)
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{
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memcpy(b, &v, sizeof(v));
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}
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void wbytes32(uint8_t *b, uint32_t v)
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{
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b[0] = (uint8_t)v;
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b[1] = (uint8_t)(v >> 8);
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b[2] = (uint8_t)(v >> 16);
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b[3] = (uint8_t)(v >> 24);
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}
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void wcopy32(uint8_t *b, uint32_t v)
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{
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memcpy(b, &v, sizeof(v));
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}
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*/
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#ifdef TUKLIB_FAST_UNALIGNED_ACCESS
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static inline uint16_t
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read16ne(const uint8_t *buf)
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{
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#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
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&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
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#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
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return *(const uint16_t *)buf;
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#else
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uint16_t num;
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@ -227,8 +333,7 @@ read16ne(const uint8_t *buf)
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static inline uint32_t
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read32ne(const uint8_t *buf)
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{
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#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
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&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
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#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
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return *(const uint32_t *)buf;
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#else
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uint32_t num;
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@ -241,8 +346,7 @@ read32ne(const uint8_t *buf)
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static inline uint64_t
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read64ne(const uint8_t *buf)
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{
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#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
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&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
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#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
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return *(const uint64_t *)buf;
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#else
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uint64_t num;
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@ -255,8 +359,7 @@ read64ne(const uint8_t *buf)
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static inline void
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write16ne(uint8_t *buf, uint16_t num)
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{
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#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
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&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
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#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
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*(uint16_t *)buf = num;
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#else
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memcpy(buf, &num, sizeof(num));
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@ -268,8 +371,7 @@ write16ne(uint8_t *buf, uint16_t num)
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static inline void
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write32ne(uint8_t *buf, uint32_t num)
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{
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#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
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&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
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#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
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*(uint32_t *)buf = num;
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#else
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memcpy(buf, &num, sizeof(num));
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@ -281,8 +383,7 @@ write32ne(uint8_t *buf, uint32_t num)
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static inline void
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write64ne(uint8_t *buf, uint64_t num)
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{
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#if defined(TUKLIB_FAST_UNALIGNED_ACCESS) \
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&& defined(TUKLIB_USE_UNSAFE_TYPE_PUNNING)
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#ifdef TUKLIB_USE_UNSAFE_TYPE_PUNNING
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*(uint64_t *)buf = num;
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#else
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memcpy(buf, &num, sizeof(num));
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@ -294,68 +395,122 @@ write64ne(uint8_t *buf, uint64_t num)
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static inline uint16_t
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read16be(const uint8_t *buf)
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{
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#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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uint16_t num = read16ne(buf);
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return conv16be(num);
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#else
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uint16_t num = ((uint16_t)buf[0] << 8) | (uint16_t)buf[1];
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return num;
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#endif
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}
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static inline uint16_t
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read16le(const uint8_t *buf)
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{
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#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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uint16_t num = read16ne(buf);
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return conv16le(num);
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#else
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uint16_t num = ((uint16_t)buf[0]) | ((uint16_t)buf[1] << 8);
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return num;
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#endif
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}
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static inline uint32_t
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read32be(const uint8_t *buf)
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{
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#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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uint32_t num = read32ne(buf);
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return conv32be(num);
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#else
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uint32_t num = (uint32_t)buf[0] << 24;
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num |= (uint32_t)buf[1] << 16;
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num |= (uint32_t)buf[2] << 8;
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num |= (uint32_t)buf[3];
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return num;
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#endif
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}
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static inline uint32_t
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read32le(const uint8_t *buf)
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{
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#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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uint32_t num = read32ne(buf);
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return conv32le(num);
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#else
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uint32_t num = (uint32_t)buf[0];
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num |= (uint32_t)buf[1] << 8;
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num |= (uint32_t)buf[2] << 16;
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num |= (uint32_t)buf[3] << 24;
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return num;
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#endif
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}
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static inline uint64_t
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read64be(const uint8_t *buf)
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{
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#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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uint64_t num = read64ne(buf);
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return conv64be(num);
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}
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static inline uint64_t
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read64le(const uint8_t *buf)
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{
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uint64_t num = read64ne(buf);
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return conv64le(num);
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}
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// NOTE: Possible byte swapping must be done in a macro to allow the compiler
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// to optimize byte swapping of constants when using glibc's or *BSD's
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// byte swapping macros. The actual write is done in an inline function
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// to make type checking of the buf pointer possible.
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#define write16be(buf, num) write16ne(buf, conv16be(num))
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#define write32be(buf, num) write32ne(buf, conv32be(num))
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#define write64be(buf, num) write64ne(buf, conv64be(num))
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#define write16le(buf, num) write16ne(buf, conv16le(num))
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#define write32le(buf, num) write32ne(buf, conv32le(num))
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#define write64le(buf, num) write64ne(buf, conv64le(num))
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#else
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#ifdef WORDS_BIGENDIAN
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# define read16ne read16be
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# define read32ne read32be
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# define read64ne read64be
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# define write16ne write16be
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# define write32ne write32be
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# define write64ne write64be
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#else
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# define read16ne read16le
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# define read32ne read32le
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# define read64ne read64le
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# define write16ne write16le
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# define write32ne write32le
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# define write64ne write64le
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#endif
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static inline uint16_t
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read16be(const uint8_t *buf)
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{
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uint16_t num = ((uint16_t)buf[0] << 8) | (uint16_t)buf[1];
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return num;
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}
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static inline uint16_t
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read16le(const uint8_t *buf)
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{
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uint16_t num = ((uint16_t)buf[0]) | ((uint16_t)buf[1] << 8);
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return num;
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}
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static inline uint32_t
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read32be(const uint8_t *buf)
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{
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uint32_t num = (uint32_t)buf[0] << 24;
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num |= (uint32_t)buf[1] << 16;
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num |= (uint32_t)buf[2] << 8;
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num |= (uint32_t)buf[3];
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return num;
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}
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static inline uint32_t
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read32le(const uint8_t *buf)
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{
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uint32_t num = (uint32_t)buf[0];
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num |= (uint32_t)buf[1] << 8;
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num |= (uint32_t)buf[2] << 16;
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num |= (uint32_t)buf[3] << 24;
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return num;
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}
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static inline uint64_t
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read64be(const uint8_t *buf)
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{
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uint64_t num = (uint64_t)buf[0] << 56;
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num |= (uint64_t)buf[1] << 48;
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num |= (uint64_t)buf[2] << 40;
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@ -365,17 +520,12 @@ read64be(const uint8_t *buf)
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num |= (uint64_t)buf[6] << 8;
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num |= (uint64_t)buf[7];
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return num;
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#endif
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}
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static inline uint64_t
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read64le(const uint8_t *buf)
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{
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#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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uint64_t num = read64ne(buf);
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return conv64le(num);
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#else
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uint64_t num = (uint64_t)buf[0];
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num |= (uint64_t)buf[1] << 8;
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num |= (uint64_t)buf[2] << 16;
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@ -385,28 +535,9 @@ read64le(const uint8_t *buf)
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num |= (uint64_t)buf[6] << 48;
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num |= (uint64_t)buf[7] << 56;
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return num;
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#endif
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}
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// NOTE: Possible byte swapping must be done in a macro to allow the compiler
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// to optimize byte swapping of constants when using glibc's or *BSD's
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// byte swapping macros. The actual write is done in an inline function
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// to make type checking of the buf pointer possible.
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#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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# define write16be(buf, num) write16ne(buf, conv16be(num))
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# define write32be(buf, num) write32ne(buf, conv32be(num))
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# define write64be(buf, num) write64ne(buf, conv64be(num))
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#endif
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#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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# define write16le(buf, num) write16ne(buf, conv16le(num))
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# define write32le(buf, num) write32ne(buf, conv32le(num))
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# define write64le(buf, num) write64ne(buf, conv64le(num))
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#endif
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#ifndef write16be
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static inline void
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write16be(uint8_t *buf, uint16_t num)
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{
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@ -414,10 +545,8 @@ write16be(uint8_t *buf, uint16_t num)
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buf[1] = (uint8_t)num;
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return;
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}
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#endif
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#ifndef write16le
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static inline void
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write16le(uint8_t *buf, uint16_t num)
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{
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@ -425,10 +554,8 @@ write16le(uint8_t *buf, uint16_t num)
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buf[1] = (uint8_t)(num >> 8);
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return;
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}
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#endif
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#ifndef write32be
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static inline void
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write32be(uint8_t *buf, uint32_t num)
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{
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@ -438,10 +565,8 @@ write32be(uint8_t *buf, uint32_t num)
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buf[3] = (uint8_t)num;
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return;
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}
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#endif
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#ifndef write32le
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static inline void
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write32le(uint8_t *buf, uint32_t num)
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{
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@ -451,10 +576,8 @@ write32le(uint8_t *buf, uint32_t num)
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buf[3] = (uint8_t)(num >> 24);
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return;
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}
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#endif
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#ifndef write64be
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static inline void
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write64be(uint8_t *buf, uint64_t num)
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{
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@ -468,10 +591,8 @@ write64be(uint8_t *buf, uint64_t num)
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buf[7] = (uint8_t)num;
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return;
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}
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#endif
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#ifndef write64le
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static inline void
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write64le(uint8_t *buf, uint64_t num)
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{
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@ -485,6 +606,7 @@ write64le(uint8_t *buf, uint64_t num)
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buf[7] = (uint8_t)(num >> 56);
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return;
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}
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#endif
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