Skip to content

Decompiler: Extend Rule2Comp2Sub to constants #8628

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
wants to merge 1 commit into
base: master
Choose a base branch
from
+43 −10
Open

Decompiler: Extend Rule2Comp2Sub to constants #8628

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
53 changes: 43 additions & 10 deletions Ghidra/Features/Decompiler/src/decompile/cpp/ruleaction.cc
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -7192,24 +7192,57 @@ int4 RuleAddUnsigned::applyOp(PcodeOp *op,Funcdata &data)
}

/// \class Rule2Comp2Sub
/// \brief Cleanup: Convert INT_ADD back to INT_SUB: `V + -W ==> V - W`
/// \brief Cleanup: Convert INT_ADD back to INT_SUB: `V + -W ==> V - W` (also for constant W's)
void Rule2Comp2Sub::getOpList(vector<uint4> &oplist) const

{
oplist.push_back(CPUI_INT_2COMP);
oplist.push_back(CPUI_INT_ADD);
}

int4 Rule2Comp2Sub::applyOp(PcodeOp *op,Funcdata &data)

{
PcodeOp *addop = op->getOut()->loneDescend();
if (addop == (PcodeOp *)0) return 0;
if (addop->code() != CPUI_INT_ADD) return 0;
if (addop->getIn(0) == op->getOut())
data.opSetInput(addop,addop->getIn(1),0);
data.opSetInput(addop,op->getIn(0),1);
data.opSetOpcode(addop,CPUI_INT_SUB);
data.opDestroy(op); // Completely remove 2COMP
Varnode *v = op->getIn(0);
Varnode *w = op->getIn(1);

// V | W | action
// + | + | nop
// - | + | INT_ADD(V, W) -> INT_SUB(W, -V)
// + | - | INT_ADD(V, W) -> INT_SUB(V, -W)
// - | - | nop

// a varnode is deemed to be negative if one of:
// - it's a constant with the highest bit set
// - it's defined by a INT_2COMP pcode op
bool v_is_neg = v->isConstant() ? ((v->getOffset() & (((uintb)1) << ((v->getSize() * 8) - 1))) != 0) : (v->isWritten() && v->getDef()->code() == CPUI_INT_2COMP);
bool w_is_neg = w->isConstant() ? ((w->getOffset() & (((uintb)1) << ((w->getSize() * 8) - 1))) != 0) : (w->isWritten() && w->getDef()->code() == CPUI_INT_2COMP);

if (v_is_neg == w_is_neg) return 0;

Varnode *var_to_negate = v_is_neg ? v : w;
Varnode *positive;

if (var_to_negate->isConstant()) {
// Case 1: It's a constant, with the highest bit set
int4 var_size = var_to_negate->getSize();
positive = data.newConstant(var_size, (-var_to_negate->getOffset()) & calc_mask(var_size));
} else {
// Case 2: It's an INT_2COMP
// just wire the input of the INT_2COMP directly into the INT_SUB-to-be
positive = var_to_negate->getDef()->getIn(0);
}

// The negated value is always the second input
// unlink the old W and insert the new W
data.opSetInput(op, positive, 1);

if (var_to_negate == v) {
// we also need to flip operands, so set w to the first input
data.opSetInput(op, w, 0);
}

// change the INT_ADD into an INT_SUB
data.opSetOpcode(op, CPUI_INT_SUB);
return 1;
}

Expand Down