Re: Another security vulnerability

Thomas Koenig wrote:

MitchAlsup1 <mitchalsup@aol.com> schrieb:

Thomas Koenig wrote:
>

Stefan Monnier <monnier@iro.umontreal.ca> schrieb:

Since each chased pointer starts back at LSQ, the cost is no different
than an explicit Prefetch instruction, except without (a),(b) and (c)
having been applied first.

>
I thought the important difference is that the decision to prefetch or
not can be done dynamically based on past history.

>

Programmers and compilers are notoriously bad at predicting
branches (except for error branches),

>
Which are always predicted to have no error.

There I mean that the programmer wrote the code::
      if( no error so far )       {
          then continue
      }
      else
      {
          deal with the error
      }
Many times, the "deal with the error" code is never even fetched.

On the second or third time, certainly.  Hmmm... given hot/cold
splitting which is fairly standard by now, do branch predictors
take this into account?

First we are talking about predicting branches at compile time and
the way the programmer writes the source code, not about the dynamic
predictions of HW.
Given that it is compile time, one either predicts it is taken
(loops) or not taken (errors and initialization) and arrange
the code such that fall through is the predicted pattern (except
for loops).
Then at run time, all these branches are predicted with the standard
predictors present in the core.
Initialization stuff is mispredicted once or twice
error code is only mispredicted when an error occurs
loops are mispredicted once or twice per entrance.
Also note:: With an ISA like My 66000, one can preform branching using
predication and neither predict the branch nor modify where FETCH is
fetching. Ideally, predication should deal with hard to predict branches
and all flow control where the then and else clauses are short. When
these are removed from the predictor, prediction should improve--maybe not in the number of predictions that are correct, but in the total time
wasted on branching (including both repair and misfetching overheads).

                                      but ought to be quite good
at predicting prefetches.

>
What makes you think programmers understand prefetches any better than
exceptions ??

Pointers are used in many common data structures; linked list,
trees, ...  A programmer who does not know about dereferencing
pointers should be kept away from computer keyboards, preferably
at a distance of at least 3 m.

3Km ??

 

                          If a pointer is loaded, chances are
very high that are it will be dereferenced.

>
What if the value loaded is NULL.

Then it should be trivially predicted that it should not be prefetched.