=============== #include <stdint.h> uint32_t load_uint32_be(uint8_t* p) { return (p[0] << 24) | (p[1] << 16) | (p[2] << 8) | p[3]; } uint32_t load_ui

=============== #include <stdint.h> uint32_t load_uint32_be(uint8_t* p) { return (p[0] << 24) | (p[1] << 16) | (p[2] << 8) | p[3]; } uint32_t load_uint32_le(uint8_t* p) { return (p[3] << 24) | (p[2] << 16) | (p[1] << 8) | p[0]; } =============== gcc -O3 -fomit-frame-pointer -S bo.c =============== load_uint32_be: movl 4(%esp), %edx movzbl (%edx), %eax movzbl 1(%edx), %ecx sall $24, %eax sall $16, %ecx orl %ecx, %eax movzbl 3(%edx), %ecx movzbl 2(%edx), %edx orl %ecx, %eax sall $8, %edx orl %edx, %eax ret load_uint32_le: movl 4(%esp), %edx movzbl 3(%edx), %eax movzbl 2(%edx), %ecx sall $24, %eax sall $16, %ecx orl %ecx, %eax movzbl (%edx), %ecx movzbl 1(%edx), %edx orl %ecx, %eax sall $8, %edx orl %edx, %eax ret GCC doesn't merge the 4 byte-level reads into one 32-bit read. Thus, it does cause some performance penalty. The true impact is probably quite low, but it does exist on x86. It is true however, that GCC will take a series of bit ops and produce a 'bswap' instruction on x86, but that requires a full 32-bit word to start with.