Richard Biener wrote, On Thursday 24 January 2013 05:28 PM:
In the program below, we have a global pointer p that has conditional
assignments before its
use on the RHS of a copy statement.
-----------------------------------------------------------------------------------------
#include<stdio.h>
int **p;
int main()
{
int *x,*y,a,c,*z;
x = &a;
z = &c;
if(a)
p = &x;
else
p = &y;
int **t = p;
printf("%d,%d",*z,*x);
}
-----------------------------------------------------------------------------------------
The assignment t=p does not seem to load p inspite of p being a global
pointer. Here's
the relevant code fragment from test.c.017t.ssa:
----------------------------------------------------------------------------------------
<bb 3>:
p = &x;
goto <bb 5>;
<bb 4>:
p = &y;
<bb 5>:
t_3 = p;
here p is loaded directly into t_3, you can consider it optimized
(copy propagated)
from
D.1234_8 = p;
t_3 = D.1234_8;
x.1_4 = x;
D.2524_5 = *x.1_4;
D.2525_6 = *z_1;
---------------------------------------------------------------------------------------
Note that p is global but it has been loaded neither loaded, nor does it
have a PHI function.
As p resides in memory it does not need a PHI - values are merged by
storing them to memory.
Yes, but because it is in memory, should it not be loaded? I thought every
memory
access is preceded by a load. At least that is how I interpreted your original
statement.
There is no way we can put the pointees of p in SSA_NAME_PTR_INFO of p in a
flow sensitive
manner.
So if a later optimization pass were to find out aliases of p or pointees of
p for the statement
t_3 = p, I don't know how will we supply the information that was discovered
by our analysis.
Your analysis should have come across that statement
t_3 = p;
and record that t_3 points to what p points to at this point of the program,
thus compute a solution for all SSA name pointers.
Yes that is easy to do. But my concern is what if there is a pass that
requires/uses pointer
information of p in the statement t_3 = p.
This is what the present points-to algorithm does, and at the end it simply
discards anything but the solutions computed for SSA name pointers
which are stored in SSA_NAME_PTR_INFO.
Oh, does it mean that the SSA_NAME_PTR_INFO would not be present for p
but would be present only for t_3 (because p is not an SSA name and t_3 is)?
If this is true, can we assume the invariant that any pass would seek pointer
info only for SSA names and not for any non-SSA name?
If that is the case, then I think there is a simple way in which we can make the
result of our analysis available to all other passes without they having to
consult our data structure.
Uday.
