C intrinsics, std, cld

Question

I have an x64 C (not C++) dll that uses memmove on occasion. In some cases, the move is forward in overlapping memory, and memmove handles it properly as documented. For int * p, the kind of move I'm talking about is memmove(p+2,p+1,n).

I have a performance issue, and I'd like to try using intrinsics __movsd and __movsq. They compile to about 4 inline instructions, so they look promising. What I'd like to try is setting and clearing the direction flag like __std; __movsd(); __cld. Alas, in x64 I can't find direction flag manipulation intrinsics, and I can't use asm. Any ideas?

Answer 1

The direction flag can be changed with __writeeflags. Check an example:

unsigned long s[8] = { 100, 101, 102, 103, 104, 105, 106, 107 };
unsigned long d[8] = { 0 };

__writeeflags( __readeflags() | ( 1 << 10 ) );
__movsd( d + 4, s + 4, 3 );

It seems to work, however it does not generate just a simple std. Maybe consider adding an asm file to your project. Visual Studio is able to compile it.