RE: precompiled probing scenarios

["Stone, Joshua I" <joshua dot i dot stone at intel dot com>]

On Tuesday, October 24, 2006 8:17 AM, David Smith wrote:
> Stone, Joshua I wrote:
>> 1. probe begin { exit() }
>> 2. probe begin { exit(); }
>> 
>> 4. probe begin, end { exit() }
>> 5. probe end, begin { exit() }
> 
> Hmm.  Just for fun, I decided to see if the pass 3 output of [1. 2.]
> or [4. 5.] would compare equally.  They don't.

That's partly my point.  Those pairings are functionally equivalent,
right?  So why should the code we generate show any differences?

The difference between 1 & 2 is basically just the line "/* null */;" in
the probe's generated C -- a useless statement.  Between 4 & 5 the only
difference is whether the begin or end is generated first.  Because
these differences are insignificant, we should be able to treat them the
same for caching purposes.

Another normalization example is with braces:

6. probe begin { if(foo) exit() }
7. probe begin { if(foo) { exit() } }
8. probe begin { if(foo) { { exit() } } }

My hope is that someday the translator will also treat less obvious
cases like these as identical:

9.  probe begin { log("foo") }
10. probe begin { if(1) log("foo") }
11. probe begin { i=1; if(i) log("foo") }
12. probe begin { while(!i++) log("foo") }

This has more to do with optimization of the generated code, but it
could help caching if the optimization is done before pass-2 output.


Josh