My understanding is that C++ reinterpret_cast and C pointer cast is a just a compile-time functionality and that it has no performance cost at all.
Is this true?
7 Answers
I was looking at my assembler code before and after reinterpret casting signed char as unsigned char. The instructions grew by about 3 or four more instructions.
int main()
{
signed char i = 0x80;
(unsigned char&)i >>= 7;
return i;
}
I was casting to unsigned char to make the compiler use SHL instruction, rather than SAR instruction, so that newly shifted shift in bits would be zer0s instead of var i signed bit value.
The compiler still and seems to always use SAR instruction. But the reinterpret casting made the compiler add more instructions. 3 to 4 more instructions!
I was concerned why my unicode function for converting UTF8 to UTF16 string was almost 3 times slower than Win32 MultiByteToWideChar(). Now I am worried that casting is one of the main factors.
Which is IRONIC, as we use reinterpret cast for speed.
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