fast floor

[Sönke Ludwig]

Am 11.10.2013 00:27, schrieb Spacen Jasset:
> Hello,
>
> I am after a fast floor function; In fact a fast truncation and
> conversion to integer. I see that std.math takes a real, and that
> std.c.math takes a double.
>
> Is there a quicker function, and what might cast(int)1.5f do?
>
> Regards,
>
> Spacen.

For a noise generator I wrote some little assembler routines for that 
some years ago. It's main speed advantage comes from the fact that it 
allows to set the rounding mode once (ftoi_init) instead of for every 
conversion (as a cast(int) does for example). However, it requires great 
care that the rounding mode isn't accidentally changed during the 
computation by some function.

Today I would use core.simd instead, though (using XMM.CVTSS2SI), but 
that requires that the whole algorithm is in SIMD for optimal performance.

This is the old code:

	void ftoi_init() {
		fesetround(FE_DOWNWARD);
	}
	void ftoi_toint2(int* dst, float src) {
		asm {
			mov EAX, dst;
			fld src;
			fistp [EAX];
		}
	}
	void ftio_fracint(int* int_part, float* frac_part, float src) {
		asm {
			mov EAX, int_part;
			mov EBX, frac_part;
			fld src;
			frndint;
			fist [EAX];
			fld src;
			fsubr;
			fstp [EBX];
		}
	}
	void ftoi_round(float* dst, float src) {
		asm {
			mov EAX, dst;
			fld src;
			frndint;
			fstp [EAX];
		}
	}