Re: += in ada

["Robert I. Eachus" <rieachus@comcast.net>]

Hyman Rosen wrote:

> In C++, I can say A += B, and then I don't need the temporary. If I say 
> A = B + C, then I do. But Ada forces you to say A = A + b for the first
>  case, and that gives you the temporary that you don't need.

Which makes Ada better for most common CPUs today, right?  In a 
processor with register renaming and OoO execution, which all Pentiums 
since the Pentium Pro and all Celerons, Athlons, and Durons have, 
register pressure can be caused by a lack of register names, but not 
registers.  So in (pseudo)code like
    load R1,A
    add R1,B
    store R1,A
the CPU will actually use a pair of renaming registers to represent R1 
in this code fragment.  And no matter whether you use the "Ada" approach 
or the "C++" approach to defining the sequence, you will still use two 
renaming registers and one register name to implement the code.  (Oh, 
and the time to execute the code will probably be dominated by the time 
to fetch A and B if they are not already in registers or L1 cache.)

Modern CPUs are so far from the finite state machines you may have been 
taught about in some Assembler class that it isn't even funny.  I 
usually say that modern processors have lots of circutry devoted to 
confusing anyone who tries to debug software using a debugger.  The 
problem is not just that the states you see in the debugger don't exist 
when your code is running normally, no states corresponding to the ISA 
exist when the processor is running normally.  In fact, no states at all 
exist when the processor is running.  If you tell the processor to 
create some sort of a state and push it on the stack it will do that. 
Just remember that the state you see was manufactured this way because 
you asked the processor to do so.
-- 
                                        Robert I. Eachus

"Quality is the Buddha. Quality is scientific reality. Quality is the 
goal of Art. It remains to work these concepts into a practical, 
down-to-earth context, and for this there is nothing more practical or 
down-to-earth than what I have been talking about all along...the repair 
of an old motorcycle."  -- from Zen and the Art of Motorcycle 
Maintenance by Robert Pirsig