C++: Add isPartialUpdate member predicate to ChiInstructions.

Author: MathiasVP

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/Instruction.qll

@@ -2055,6 +2055,13 @@ class ChiInstruction extends Instruction {
   final predicate getUpdatedInterval(int startBit, int endBit) {
     Construction::getIntervalUpdatedByChi(this, startBit, endBit)
   }
+
+  /**
+   * Holds if the `ChiPartialOperand` totally, but not exactly, overlaps with the `ChiTotalOperand`.
+   * This means that the `ChiPartialOperand` will not override the entire memory associated with the
+   * `ChiTotalOperand`.
+   */
+  final predicate isPartialUpdate() { Construction::chiOnlyPartiallyUpdatesLocation(this) }
 }
 
 /**

cpp/ql/src/semmle/code/cpp/ir/implementation/aliased_ssa/internal/SSAConstruction.qll

@@ -178,6 +178,22 @@ private module Cached {
     )
   }
 
+  /**
+   * Holds if the `ChiPartialOperand` totally, but not exactly, overlaps with the `ChiTotalOperand`.
+   * This means that the `ChiPartialOperand` will not override the entire memory associated with the
+   * `ChiTotalOperand`.
+   */
+  cached
+  predicate chiOnlyPartiallyUpdatesLocation(ChiInstruction chi) {
+    exists(Alias::MemoryLocation location, OldInstruction oldInstruction |
+      oldInstruction = getOldInstruction(chi.getPartial()) and
+      location = Alias::getResultMemoryLocation(oldInstruction)
+    |
+      Alias::getStartBitOffset(location) != 0 or
+      Alias::getEndBitOffset(location) != 8 * location.getType().getByteSize()
+    )
+  }
+
   /**
    * Holds if `instr` is part of a cycle in the operand graph that doesn't go
    * through a phi instruction and therefore should be impossible.