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 2019-06-01 18:41:16 +03:00
parent 596ed3de44
commit 29afef0348
1 changed files with 168 additions and 12 deletions

View File

@ -42,6 +42,7 @@
#define TUKLIB_INTEGER_H #define TUKLIB_INTEGER_H
#include "tuklib_common.h" #include "tuklib_common.h"
#include <string.h>
//////////////////////////////////////// ////////////////////////////////////////
@ -274,61 +275,211 @@ write64ne(uint8_t *buf, uint64_t num)
// Unaligned reads and writes // // Unaligned reads and writes //
//////////////////////////////// ////////////////////////////////
// The traditional way of casting e.g. *(const uint16_t *)uint8_pointer
// is bad (at least) because compilers can emit vector instructions that
// require aligned pointers even if non-vector instructions work with
// unaligned pointers.
//
// Using memcpy() is the standard compliant way to do unaligned access.
// Many modern compilers inline it so there is no function call overhead.
//
// However, it seems that some compilers generate better code with a cast
// to a packed struct than with memcpy():
// - Old GCC versions (early 4.x and older) on x86
// - GCC <= 8.2 (and possibly newer) on ARMv5 (but ARMv5 is old and maybe
// doesn't matter so much)
// - GCC <= 5.x on ARMv7 (on 4.x neither is great but packed is less bad)
// - Intel C Compiler <= 19 (and possibly newer)
//
// GCC on ARMv6 is weird:
// - GCC >= 6.x is better with memcpy() than with a packed struct.
// - On GCC < 6 neither method is good, but packed seems less bad.
//
// https://gcc.godbolt.org/ was useful for seeing what kind of code is
// generated by different compilers on different archs. Note that one
// may need to try a little less trivial code than than these functions
// alone to spot differences. For example this is better with packed method
// on Intel C Compiler 19:
//
// int foo(const uint8_t *a, const uint8_t *b)
// {
// return unaligned_read16ne(a) == unaligned_read16ne(b);
// }
//
// Based on the above information, prefer the memcpy() method in
// general (including all Clang versions), but use the packed struct
// with GCC 5.x and older and with the Intel C Compiler. This isn't
// optimal but at least it covers some known special cases.
#if defined(__GNUC__) && !defined(__clang__) \
&& (__GNUC__ < 6 || defined(__INTEL_COMPILER))
# define TUKLIB_UNALIGNED_WITH_PACKED 1
#endif
static inline uint16_t
unaligned_read16ne(const uint8_t *buf)
{
#ifdef TUKLIB_UNALIGNED_WITH_PACKED
struct __attribute__((__packed__)) s { uint16_t v; };
const struct s *p = (const struct s *)buf;
return p->v;
#else
uint16_t num;
memcpy(&num, buf, sizeof(num));
return num;
#endif
}
static inline uint32_t
unaligned_read32ne(const uint8_t *buf)
{
#ifdef TUKLIB_UNALIGNED_WITH_PACKED
struct __attribute__((__packed__)) s { uint32_t v; };
const struct s *p = (const struct s *)buf;
return p->v;
#else
uint32_t num;
memcpy(&num, buf, sizeof(num));
return num;
#endif
}
static inline uint64_t
unaligned_read64ne(const uint8_t *buf)
{
#ifdef TUKLIB_UNALIGNED_WITH_PACKED
struct __attribute__((__packed__)) s { uint64_t v; };
const struct s *p = (const struct s *)buf;
return p->v;
#else
uint64_t num;
memcpy(&num, buf, sizeof(num));
return num;
#endif
}
static inline void
unaligned_write16ne(uint8_t *buf, uint16_t num)
{
#ifdef TUKLIB_UNALIGNED_WITH_PACKED
struct __attribute__((__packed__)) s { uint16_t v; };
struct s *p = (struct s *)buf;
p->v = num;
#else
memcpy(buf, &num, sizeof(num));
#endif
return;
}
static inline void
unaligned_write32ne(uint8_t *buf, uint32_t num)
{
#ifdef TUKLIB_UNALIGNED_WITH_PACKED
struct __attribute__((__packed__)) s { uint32_t v; };
struct s *p = (struct s *)buf;
p->v = num;
#else
memcpy(buf, &num, sizeof(num));
#endif
return;
}
static inline void
unaligned_write64ne(uint8_t *buf, uint64_t num)
{
#ifdef TUKLIB_UNALIGNED_WITH_PACKED
struct __attribute__((__packed__)) s { uint64_t v; };
struct s *p = (struct s *)buf;
p->v = num;
#else
memcpy(buf, &num, sizeof(num));
#endif
return;
}
// NOTE: TUKLIB_FAST_UNALIGNED_ACCESS indicates only support for 16-bit and // NOTE: TUKLIB_FAST_UNALIGNED_ACCESS indicates only support for 16-bit and
// 32-bit unaligned integer loads and stores. It's possible that 64-bit // 32-bit unaligned integer loads and stores. It's possible that 64-bit
// unaligned access doesn't work or is slower than byte-by-byte access. // unaligned access doesn't work or is slower than byte-by-byte access.
// Since unaligned 64-bit is probably not needed as often as 16-bit or // Since unaligned 64-bit is probably not needed as often as 16-bit or
// 32-bit, we simply don't support 64-bit unaligned access for now. // 32-bit, we simply don't support 64-bit unaligned access for now.
#ifdef TUKLIB_FAST_UNALIGNED_ACCESS
# define unaligned_read16be read16be
# define unaligned_read16le read16le
# define unaligned_read32be read32be
# define unaligned_read32le read32le
# define unaligned_write16be write16be
# define unaligned_write16le write16le
# define unaligned_write32be write32be
# define unaligned_write32le write32le
#else
static inline uint16_t static inline uint16_t
unaligned_read16be(const uint8_t *buf) unaligned_read16be(const uint8_t *buf)
{ {
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
return conv16be(unaligned_read16ne(buf));
#else
uint16_t num = ((uint16_t)buf[0] << 8) | (uint16_t)buf[1]; uint16_t num = ((uint16_t)buf[0] << 8) | (uint16_t)buf[1];
return num; return num;
#endif
} }
static inline uint16_t static inline uint16_t
unaligned_read16le(const uint8_t *buf) unaligned_read16le(const uint8_t *buf)
{ {
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
return conv16le(unaligned_read16ne(buf));
#else
uint16_t num = ((uint16_t)buf[0]) | ((uint16_t)buf[1] << 8); uint16_t num = ((uint16_t)buf[0]) | ((uint16_t)buf[1] << 8);
return num; return num;
#endif
} }
static inline uint32_t static inline uint32_t
unaligned_read32be(const uint8_t *buf) unaligned_read32be(const uint8_t *buf)
{ {
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
return conv32be(unaligned_read32ne(buf));
#else
uint32_t num = (uint32_t)buf[0] << 24; uint32_t num = (uint32_t)buf[0] << 24;
num |= (uint32_t)buf[1] << 16; num |= (uint32_t)buf[1] << 16;
num |= (uint32_t)buf[2] << 8; num |= (uint32_t)buf[2] << 8;
num |= (uint32_t)buf[3]; num |= (uint32_t)buf[3];
return num; return num;
#endif
} }
static inline uint32_t static inline uint32_t
unaligned_read32le(const uint8_t *buf) unaligned_read32le(const uint8_t *buf)
{ {
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
return conv32le(unaligned_read32ne(buf));
#else
uint32_t num = (uint32_t)buf[0]; uint32_t num = (uint32_t)buf[0];
num |= (uint32_t)buf[1] << 8; num |= (uint32_t)buf[1] << 8;
num |= (uint32_t)buf[2] << 16; num |= (uint32_t)buf[2] << 16;
num |= (uint32_t)buf[3] << 24; num |= (uint32_t)buf[3] << 24;
return num; return num;
#endif
} }
#if defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
// Like in the aligned case, these need to be macros.
# define unaligned_write16be(buf, num) \
unaligned_write16ne(buf, conv16be(num))
# define unaligned_write32be(buf, num) \
unaligned_write32ne(buf, conv32be(num))
#endif
#if !defined(WORDS_BIGENDIAN) || defined(TUKLIB_FAST_UNALIGNED_ACCESS)
# define unaligned_write16le(buf, num) \
unaligned_write16ne(buf, conv16le(num))
# define unaligned_write32le(buf, num) \
unaligned_write32ne(buf, conv32le(num))
#endif
#ifndef unaligned_write16be
static inline void static inline void
unaligned_write16be(uint8_t *buf, uint16_t num) unaligned_write16be(uint8_t *buf, uint16_t num)
{ {
@ -336,8 +487,10 @@ unaligned_write16be(uint8_t *buf, uint16_t num)
buf[1] = (uint8_t)num; buf[1] = (uint8_t)num;
return; return;
} }
#endif
#ifndef unaligned_write16le
static inline void static inline void
unaligned_write16le(uint8_t *buf, uint16_t num) unaligned_write16le(uint8_t *buf, uint16_t num)
{ {
@ -345,8 +498,10 @@ unaligned_write16le(uint8_t *buf, uint16_t num)
buf[1] = (uint8_t)(num >> 8); buf[1] = (uint8_t)(num >> 8);
return; return;
} }
#endif
#ifndef unaligned_write32be
static inline void static inline void
unaligned_write32be(uint8_t *buf, uint32_t num) unaligned_write32be(uint8_t *buf, uint32_t num)
{ {
@ -356,8 +511,10 @@ unaligned_write32be(uint8_t *buf, uint32_t num)
buf[3] = (uint8_t)num; buf[3] = (uint8_t)num;
return; return;
} }
#endif
#ifndef unaligned_write32le
static inline void static inline void
unaligned_write32le(uint8_t *buf, uint32_t num) unaligned_write32le(uint8_t *buf, uint32_t num)
{ {
@ -367,7 +524,6 @@ unaligned_write32le(uint8_t *buf, uint32_t num)
buf[3] = (uint8_t)(num >> 24); buf[3] = (uint8_t)(num >> 24);
return; return;
} }
#endif #endif