Proposed doc update for Explicit Reg Vars 3/3

[Segher Boessenkool]

On Mon, Oct 12, 2015 at 04:09:34PM -0700, David Wohlferd wrote:
> I was hoping to modify the text to say that local register variables can 
> "only" be used to call Extended asm.  This would greatly simplify this 
> section.

But it is not true: they can be used anywhere any variable can be used.
Only in asm operands is it guaranteed that they end up in certain regs
though.

> -This option does not guarantee that GCC generates code that has
> -this variable in the register you specify at all times.  You may not
> -code an explicit reference to this register in the assembler
> -instruction template part of an @code{asm} statement and assume it
> -always refers to this variable.
> -However, using the variable as an input or output operand to the @code{asm}
> -guarantees that the specified register is used for that operand.  
> -@xref{Extended Asm}, for more information.

... as this correctly said.

> +Defining a register variable does not reserve the register; it 
> +remains available for other uses in places where flow control determines 
> +the variable's value is not live.  For this reason, the following uses 

This is misleading -- people will try to keep the variable "live" so it
stays in the wanted reg.  Which is not something guaranteed to work either.


Segher