mirror of https://git.tukaani.org/xz.git
tuklib_integer: Improve unaligned memory access.
Now memcpy() or GNU C packed structs for unaligned access instead of type punning. See the comment in this commit for details. Avoiding type punning with unaligned access is needed to silence gcc -fsanitize=undefined. New functions: unaliged_readXXne and unaligned_writeXXne where XX is 16, 32, or 64.
This commit is contained in:
Lasse Collin
parent
2a22de439e
commit
3bc112c2d3
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@ -42,6 +42,7 @@
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#define TUKLIB_INTEGER_H
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#include "tuklib_common.h"
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#include <string.h>
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////////////////////////////////////////
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@ -274,61 +275,211 @@ write64ne(uint8_t *buf, uint64_t num)
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// Unaligned reads and writes //
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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 (at least) because compilers can emit vector instructions that
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// require aligned pointers even if non-vector instructions work with
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// unaligned pointers.
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//
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// Using memcpy() is the standard compliant way to do unaligned access.
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// Many modern compilers inline it so there is no function call overhead.
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//
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// However, it seems that some compilers generate better code with a cast
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// to a packed struct than with memcpy():
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// - Old GCC versions (early 4.x and older) on x86
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// - GCC <= 8.2 (and possibly newer) on ARMv5 (but ARMv5 is old and maybe
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// doesn't matter so much)
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// - GCC <= 5.x on ARMv7 (on 4.x neither is great but packed is less bad)
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// - Intel C Compiler <= 19 (and possibly newer)
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//
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// GCC on ARMv6 is weird:
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// - GCC >= 6.x is better with memcpy() than with a packed struct.
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// - On GCC < 6 neither method is good, but packed seems less bad.
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//
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// https://gcc.godbolt.org/ was useful for seeing what kind of code is
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// generated by different compilers on different archs. Note that one
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// may need to try a little less trivial code than than these functions
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// alone to spot differences. For example this is better with packed method
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// on Intel C Compiler 19:
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//
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// int foo(const uint8_t *a, const uint8_t *b)
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// {
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// return unaligned_read16ne(a) == unaligned_read16ne(b);
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// }
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//
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// Based on the above information, prefer the memcpy() method in
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// general (including all Clang versions), but use the packed struct
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// with GCC 5.x and older and with the Intel C Compiler. This isn't
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// optimal but at least it covers some known special cases.
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#if defined(__GNUC__) && !defined(__clang__) \
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&& (__GNUC__ < 6 || defined(__INTEL_COMPILER))
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# define TUKLIB_UNALIGNED_WITH_PACKED 1
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#endif
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static inline uint16_t
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unaligned_read16ne(const uint8_t *buf)
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{
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#ifdef TUKLIB_UNALIGNED_WITH_PACKED
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struct __attribute__((__packed__)) s { uint16_t v; };
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const struct s *p = (const struct s *)buf;
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return p->v;
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#else
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uint16_t num;
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memcpy(&num, buf, sizeof(num));
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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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unaligned_read32ne(const uint8_t *buf)
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{
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#ifdef TUKLIB_UNALIGNED_WITH_PACKED
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struct __attribute__((__packed__)) s { uint32_t v; };
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const struct s *p = (const struct s *)buf;
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return p->v;
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#else
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uint32_t num;
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memcpy(&num, buf, sizeof(num));
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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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unaligned_read64ne(const uint8_t *buf)
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{
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#ifdef TUKLIB_UNALIGNED_WITH_PACKED
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struct __attribute__((__packed__)) s { uint64_t v; };
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const struct s *p = (const struct s *)buf;
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return p->v;
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#else
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uint64_t num;
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memcpy(&num, buf, sizeof(num));
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return num;
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#endif
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}
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static inline void
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unaligned_write16ne(uint8_t *buf, uint16_t num)
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{
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#ifdef TUKLIB_UNALIGNED_WITH_PACKED
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struct __attribute__((__packed__)) s { uint16_t v; };
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struct s *p = (struct s *)buf;
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p->v = num;
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#else
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memcpy(buf, &num, sizeof(num));
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#endif
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return;
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}
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static inline void
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unaligned_write32ne(uint8_t *buf, uint32_t num)
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{
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#ifdef TUKLIB_UNALIGNED_WITH_PACKED
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struct __attribute__((__packed__)) s { uint32_t v; };
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struct s *p = (struct s *)buf;
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p->v = num;
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#else
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memcpy(buf, &num, sizeof(num));
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#endif
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return;
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}
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static inline void
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unaligned_write64ne(uint8_t *buf, uint64_t num)
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{
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#ifdef TUKLIB_UNALIGNED_WITH_PACKED
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struct __attribute__((__packed__)) s { uint64_t v; };
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struct s *p = (struct s *)buf;
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p->v = num;
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#else
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memcpy(buf, &num, sizeof(num));
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#endif
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return;
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}
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// NOTE: TUKLIB_FAST_UNALIGNED_ACCESS indicates only support for 16-bit and
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// 32-bit unaligned integer loads and stores. It's possible that 64-bit
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// unaligned access doesn't work or is slower than byte-by-byte access.
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// Since unaligned 64-bit is probably not needed as often as 16-bit or
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// 32-bit, we simply don't support 64-bit unaligned access for now.
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#ifdef TUKLIB_FAST_UNALIGNED_ACCESS
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# define unaligned_read16be read16be
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# define unaligned_read16le read16le
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# define unaligned_read32be read32be
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# define unaligned_read32le read32le
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# define unaligned_write16be write16be
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# define unaligned_write16le write16le
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# define unaligned_write32be write32be
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# define unaligned_write32le write32le
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#else
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static inline uint16_t
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unaligned_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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return conv16be(unaligned_read16ne(buf));
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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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unaligned_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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return conv16le(unaligned_read16ne(buf));
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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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unaligned_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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return conv32be(unaligned_read32ne(buf));
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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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unaligned_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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return conv32le(unaligned_read32ne(buf));
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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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#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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// Like in the aligned case, these need to be macros.
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# define unaligned_write16be(buf, num) \
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unaligned_write16ne(buf, conv16be(num))
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# define unaligned_write32be(buf, num) \
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unaligned_write32ne(buf, conv32be(num))
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#endif
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#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
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# define unaligned_write16le(buf, num) \
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unaligned_write16ne(buf, conv16le(num))
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# define unaligned_write32le(buf, num) \
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unaligned_write32ne(buf, conv32le(num))
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#endif
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#ifndef unaligned_write16be
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static inline void
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unaligned_write16be(uint8_t *buf, uint16_t num)
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{
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@ -336,8 +487,10 @@ unaligned_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 unaligned_write16le
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static inline void
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unaligned_write16le(uint8_t *buf, uint16_t num)
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{
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@ -345,8 +498,10 @@ unaligned_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 unaligned_write32be
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static inline void
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unaligned_write32be(uint8_t *buf, uint32_t num)
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{
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@ -356,8 +511,10 @@ unaligned_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 unaligned_write32le
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static inline void
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unaligned_write32le(uint8_t *buf, uint32_t num)
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{
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@ -367,7 +524,6 @@ unaligned_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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