Re: MSDN volatile sample

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Hi Alex,


Your sample is great. I have stuied your optimization code and I think the
optimization approach you are using to optimize while loop to for loop is a
general solution, not specific code for the special sample on MSDN.

If yes, I think such type of optimization is not safe if the control
condition if while loop -- in this sample is variable Sentinel may be changed
to other value by other threads. Any comments?


regards,
George

"Alex Blekhman" wrote:

"George" wrote:
Wny code like this,

if (Sentinel)
for (;;)
Sleep(0);

is optimized compared with original code,

while (Sentinel)
Sleep(0);

I do not understand why the former is faster?

Because first case generates less instructions. It looks something
like this in pseudocode:

// original code

if (Sentinel)
for (;;)
Sleep(0);

// generated assembly code

load value from memory to register
if not zero jump to STARTFOR address
STARTFOR:
push parameter on stack
call Sleep
jump to STARTFOR address

Now, this code:

while (Sentinel)
Sleep(0);

will generate following instructions:

STARTWHILE:
load value from memory to register
if zero jump to ENDWHILE address
push parameter on stack
call Sleep
jump to STARTWHILE address
ENDWHILE:
...

As you can see in first case costly memory access is factored out
of for-loop's body. In the second case the memory where `Sentinel'
variable is resided is accessed every cycle of while-loop.

HTH
Alex



.

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