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Re: "better" version of Iscale functions [message #15140 is a reply to message #15138]

Wed, 02 April 2008 16:48

mdelfede
Messages: 1308
Registered: September 2007

Ultimate Contributor

luzr wrote on Wed, 02 April 2008 15:11

mdelfede wrote on Tue, 01 April 2008 17:10

BTW, we could completely drop the assembly code, as-is it's not portable between compilers with greater integer width.

Yes, but int64 does not come cheap on non-64 architecture. Maybe even that FP computation could be faster. Of course, as long as FP is performed by HW. For ARM this new iscale can be good.

I thought you were in holydays Smile

Back to Iscale, I don't know about modern processors that does have an hardware ftp and doesn't have 32x32->64 bit mul and 64/32 ->32 bit div core instructions... but I can be wrong.
Yet, I don't remember if intel ones works just with unsigned or signed or both integers...
BTW, I noticed that my iscale needs to work with 32 bit result; if not it'll use full 64 bit math for the multiply (in iscalefloor and iscaleceil) which can be slow.
I guess that using 32x32 multiply and 64/32 division, GCC translates it directly in DIV and MUL, but I've not checked yet.

Quote:

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.

IMO, that really is not that bug trouble, as any serious portable code should work with 32-bit int.

Mirek[/quote]

I can agree, but I think more and more that the lack of width specs in C++ is really a nasty stuff.
Now it's too late, but if I'd have to write a framework from scratch, I'd use some typedef'd int8, int16, int32 and so on stuffs.

BTW, our Iscale is much better than micro$oft's one, MulDiv, which returns -1 on 0 divisor.... so on both

MulDiv(1,-1,1)
MulDiv(a, b,0)

you get -1 as result. Smile

Another possibility would be to use GCC built in asm, which is much different in syntax from Intel one (MS), but it's quite complicated, even if much more powerful.

Max

[Updated on: Wed, 02 April 2008 16:49]

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