Re: Incrementing volatiles?

[Carlo Wood <carlo@runaway.xs4all.nl>]

| > bar:
| >     movl  foo,%eax
| >     incl  foo
| >     ret
| > 
| Wrong. The foo++ means "read it, perhaps do something with the value,
| increment the value, put it back". Your assembly code reads the value
| twice.
| 
| > Now, if I'm really off track, can someone please give me some pointers
| > to information that will set me right?
| > 
| "volatile" is massively undefined. My off-the-seat-of-my-pants definition
| is that volatile variables are _always_ accessed exactly as many times, and
| in exactly that order, as described in the C source code.
| 
| How to tell the backend (Intel or otherwise) that sometimes(!) it can
| combine a read-add_one-write insn sequence into one "incr", even if the
| to-be-incremented thing in question is marked as volatile, is an
| interesting question. IMHO, however, "two volatiles never match" is a bit
| too strong.

Thanks for the definition :)

Let us assume that volatile `foo' in the above example is a r/w register
of some hardware IC that generates pseudo random numbers from its own
value every time it is being read; you are allowed to write to it however
to set a 'seed'.  That is an example that would make "volatile" pretty
clear, if I understood it well.  It shows clearly that

 volatile int foo;

 int random(void) {
   return foo++;
 }

Should read one pseudo random number, increment it and write it back as seed.
That means indeed that 

random:
	movl foo,%eax
	incl foo
	ret

is not a correct way of generating the assembly code.
It should be generated like:

random:
	movl foo,%eax
	movl %eax,%edx
	incl %edx
	movl %edx,foo
	ret

Note that currently, with -O9 -fomit-frame-pointer it generates less
optimized code:

random:
	movl foo,%eax
	movl %eax,%edx
	incl %eax
	movl %eax,foo
	movl %edx,%eax
	ret

And without -fomit-frame-pointer it does something totally redundant
things with %ebp and the stack :/

random:
        pushl %ebp		}  Do we want this with -O9 ?
        movl %esp,%ebp		}
        movl foo,%eax
        movl %eax,%edx
        incl %eax
        movl %eax,foo
        movl %edx,%eax
        movl %ebp,%esp		}
        popl %ebp		}
        ret

How hard would it be to optimize:

	movl %eax,%edx
	incl %eax
	movl %eax,foo
	movl %edx,%eax
	ret

to:

	movl %eax,%edx
	incl %edx
	movl %edx,foo
	ret

-- 
 Carlo Wood  <carlo@runaway.xs4all.nl>