C++: Use shortestDistances HOP for IR BB indexes

This doesn't make it much faster, but it reduces the debug output
volume. It also simplifies the code.

I've found this change necessary when I compute the full IR on a
Wireshark snapshot in QL4E. Without it, Eclipse runs out of memory
because the console log is too large.

Author: jbj

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

@@ -159,24 +159,13 @@ private cached module Cached {
     not startsBasicBlock(i2)
   }
 
-  /** Gets the index of `i` in its `IRBlock`. */
-  private int getMemberIndex(Instruction i) {
-    startsBasicBlock(i) and
-    result = 0
-    or
-    exists(Instruction iPrev |
-      adjacentInBlock(iPrev, i) and
-      result = getMemberIndex(iPrev) + 1
-    )
-  }
+  /** Holds if `i` is the `index`th instruction the block starting with `first`. */
+  private Instruction getInstructionFromFirst(Instruction first, int index) =
+    shortestDistances(startsBasicBlock/1, adjacentInBlock/2)(first, result, index)
 
   /** Holds if `i` is the `index`th instruction in `block`. */
   cached Instruction getInstruction(TIRBlock block, int index) {
-    exists(Instruction first |
-      block = MkIRBlock(first) and
-      index = getMemberIndex(result) and
-      adjacentInBlock*(first, result)
-    )
+    result = getInstructionFromFirst(getFirstInstruction(block), index)
   }
 
   cached int getInstructionCount(TIRBlock block) {

cpp/ql/src/semmle/code/cpp/ir/implementation/raw/IRBlock.qll

@@ -159,24 +159,13 @@ private cached module Cached {
     not startsBasicBlock(i2)
   }
 
-  /** Gets the index of `i` in its `IRBlock`. */
-  private int getMemberIndex(Instruction i) {
-    startsBasicBlock(i) and
-    result = 0
-    or
-    exists(Instruction iPrev |
-      adjacentInBlock(iPrev, i) and
-      result = getMemberIndex(iPrev) + 1
-    )
-  }
+  /** Holds if `i` is the `index`th instruction the block starting with `first`. */
+  private Instruction getInstructionFromFirst(Instruction first, int index) =
+    shortestDistances(startsBasicBlock/1, adjacentInBlock/2)(first, result, index)
 
   /** Holds if `i` is the `index`th instruction in `block`. */
   cached Instruction getInstruction(TIRBlock block, int index) {
-    exists(Instruction first |
-      block = MkIRBlock(first) and
-      index = getMemberIndex(result) and
-      adjacentInBlock*(first, result)
-    )
+    result = getInstructionFromFirst(getFirstInstruction(block), index)
   }
 
   cached int getInstructionCount(TIRBlock block) {

cpp/ql/src/semmle/code/cpp/ir/implementation/unaliased_ssa/IRBlock.qll

@@ -159,24 +159,13 @@ private cached module Cached {
     not startsBasicBlock(i2)
   }
 
-  /** Gets the index of `i` in its `IRBlock`. */
-  private int getMemberIndex(Instruction i) {
-    startsBasicBlock(i) and
-    result = 0
-    or
-    exists(Instruction iPrev |
-      adjacentInBlock(iPrev, i) and
-      result = getMemberIndex(iPrev) + 1
-    )
-  }
+  /** Holds if `i` is the `index`th instruction the block starting with `first`. */
+  private Instruction getInstructionFromFirst(Instruction first, int index) =
+    shortestDistances(startsBasicBlock/1, adjacentInBlock/2)(first, result, index)
 
   /** Holds if `i` is the `index`th instruction in `block`. */
   cached Instruction getInstruction(TIRBlock block, int index) {
-    exists(Instruction first |
-      block = MkIRBlock(first) and
-      index = getMemberIndex(result) and
-      adjacentInBlock*(first, result)
-    )
+    result = getInstructionFromFirst(getFirstInstruction(block), index)
   }
 
   cached int getInstructionCount(TIRBlock block) {