C++: Support 'if(!p)' for C programs in IRGuards.

cpp/ql/lib/semmle/code/cpp/controlflow/IRGuards.qll

@@ -565,7 +565,7 @@ class IRGuardCondition extends Instruction {
   /** Holds if (determined by this guard) `op == k` evaluates to `areEqual` if this expression evaluates to `value`. */
   cached
   predicate comparesEq(Operand op, int k, boolean areEqual, AbstractValue value) {
-    unary_compares_eq(this, op, k, areEqual, value)
+    unary_compares_eq(this, op, k, areEqual, false, value)
   }
 
   /**
@@ -586,7 +586,7 @@ class IRGuardCondition extends Instruction {
   cached
   predicate ensuresEq(Operand op, int k, IRBlock block, boolean areEqual) {
     exists(AbstractValue value |
-      unary_compares_eq(this, op, k, areEqual, value) and this.valueControls(block, value)
+      unary_compares_eq(this, op, k, areEqual, false, value) and this.valueControls(block, value)
     )
   }
 
@@ -611,7 +611,7 @@ class IRGuardCondition extends Instruction {
   cached
   predicate ensuresEqEdge(Operand op, int k, IRBlock pred, IRBlock succ, boolean areEqual) {
     exists(AbstractValue value |
-      unary_compares_eq(this, op, k, areEqual, value) and
+      unary_compares_eq(this, op, k, areEqual, false, value) and
       this.valueControlsEdge(pred, succ, value)
     )
   }
@@ -739,24 +739,30 @@ private predicate compares_eq(
 
 /** Holds if `op == k` is `areEqual` given that `test` is equal to `value`. */
 private predicate unary_compares_eq(
-  Instruction test, Operand op, int k, boolean areEqual, AbstractValue value
+  Instruction test, Operand op, int k, boolean areEqual, boolean inNonZeroCase, AbstractValue value
 ) {
   /* The simple case where the test *is* the comparison so areEqual = testIsTrue xor eq. */
-  exists(AbstractValue v | unary_simple_comparison_eq(test, op, k, v) |
+  exists(AbstractValue v | unary_simple_comparison_eq(test, op, k, inNonZeroCase, v) |
     areEqual = true and value = v
     or
     areEqual = false and value = v.getDualValue()
   )
   or
-  unary_complex_eq(test, op, k, areEqual, value)
+  unary_complex_eq(test, op, k, areEqual, inNonZeroCase, value)
   or
   /* (x is true => (op == k)) => (!x is false => (op == k)) */
-  exists(AbstractValue dual | value = dual.getDualValue() |
-    unary_compares_eq(test.(LogicalNotInstruction).getUnary(), op, k, areEqual, dual)
+  exists(AbstractValue dual, boolean inNonZeroCase0 |
+    value = dual.getDualValue() and
+    unary_compares_eq(test.(LogicalNotInstruction).getUnary(), op, k, inNonZeroCase0, areEqual, dual)
+  |
+    k = 0 and inNonZeroCase = inNonZeroCase0
+    or
+    k != 0 and inNonZeroCase = true
   )
   or
   // ((test is `areEqual` => op == const + k2) and const == `k1`) =>
   // test is `areEqual` => op == k1 + k2
+  inNonZeroCase = false and
   exists(int k1, int k2, ConstantInstruction const |
     compares_eq(test, op, const.getAUse(), k2, areEqual, value) and
     int_value(const) = k1 and
@@ -781,37 +787,84 @@ private predicate simple_comparison_eq(
   value.(BooleanValue).getValue() = false
 }
 
-/** Rearrange various simple comparisons into `op == k` form. */
+/**
+ * Holds if `test` is an instruction that is part of test that eventually is
+ * used in a conditional branch.
+ */
+private predicate relevantUnaryComparison(Instruction test) {
+  not test instanceof CompareInstruction and
+  exists(IRType type, ConditionalBranchInstruction branch |
+    type instanceof IRAddressType or type instanceof IRIntegerType
+  |
+    type = test.getResultIRType() and
+    branch.getCondition() = test
+  )
+  or
+  exists(LogicalNotInstruction logicalNot |
+    relevantUnaryComparison(logicalNot) and
+    test = logicalNot.getUnary()
+  )
+}
+
+/**
+ * Rearrange various simple comparisons into `op == k` form.
+ *
+ * The boolean `inNonZeroCase` tracks whether the value of `k` is restricted
+ * to the range `[0, 1]`.
+ *
+ * If `inNonZeroCase` is `true`, then any non-zero value of `k` should be
+ * interpreted as "we only know that `k` is non-zero".
+ *
+ * Ideally, this would be represented as an `Option<int>` type, but QL does
+ * not yet allow infinite IPA branches (which is what `Option` uses under the
+ * hood).
+ *
+ * To see why `inNonZeroCase` is needed consider the following C program:
+ * ```c
+ * char* p = ...;
+ * if(!p) {
+ *   use(p);
+ * }
+ * ```
+ * in C++ there would be an int-to-bool conversion on `p` before it is
+ * passed to `!`. However, since C does not have booleans there is no
+ * int-to-bool conversion. We should be able to conclude that `p` is non-zero
+ * in the true branch, so we need to give `k` some value. However, simply
+ * setting `k = 1` would make the rest of the analysis think that `k == 1`
+ * holds inside the branch. So we distinquish between the above case and
+ * ```c
+ * if(p == 1) {
+ *   use(p)
+ * }
+ * ```
+ * by setting `inNonZeroCase` to `true` in the former case, but not in the
+ * latter.
+ */
 private predicate unary_simple_comparison_eq(
-  Instruction test, Operand op, int k, AbstractValue value
+  Instruction test, Operand op, int k, boolean inNonZeroCase, AbstractValue value
 ) {
   exists(SwitchInstruction switch, CaseEdge case |
     test = switch.getExpression() and
     op.getDef() = test and
     case = value.(MatchValue).getCase() and
     exists(switch.getSuccessor(case)) and
-    case.getValue().toInt() = k
+    case.getValue().toInt() = k and
+    inNonZeroCase = false
   )
   or
   // There's no implicit CompareInstruction in files compiled as C since C
   // doesn't have implicit boolean conversions. So instead we check whether
   // there's a branch on a value of pointer or integer type.
-  exists(ConditionalBranchInstruction branch, IRType type |
-    not test instanceof CompareInstruction and
-    type = test.getResultIRType() and
-    (type instanceof IRAddressType or type instanceof IRIntegerType) and
-    test = branch.getCondition() and
-    op.getDef() = test
-  |
-    // We'd like to also include a case such as:
-    // ```
-    // k = 1 and
-    // value.(BooleanValue).getValue() = true
-    // ```
-    // but all we know is that the value is non-zero in the true branch.
-    // So we can only conclude something in the false branch.
+  relevantUnaryComparison(test) and
+  op.getDef() = test and
+  (
+    k = 1 and
+    value.(BooleanValue).getValue() = true and
+    inNonZeroCase = true
+    or
     k = 0 and
-    value.(BooleanValue).getValue() = false
+    value.(BooleanValue).getValue() = false and
+    inNonZeroCase = false
   )
 }
 
@@ -824,11 +877,11 @@ private predicate complex_eq(
 }
 
 private predicate unary_complex_eq(
-  Instruction test, Operand op, int k, boolean areEqual, AbstractValue value
+  Instruction test, Operand op, int k, boolean areEqual, boolean inNonZeroCase, AbstractValue value
 ) {
-  unary_sub_eq(test, op, k, areEqual, value)
+  unary_sub_eq(test, op, k, areEqual, inNonZeroCase, value)
   or
-  unary_add_eq(test, op, k, areEqual, value)
+  unary_add_eq(test, op, k, areEqual, inNonZeroCase, value)
 }
 
 /*
@@ -1093,17 +1146,19 @@ private predicate sub_eq(
 
 // op - x == c => op == (c+x)
 private predicate unary_sub_eq(
-  Instruction test, Operand op, int k, boolean areEqual, AbstractValue value
+  Instruction test, Operand op, int k, boolean areEqual, boolean inNonZeroCase, AbstractValue value
 ) {
+  inNonZeroCase = false and
   exists(SubInstruction sub, int c, int x |
-    unary_compares_eq(test, sub.getAUse(), c, areEqual, value) and
+    unary_compares_eq(test, sub.getAUse(), c, areEqual, inNonZeroCase, value) and
     op = sub.getLeftOperand() and
     x = int_value(sub.getRight()) and
     k = c + x
   )
   or
+  inNonZeroCase = false and
   exists(PointerSubInstruction sub, int c, int x |
-    unary_compares_eq(test, sub.getAUse(), c, areEqual, value) and
+    unary_compares_eq(test, sub.getAUse(), c, areEqual, inNonZeroCase, value) and
     op = sub.getLeftOperand() and
     x = int_value(sub.getRight()) and
     k = c + x
@@ -1158,11 +1213,12 @@ private predicate add_eq(
 
 // left + x == right + c => left == right + (c-x)
 private predicate unary_add_eq(
-  Instruction test, Operand left, int k, boolean areEqual,
+  Instruction test, Operand left, int k, boolean areEqual, boolean inNonZeroCase,
   AbstractValue value
 ) {
+  inNonZeroCase = false and
   exists(AddInstruction lhs, int c, int x |
-    unary_compares_eq(test, lhs.getAUse(), c, areEqual, value) and
+    unary_compares_eq(test, lhs.getAUse(), c, areEqual, inNonZeroCase, value) and
     (
       left = lhs.getLeftOperand() and x = int_value(lhs.getRight())
       or
@@ -1171,9 +1227,9 @@ private predicate unary_add_eq(
     k = c - x
   )
   or
+  inNonZeroCase = false and
   exists(PointerAddInstruction lhs, int c, int x |
-
-    unary_compares_eq(test, lhs.getAUse(), c, areEqual, value) and
+    unary_compares_eq(test, lhs.getAUse(), c, areEqual, inNonZeroCase, value) and
     (
       left = lhs.getLeftOperand() and x = int_value(lhs.getRight())
       or