
In put_be32(), we right-shift a uint32_t value various amounts and then assign the low 8-bits to individual "unsigned char" bytes, throwing away the high bits. For shifts smaller than 24 bits, those thrown away bits will be arbitrary bits from the original uint32_t. This works exactly as we want, but if you feed a constant, then sparse complains. For example if we write this (which we plan to do in a future patch): put_be32(hdr, PACK_SIGNATURE); then "make sparse" produces: compat/bswap.h:175:22: error: cast truncates bits from constant value (5041 becomes 41) compat/bswap.h:176:22: error: cast truncates bits from constant value (504143 becomes 43) compat/bswap.h:177:22: error: cast truncates bits from constant value (5041434b becomes 4b) And the same issue exists in the other put_be*() functions, when used with a constant. We can silence this warning by explicitly masking off the truncated bits. The compiler is smart enough to know the result is the same, and the asm generated by gcc (with both -O0 and -O2) is identical. Curiously this line already exists: put_be32(&hdr_version, INDEX_EXTENSION_VERSION2); in the fsmonitor.c file, but it does not get flagged because the CPP macro expands to a small integer (2). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>
194 lines
4.5 KiB
C
194 lines
4.5 KiB
C
#ifndef COMPAT_BSWAP_H
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#define COMPAT_BSWAP_H
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/*
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* Let's make sure we always have a sane definition for ntohl()/htonl().
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* Some libraries define those as a function call, just to perform byte
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* shifting, bringing significant overhead to what should be a simple
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* operation.
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*/
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/*
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* Default version that the compiler ought to optimize properly with
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* constant values.
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*/
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static inline uint32_t default_swab32(uint32_t val)
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{
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return (((val & 0xff000000) >> 24) |
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((val & 0x00ff0000) >> 8) |
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((val & 0x0000ff00) << 8) |
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((val & 0x000000ff) << 24));
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}
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static inline uint64_t default_bswap64(uint64_t val)
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{
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return (((val & (uint64_t)0x00000000000000ffULL) << 56) |
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((val & (uint64_t)0x000000000000ff00ULL) << 40) |
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((val & (uint64_t)0x0000000000ff0000ULL) << 24) |
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((val & (uint64_t)0x00000000ff000000ULL) << 8) |
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((val & (uint64_t)0x000000ff00000000ULL) >> 8) |
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((val & (uint64_t)0x0000ff0000000000ULL) >> 24) |
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((val & (uint64_t)0x00ff000000000000ULL) >> 40) |
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((val & (uint64_t)0xff00000000000000ULL) >> 56));
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}
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#undef bswap32
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#undef bswap64
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#if defined(__GNUC__) && (defined(__i386__) || defined(__x86_64__))
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#define bswap32 git_bswap32
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static inline uint32_t git_bswap32(uint32_t x)
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{
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uint32_t result;
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if (__builtin_constant_p(x))
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result = default_swab32(x);
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else
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__asm__("bswap %0" : "=r" (result) : "0" (x));
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return result;
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}
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#define bswap64 git_bswap64
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#if defined(__x86_64__)
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static inline uint64_t git_bswap64(uint64_t x)
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{
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uint64_t result;
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if (__builtin_constant_p(x))
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result = default_bswap64(x);
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else
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__asm__("bswap %q0" : "=r" (result) : "0" (x));
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return result;
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}
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#else
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static inline uint64_t git_bswap64(uint64_t x)
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{
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union { uint64_t i64; uint32_t i32[2]; } tmp, result;
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if (__builtin_constant_p(x))
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result.i64 = default_bswap64(x);
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else {
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tmp.i64 = x;
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result.i32[0] = git_bswap32(tmp.i32[1]);
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result.i32[1] = git_bswap32(tmp.i32[0]);
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}
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return result.i64;
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}
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#endif
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#elif defined(_MSC_VER) && (defined(_M_IX86) || defined(_M_X64) || defined(_M_ARM64))
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#include <stdlib.h>
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#define bswap32(x) _byteswap_ulong(x)
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#define bswap64(x) _byteswap_uint64(x)
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#endif
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#if defined(bswap32)
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#undef ntohl
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#undef htonl
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#define ntohl(x) bswap32(x)
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#define htonl(x) bswap32(x)
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#endif
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#if defined(bswap64)
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#undef ntohll
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#undef htonll
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#define ntohll(x) bswap64(x)
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#define htonll(x) bswap64(x)
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#else
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#undef ntohll
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#undef htonll
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#if defined(__BYTE_ORDER) && defined(__LITTLE_ENDIAN) && defined(__BIG_ENDIAN)
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# define GIT_BYTE_ORDER __BYTE_ORDER
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# define GIT_LITTLE_ENDIAN __LITTLE_ENDIAN
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# define GIT_BIG_ENDIAN __BIG_ENDIAN
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#elif defined(BYTE_ORDER) && defined(LITTLE_ENDIAN) && defined(BIG_ENDIAN)
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# define GIT_BYTE_ORDER BYTE_ORDER
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# define GIT_LITTLE_ENDIAN LITTLE_ENDIAN
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# define GIT_BIG_ENDIAN BIG_ENDIAN
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#else
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# define GIT_BIG_ENDIAN 4321
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# define GIT_LITTLE_ENDIAN 1234
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# if defined(_BIG_ENDIAN) && !defined(_LITTLE_ENDIAN)
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# define GIT_BYTE_ORDER GIT_BIG_ENDIAN
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# elif defined(_LITTLE_ENDIAN) && !defined(_BIG_ENDIAN)
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# define GIT_BYTE_ORDER GIT_LITTLE_ENDIAN
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# elif defined(__THW_BIG_ENDIAN__) && !defined(__THW_LITTLE_ENDIAN__)
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# define GIT_BYTE_ORDER GIT_BIG_ENDIAN
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# elif defined(__THW_LITTLE_ENDIAN__) && !defined(__THW_BIG_ENDIAN__)
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# define GIT_BYTE_ORDER GIT_LITTLE_ENDIAN
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# else
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# error "Cannot determine endianness"
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# endif
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#endif
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#if GIT_BYTE_ORDER == GIT_BIG_ENDIAN
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# define ntohll(n) (n)
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# define htonll(n) (n)
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#else
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# define ntohll(n) default_bswap64(n)
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# define htonll(n) default_bswap64(n)
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#endif
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#endif
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static inline uint16_t get_be16(const void *ptr)
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{
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const unsigned char *p = ptr;
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return (uint16_t)p[0] << 8 |
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(uint16_t)p[1] << 0;
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}
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static inline uint32_t get_be32(const void *ptr)
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{
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const unsigned char *p = ptr;
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return (uint32_t)p[0] << 24 |
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(uint32_t)p[1] << 16 |
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(uint32_t)p[2] << 8 |
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(uint32_t)p[3] << 0;
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}
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static inline uint64_t get_be64(const void *ptr)
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{
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const unsigned char *p = ptr;
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return (uint64_t)get_be32(&p[0]) << 32 |
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(uint64_t)get_be32(&p[4]) << 0;
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}
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static inline void put_be32(void *ptr, uint32_t value)
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{
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unsigned char *p = ptr;
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p[0] = (value >> 24) & 0xff;
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p[1] = (value >> 16) & 0xff;
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p[2] = (value >> 8) & 0xff;
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p[3] = (value >> 0) & 0xff;
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}
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static inline void put_be64(void *ptr, uint64_t value)
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{
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unsigned char *p = ptr;
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p[0] = (value >> 56) & 0xff;
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p[1] = (value >> 48) & 0xff;
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p[2] = (value >> 40) & 0xff;
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p[3] = (value >> 32) & 0xff;
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p[4] = (value >> 24) & 0xff;
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p[5] = (value >> 16) & 0xff;
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p[6] = (value >> 8) & 0xff;
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p[7] = (value >> 0) & 0xff;
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}
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#endif /* COMPAT_BSWAP_H */
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