In cabs2cil's doExp function, the case for &&, ||, and ! includes
comments saying that the results must be normalized to 0 or 1. Then, in
the case where the expression is UnOp(LNot,_,_), there is a comment
saying that the result is already normalized. Isn't this also true for
binary operators that evaluate to boolean values? That is, couldn't we
replace line 3827 in cabs2cil.ml (revision 10610):
| CEExp (se, (UnOp(LNot, _, _) as e)) ->
with
| CEExp (se, ((UnOp(LNot, _, _)|BinOp((Eq|Ne|Le|Lt|Ge|Gt|LAnd|
LOr),_,_,_)) as e)) ->
Right now, pattern matching falls through to the next case, causing some
superfluous occurrences of '!= 0' to get inserted into code. For
example, this code
int main() {
int x = 0;
return 1 && x == 4;
}
gets transformed into
int main(void)
{ int x ;
{
x = 0;
return ((x == 4) != 0);
}
}
This might not come up so often---it seems that this only happens when
an && or || has a constant value as an operand, or if you set CIL's
useLogicalOperators flag to true---but if it is simple to stop
generating unnecessary extra code, why not do it?
Speaking of useLogicalOperators, here is a second question:
When useLogicalOperators is set to true, cabs2cil.ml's doCondExp
function keeps &&s and ||s (rather than transforming them to explicit
conditionals) if neither operand has side-effects. However, notice that
the *first* operand will be evaluated regardless of whether short-
circuiting occurs; this means that we only need to check if the *second*
operand has side-effects to see if we can keep the && or ||. This means
we can change lines 4474-4478 in cabs2cil.ml:
| CEExp(se1, e1'), CEExp (se2, e2') when
- !useLogicalOperators && isEmpty se1 && isEmpty se2 ->
- CEExp (empty, BinOp(LAnd,
+ !useLogicalOperators && isEmpty se2 ->
+ CEExp (se1, BinOp(LAnd,
makeCast e1' intType,
makeCast e2' intType, intType))
and similarly for the LOr case.
Is my reasoning correct? Or is there a reason *not* to keep &&s and ||s
in such cases?
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