Re: Request for discussion: Rewrite of inline assembler docs

[dw <limegreensocks at yahoo dot com>]

> Sorry for the slow response.

Thanks for getting back to me.  I was pretty sure I didn't have this all 
quite right yet.

>    asm ("" : "=m" (*x), "=r" (y));
>
> you have to assume that the address in %0 might use the same register as %1

Ok, now I'm getting there.  It helps that I've compiled some examples 
and can see what is happening.  This one is subtle.  I'm going to have 
to go back and review my code to see if I've ever done this.

So, the existing text (which only talks about overlaps with input 
parameters) reads:

"Unless an output operand has the '&' constraint modifier (see 
Modifiers), GCC may allocate it in the same register as an unrelated 
input operand, on the assumption that the assembler code will consume 
its inputs before producing outputs. This assumption may be false if the 
assembler code actually consists of more than one instruction. In this 
case, use '&' for each output operand that must not overlap an input."

I'm thinking about adding something like this after it:

"The same problem can occur if one of the output parameters allows a 
register constraint and contains an address.  In this case, GCC may use 
the same register for this parameter as it does for other output 
parameters that allow a memory constraint.  This can produce 
inconsistent results if the register address is updated before updating 
the memory address.  Combining the '&' constraint with the register 
constraint prevents this overlap and resolves the inconsistency."

That's as clear as I can come up with.  Better?

dw