[sdcc-devel] [ sdcc-Bugs-690781 ] False optimization (key confusion)

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Bugs item #690781, was opened at 2003-02-21 17:38
You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=100599&aid=690781&group_id=599

Category: C-Front End
>Group: fixed
>Status: Closed
Resolution: None
Priority: 5
Submitted By: Erik Petrich (epetrich)
>Assigned to: Johan Knol (johanknol)
Summary: False optimization (key confusion)

Initial Comment:
This one is a cousin to bug #628025 fixed earlier. The key assigned 
to a parameter passed to a function is now getting confused with 
another iTemp in the caller that happens to have the same 
key.

Since SDCCcse.c is riddled with key comparisions, 
rather than sprinkling more IS_OTHERSPARM marcros around, 
the keys to other function's parameters could be assigned a 
nonoverlapping range with the current function's operands (for 
example, make them all negative). In any case, I don't feel 
confident enough to make such drastic changes on my own at this 
time.

Here's some sample c code:

extern char 
func1(char x, char y);

char func2(void) {
 if (!func1(1,2)) 
return 1;
 if (!func1(2,3)) return 1;
 return 0;
}

char 
func3(void) {
 if (!func1(1,2)) return 1; // <-bug
 if (!func1(2,3)) 
return 1;
 return 0;
}

Here's the iCode for the 
problematic line:

send 0x1 {literal char}{argreg = 
1}
_func1_PARM_2 [k3 lr0:0 so:0]{ ia1 re0 rm0 nos0 ru0 
dp0}{char} := 0x2 {literal char}
iTemp0 [k3 lr0:0 so:0]{ ia0 re0 
rm0 nos0 ru0 dp0}{char} = call _func1 [k2 lr0:0 so:0]{ ia0 re0 rm0 
nos0 ru0 dp0}{char function    }
if iTemp0 [k3 lr0:0 so:0]{ ia0 re0 
rm0 nos0 ru0 dp0}{char} != 0 goto _iffalse_2($2)

The 
second parameter and the function result both have key 3, so the 
optimizer effectively turns this into

 func1(1,2); if (!2) return 
1;

Key confusion aside, this also seems a bad optimization 
in that the literal 2 is stale by this point.


----------------------------------------------------------------------

>Comment By: Johan Knol (johanknol)
Date: 2003-02-24 12:49

Message:
Logged In: YES 
user_id=63512

The IS_OTHERS_PARM was a bad fix. The keys of function 
parameters needs to be reset for every new function, but that 
didn't happen for prototypes.
I fixed that in SDCCsymt.c:141 and removed the bad fix.

----------------------------------------------------------------------

You can respond by visiting: 
https://sourceforge.net/tracker/?func=detail&atid=100599&aid=690781&group_id=599