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Subject: Re: Volatile happens before question
Date: Wed, 18 Jan 2012 17:17:32 -0800
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On 1/18/2012 4:34 PM, Patricia Shanahan wrote:
>
> I'm curious about the "values out of thin air" statement about hardware.
> Could you give some information about how it happens?


<http://vimeo.com/3757991>

If I understand correctly:

x & y are global integers initialized to 0. r1 and r2 are temporary 
variables.

Thread 1:

   int r1 = x;
   y = r1;

Thread 2:

   int r2 = y;
   x = r2;


What happens is during the read of x, it is not found in cache.  So a 
bus cycle is started to read the value of x, but at the same time the 
cpu has nothing to do while waiting, so it speculates as to the value of 
x and continues processing.  Let's say the cpu speculates that x is 42.

  r1 = 42;

Now since r1 is 42, when y is written it also gets a speculative write 
of the value 42;

  y = 42;

This doesn't actually go on the memory bus, it's held in an output queue 
of writes.

Next, Thread 2 comes along and also tries to read y.  It also can't and 
like thread 1 decides to speculate on the value of y while waiting for 
main memory.  It speculates y is 42.

  r2 = 42;

Next x is written from the value of r2.

  x = 42;

At this point, the internal cpu bus sees a write of 42 to the memory 
location of x and thinks its speculation was correct, cancels the memory 
read, and commits the value of y to memory.

That's what I understood from that talk on Vimeo.  I'm starting to 
wonder though exactly what sort of problem Bartosz Milewski is 
describing there.  I thought he was describing what could actually 
happen in the absence of synchronization; possibly he is speculating on 
some kind of hardware issue however.