ASM Community

I don't get it.
Could you explain?
I thought of integer values as three groups.
1. x <> 0 and s bit = 0 (x>0)
2. x <> 0 and s bit = 1 (x<0)
3. x = 0
And that f(x) should return {1,-1,0} respectivly.

If I was wrong - what was the task?
And what format is data it eax?

Thanx for claryfication, bitRake.
We can do it with four instructions also,
and very few bytes of opcode:

	add eax,eax ;2

	je @F	    ;2

	or al,1    ;2

	salc	    ;1 = 7 bytes

@@:

We edit our posts sulmanteniously :)
And about not jcc -
why people don't like jcc - misprediction
You know chances of misprediction in this case?
1/ (2^31)
:)

Well, if here was such a condition (no jcc) then you may
consider my code as "out of the challange free-style idea".
:)

IMHO, from practicle point of view your code is for sure improvement from the original.
1. sbb would run faster then salc
2. Return in eax is better for use f(x) as index pointer.
For example f(x) returns 3 values -1,0,1 This array intself
has relation as a[0]=-1 and a=a[-n]+1
so we have leanear function where int can be used as index.
For example we have 3 different procs or 3 different blocks
of code that respectively handle (x<0),(x=0),(x>0) cases.

caseXless0:
.....
caseXeq0:
.....
caseXgr0:

;we can:

handles dd caseXless0, caseXeq0,caseXgr0
.....
.code

add eax,eax
je @F
sbb eax,eax
or eax,1
@@:
jmp

......

In code with salc the return is in al
we would need additinal movsx eax,al
to use the return for index.

The only difference in favor about salc that code with
salc is 1 byte shorter. But it is true only if return value
is not used as pointer. Otherwize using sbb again would
finally lead to shorter code.

I don't have the time to search for this right now,
but do a search on the web for "superoptimizer".
About 10-15 years ago someone used the signum
function has input to their superoptimizer program
and came up with a three-instruction sequence (IIRC)
to do this trick. Very efficient on CPUs of that day.
I suspect it's still pretty good on modern CPUs.
It took the value in EAX and left the result in EDX,
as I recall.
Cheers,
Randy Hyde

It returns -1 with 80000000h in eax

FPU signum, how?

1. You can use FTST to check if something is 0 or unordered. FTST will make your fn shorter.

2. FNSTSW AX would make your fn faster and shorter.

3. Sometimes, using AND instead of SAHF is faster. This is one of those cases.

4. Finally, you can use all of the above discussion to create the result -1,0,1 and, you can just

push eax

fild dword ptr [esp]

pop eax

at the end, which is certainly faster than using FDIV.

fsgn MACRO

	ftst

   	fnstsw AX

   	and ah,40h

   	jnz @F ; (the zero flag is inverted!)

	fld st

	fabs		

	fdiv

@@:

	;donothing because x=0 already and at top of the stack if jumped here.

	

endm

Is this better? I think it is. But i don't understand how to eliminate fdiv.