Optimal and portable conversion of endian in c/c++
Given a binary file with 32-bit little-endian fields that I need to parse, I want to write parsing code that compiles correctly independent of endianness of machine that executes that code. Currently I use
uint32_t fromLittleEndian(const char* data){
return uint32_t(data[3]) << (CHAR_BIT*3) |
uint32_t(data[2]) << (CHAR_BIT*2) |
uint32_t(data[1]) << CHAR_BIT |
data[0];
}
this, however generate inoptimal assembly. On my machine g++ -O3 -S produces:
_Z16fromLittleEndianPKc:
.LFB4:
.cfi_startproc
movsbl 3(%rdi), %eax
sall $24, %eax
movl %eax, %edx
movsbl 2(%rdi), %eax
sall $16, %eax
orl %edx, %eax
movsbl (%rdi), %edx
orl %edx, %eax
movsbl 1(%rdi), %edx
sall $8, %edx
orl %edx, %eax
ret
.cfi_endproc
why is this happening? How could I convince it to produce optimal code when compiled on little endian machines :
_Z17fromLittleEndian2PKc:
.LFB5:
.cfi_startproc
movl (%rdi), %eax
ret
.cfi_endproc
which I have gotten by compiling:
uint32_t fromLittleEndian2(const char* data){
return *reinterpret_cast<const uint32_t*>(data);
}
Since I know my machine is little-endian, I know that above assembly is optimal, but it will fail if compiled on big-endian machine. It also violates strict-aliasing rules, so if inlined it might produce UB even on little endian machines. Is there a valid code that will be compiled to optimal assembly if possible ?
Since I expect my function to be inlined a lot, any kind of runtime endian detection is out of the question. The only alternative to writing optimal C/C++ code is to use compile time endian detection, and use template s or #define s to fall back to the inefficient code if target endian is not little-endian. This however seems to be quite difficult to be done portably.
Various platform libraries that I know of do this by #defining macros for the endian-swapping routines, based on the value of #define BIG_ENDIAN. In the cases where the source endianness matches your target endianness, you can just:
#ifdef LITTLE_ENDIAN
#define fromLittleEndian(x) (x)
#else
#define fromLittleEndian(x) _actuallySwapLittle((x))
#endif
For example:
http://man7.org/linux/man-pages/man3/endian.3.html
http://fxr.watson.org/fxr/source/sys/endian.h
简短的答案 - 使用htonl - 它将会在wazzoo中得到优化
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