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"better" version of Iscale functions [message #15129]

Tue, 01 April 2008 23:10

mdelfede
Messages: 1307
Registered: September 2007

Ultimate Contributor

In file 'mathutils.cpp' the Iscale...() functions use some floating point math, when assembly is unavailable OR when it's not compatible with intel syntax (I.E. GCC and MinGW).
That makes it slow and not showing divide-by-0 errors when third argument is 0.
So, here an (IMHO) better version of such functions :

#include "Core.h"

// iscale: computes x * y / z.

#ifdef flagGCC
#define __USE_64BIT_MATH__
#endif

NAMESPACE_UPP

int iscale(int x, int y, int z)
{
#ifdef __NOASSEMBLY__
#ifndef __USE_64BIT_MATH__
	return int(x * (double)y / z);
#else
	int64_t res = x;
	res *= y;
	res /= z;
	return (int)res;
#endif
#else
	__asm
	{
		mov		eax, [x]
		imul	[y]
		idiv	[z]
	}
#endif
}

// iscalefloor: computes x * y / z, rounded towards -infty.

int iscalefloor(int x, int y, int z)
{
#ifdef __NOASSEMBLY__
#ifndef __USE_64BIT_MATH__
	return (int)ffloor(x * (double)y / z);
#else
	int64_t res = x;
	int64_t mulres = res * y;
	res = mulres / z;
	if(res * z != mulres)
		res--;
	return (int)res;
#endif
#else
	__asm
	{
		mov		eax, [x]
		imul	[y]
		idiv	[z]
		and		edx, edx
		jge		__1
		dec		eax
	__1:
	}
#endif
}

// iscaleceil: computes x * y / z, rounded towards +infty.

int iscaleceil(int x, int y, int z)
{
#ifdef __NOASSEMBLY__
#ifndef __USE_64BIT_MATH__
	return fceil(x * (double)y / z);
#else
	int64_t res = x;
	int64_t mulres = res * y;
	res = mulres / z;
	if(res * z != mulres)
		res++;
	return (int)res;
#endif
#else
	__asm
	{
		mov		eax, [x]
		imul	[y]
		idiv	[z]
		and		edx, edx
		jle		__1
		inc		eax
	__1:
	}
#endif
}

BTW, we could completely drop the assembly code, as-is it's not portable between compilers with greater integer width.
My version is also *not* portable on compilers with 64 bit wide integers, but can be made ok just changing function prototype :

int32_t iscale(int32_t x, int32_t y, int32_t z)

Leaving so to the compiler the integer width check and warnings.

Attached here the patched 'mathutil.cpp' (NO patched function prototype, as it'll require Core.h patch too).

Ciao

Max

Attachment: mathutil.cpp
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