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Decompiler: Simplify comparisons between INT_OR and zero.

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At -O1, gcc combines several values that all need to be compared against zero by
combining them using `INT_OR` and only comparing the combined result against
zero. With this rule, the decompiler is able to break these `INT_OR` chains
apart and simplify the individual links.
This commit is contained in:
Luke Serné 2024-05-25 12:11:34 +02:00
parent fb844be1f6
commit c79c9606bc
3 changed files with 70 additions and 0 deletions
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1
Ghidra/Features/Decompiler/src/decompile/cpp/coreaction.cc
View file

@ -5518,6 +5518,7 @@ void ActionDatabase::universalAction(Architecture *conf)
actprop->addRule( new RuleOrMultiBool("analysis") );
actprop->addRule( new RuleXorSwap("analysis") );
actprop->addRule( new RuleLzcountShiftBool("analysis") );
actprop->addRule( new RuleOrCompare("analysis") );
actprop->addRule( new RuleSubvarAnd("subvar") );
actprop->addRule( new RuleSubvarSubpiece("subvar") );
actprop->addRule( new RuleSplitFlow("subvar") );

58
Ghidra/Features/Decompiler/src/decompile/cpp/ruleaction.cc
View file

@ -10434,4 +10434,62 @@ int4 RuleLzcountShiftBool::applyOp(PcodeOp *op,Funcdata &data)
return 0;
}
/// \class RuleOrCompare
/// \brief Simplify INT_OR in comparisons with 0.
/// `(V | W) == 0` => '(V == 0) && (W == 0)'
/// `(V | W) != 0` => '(V != 0) || (W != 0)'
void RuleOrCompare::getOpList(vector<uint4> &oplist) const
{
oplist.push_back(CPUI_INT_EQUAL);
oplist.push_back(CPUI_INT_NOTEQUAL);
}
int4 RuleOrCompare::applyOp(PcodeOp *op,Funcdata &data)
{
// make sure the comparison is against 0
if (! op->getIn(1)->constantMatch(0)) return 0;
// make sure the other operand is an INT_OR
PcodeOp *or_op = op->getIn(0)->getDef();
if (or_op == (PcodeOp *)0) return 0;
if (or_op->code() != CPUI_INT_OR) return 0;
Varnode* V = or_op->getIn(0);
Varnode* W = or_op->getIn(1);
// make sure V and W are in SSA form
if (V->isFree()) return 0;
if (W->isFree()) return 0;
// construct the new segment:
// if the original condition was INT_EQUAL: BOOL_AND(INT_EQUAL(V, 0:|V|), INT_EQUAL(W, 0:|W|))
// if the original condition was INT_NOTEQUAL: BOOL_OR(INT_NOTEQUAL(V, 0:|V|), INT_NOTEQUAL(W, 0:|W|))
Varnode* zero_V = data.newConstant(V->getSize(), 0);
Varnode* zero_W = data.newConstant(W->getSize(), 0);
PcodeOp* eq_V = data.newOp(2, op->getAddr());
data.opSetOpcode(eq_V, op->code());
data.opSetInput(eq_V, V, 0);
data.opSetInput(eq_V, zero_V, 1);
PcodeOp* eq_W = data.newOp(2, op->getAddr());
data.opSetOpcode(eq_W, op->code());
data.opSetInput(eq_W, W, 0);
data.opSetInput(eq_W, zero_W, 1);
Varnode* eq_V_out = data.newUniqueOut(1, eq_V);
Varnode* eq_W_out = data.newUniqueOut(1, eq_W);
// make sure the comparisons' output is already defined
data.opInsertBefore(eq_V, op);
data.opInsertBefore(eq_W, op);
// change the original INT_EQUAL into a BOOL_AND, and INT_NOTEQUAL becomes BOOL_OR
data.opSetOpcode(op, op->code() == CPUI_INT_EQUAL ? CPUI_BOOL_AND : CPUI_BOOL_OR);
data.opSetInput(op, eq_V_out, 0);
data.opSetInput(op, eq_W_out, 1);
return 1;
}
} // End namespace ghidra

11
Ghidra/Features/Decompiler/src/decompile/cpp/ruleaction.hh
View file

@ -1643,5 +1643,16 @@ public:
virtual int4 applyOp(PcodeOp *op,Funcdata &data);
};
class RuleOrCompare : public Rule {
public:
RuleOrCompare(const string &g) : Rule( g, 0, "ruleorcompare") {} ///< Constructor
virtual Rule *clone(const ActionGroupList &grouplist) const {
if (!grouplist.contains(getGroup())) return (Rule *)0;
return new RuleOrCompare(getGroup());
}
virtual void getOpList(vector<uint4> &oplist) const;
virtual int4 applyOp(PcodeOp *op,Funcdata &data);
};
} // End namespace ghidra
#endif