Trying to backport alias fix to 4.2

["Ned Gill" <thomas dot gill at csr dot com>]

I'm having a problem with code like the following on all 4.2.x versions of  
gcc (at least on intel and with my port):

unsigned z = 1;
unsigned *foo()
{
    return &z;
}

int main(void)
{
    z = 1;
    *foo() = 0;
    return z;
}

When optimisation is enabled, but inlining disabled (-O1 for example) the  
compiler appears to fail to recognise that z clobbered by *foo()=0, and  
produces code that returns a constant 1.  The problem appears to be caused  
by foo being marked pure; if I move foo to another compilation unit, to  
problem goes away unless I mark the prototype with __attribute__((pure)).   
Also the problem goes away if I add a side-effect to foo.  Looking at the  
tree dumps reveals that this problem appears in the FRE pass - although  
I'd guess that the problem could well originate in the earlier alias  
analysis pass.

This problem is fixed in 4.3.  A binary search through trunk GCC led me to  
revision 119502 (Daniel Berlin, 2006-12-04).  However, it doesn't look  
like it's possible to back-port this change to 4.2, because of the  
intervening changes to the files involved.  I naively attempted to patch  
across the relitively simple changes to tree-ssa-alias.c in 119502, but  
not suprisingly that didn't help on its own.

Does anyone recognise this problem?  Any help tracking down the cause of  
the problem would be very gratefully received - if nothing else I'll have  
to re-fix the problem in 4.2.  However, my experience hacking the tree  
passes is pretty much zero :(

In the long run I can upgrade to 4.3 or 4.4.  However, updating my port  
and re-validating it is a fair amount of work that I'm not keen to tackle  
immediately.


Thanks,
Ned.

--