I wonder if it is possible to copy four uint8_t
values stored in one uint32_t
into proper places in uint64_t
as fast as possible. I am looking for equivalent of:
union
{
struct {uint8_t a; uint8_t b; uint8_t c; uint8_t d};
uint32_t whole;
} x32;
union
{
struct {int16_t a; int16_t b; int16_t c; int16_t d};
uint64_t whole;
} x64;
x64.a=x32.a;
x64.b=x32.b;
x64.c=x32.c;
x64.d=x32.d;
The problem is: I cannot use MMX/SSE.
不会。没有其他方法可以像执行操作那样移动数据并将其零扩展 。
不,这是不可能的,因为几乎没有硬件会提供这种(非常特定的)组装指令。
Type punning through union does not have support in the C++ standard. Instead, use ors and shifts to compose the value together. Correctness is more important than fast but broken code.
uint8_t a,b,c,d;
uint64_t whole;
whole = a | (uint64_t (b) << 1*16) | (uint64_t (c) << 2*16) | (uint64_t (d) << 3*16)
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