Machine Code..._(re)

Thu, 5 Jun 1997 14:10:06 +0300 (EET DST)

On Fri, 2 May 1997, PredatorZeta wrote:

> 1)_The reserved word "result" ISN'T explained properly in the info
> documentation. (at least in
> my version!!???). It work, but with this stupid warning:

	'result' is not a reserved word. In extended pascal, you may
	declare a return variable, which in this case happens
	to be called 'result'.

> Warning:  return value of function not assigned
> 
> I think because GPC can't consider the Asm assignment as a real
> assignment....

	Right, gpc does not handle this properly. But I had
	the impression the compiler flow analysis would recognize
	this, if the output section of the asm() uses the result
	variable...

> 2)_The code outputted from your proc with -O2 switch activated is:
> 
> 1	pushl %ebp
> 2	movl %esp,%ebp
> 3	subl $12,%esp
> 4	movl %ecx,-4(%ebp)
> 5	movl 8(%ebp),%edx
> /APP
> 6	inb %dx, %al
> /NOAPP
> 7	movb %al,-5(%ebp)                            
> 8	movb -5(%ebp),%al                            |
> 9	leave
> 10	ret
> 
> =8-||||| AAAAAAAARRRRGGGHHH!!!8-)) Whatta  the lines 7 and 8 do??????
> NOTHING!!!
> If I cutout these lines the code works IDeNtIcAL!! Also this cost >3<
> clock cycles on Pentium, for the AGI stall...

	I think this is because the return value of the function
	is declared volatile to prevent optimization screwing
	it up. I remember having problems with this, so please don't
	remove the volatile stuff until the real problem is tracked.
	In this case (when the return variable exists) the code
	is of course quite stupid. (but I don't remember the intel
	assembler well enough to be sure about the problem)

						Juki
						jtv@hut.fi

	ps. Thanks for the asm manual! Although quite PC specific ":-)"
	it is a lot simpler to understand than the GCC documentation.
	Please keep working with it.

Jukka Virtanen (jtv@cc.hut.fi)