Trying to backport alias fix to 4.2

[Ned Gill]

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.

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Re: Trying to backport alias fix to 4.2

[Ned Gill]

On Tue, 18 Aug 2009 10:14:37 +0100, Richard Guenther  
 wrote:



It sounds like a bug in points-to analysis where it doesn't correctly
handle return values of pure/const functions.  I would just
add a constraint from anything to the lhs of the call to fix it.


Thanks for the quick reply.  Could you expand a little - should I be  
looking at tree-ssa-alias.c or tree-ssa-pre.c?  I'm not really familiar  
with this part of the compiler.  What do you mean by "constraint" in this  
context?


Thanks,
Ned.

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Re: Trying to backport alias fix to 4.2

[Ned Gill]

FWIW, I'm still looking at this on and off.  I beleive the pure function  
was a red herring, as this example also does the wrong thing:


extern int z;

int foo(int *y)
{
z = 1;
*y = 0;
return z;  // returns 1, even though y could point to z :(
}

Not much joy tracking it down as yet.  I've attached the alias information  
tree dump in case anyone fancies a look.  It looks ok to me, but I don't  
really understand it fully.


As a reminder: this bug is fixed from r119502 on trunk. I'd like to fix it  
on 4.2.x.



Ned.


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bbb.c.030t.alias1
Description: Binary data