EssentialTypes: Address performance issues with stlr/utlr

The calculatons for signed and unsigned type of lowest rank were
expensive when there were a large number of constant expressions in the
program. This commit improves performance by:
 1. Creating classes to represent the set of integer constant
expressions and integer constants in the program.
 2. Creating a candidate table of IntegralTypes for each known constant
integer value in the program, to avoid duplicate work.
 3. Calculating the stlr and utlr for each constant value, not each
constant expression.

Author: lcartey

c/misra/src/codingstandards/c/misra/EssentialTypes.qll

@@ -31,45 +31,83 @@ class EssentialTypeCategory extends TEssentialTypeCategory {
   }
 }
 
+/**
+ * An expression in the program that evaluates to a compile time constant signed or unsigned integer.
+ */
+private class ConstantIntegerExpr extends Expr {
+  pragma[noinline]
+  ConstantIntegerExpr() {
+    getEssentialTypeCategory(this.getType()) =
+      [
+        EssentiallyUnsignedType().(EssentialTypeCategory),
+        EssentiallySignedType().(EssentialTypeCategory)
+      ] and
+    exists(this.getValue().toFloat()) and
+    not this instanceof Conversion
+  }
+}
+
+/** A `float` which represents an integer constant in the program. */
+private class IntegerConstantAsFloat extends float {
+  IntegerConstantAsFloat() { exists(ConstantIntegerExpr ce | this = ce.getValue().toFloat()) }
+}
+
+/**
+ * Identifies which integral types from which type categories can represent a given integer constant
+ * in the program.
+ */
+pragma[nomagic]
+private predicate isCandidateIntegralType(
+  EssentialTypeCategory cat, IntegralType it, IntegerConstantAsFloat c
+) {
+  getEssentialTypeCategory(it) = cat and
+  c = any(ConstantIntegerExpr ce).getValue().toFloat() and
+  // As with range analysis, we assume two's complement representation
+  typeLowerBound(it) <= c and
+  typeUpperBound(it) >= c
+}
+
 /**
  * Gets the unsigned type of lowest rank that can represent the value of the given expression,
  * assuming that the expression is essentially unsigned.
  */
-private IntegralType utlr(Expr const) {
+pragma[nomagic]
+private IntegralType utlr(ConstantIntegerExpr const) {
   getEssentialTypeCategory(const.getType()) = EssentiallyUnsignedType() and
-  getEssentialTypeCategory(result) = EssentiallyUnsignedType() and
-  exists(float c | c = const.getValue().toFloat() |
-    // As with range analysis, we assume two's complement representation
-    typeLowerBound(result) <= c and
-    typeUpperBound(result) >= c and
-    forall(IntegralType it |
-      getEssentialTypeCategory(it) = EssentiallyUnsignedType() and
-      typeLowerBound(it) <= c and
-      typeUpperBound(it) >= c
-    |
-      result.getSize() <= it.getSize()
-    )
+  result = utlr_c(const.getValue().toFloat())
+}
+
+/**
+ * Given an integer constant that appears in the program, gets the unsigned type of lowest rank
+ * that can hold it.
+ */
+pragma[nomagic]
+private IntegralType utlr_c(IntegerConstantAsFloat c) {
+  isCandidateIntegralType(EssentiallyUnsignedType(), result, c) and
+  forall(IntegralType it | isCandidateIntegralType(EssentiallyUnsignedType(), it, c) |
+    result.getSize() <= it.getSize()
   )
 }
 
 /**
  * Gets the signed type of lowest rank that can represent the value of the given expression,
  * assuming that the expression is essentially signed.
  */
-private IntegralType stlr(Expr const) {
+pragma[nomagic]
+private IntegralType stlr(ConstantIntegerExpr const) {
   getEssentialTypeCategory(const.getType()) = EssentiallySignedType() and
-  getEssentialTypeCategory(result) = EssentiallySignedType() and
-  exists(float c | c = const.getValue().toFloat() |
-    // As with range analysis, we assume two's complement representation
-    typeLowerBound(result) <= c and
-    typeUpperBound(result) >= c and
-    forall(IntegralType it |
-      getEssentialTypeCategory(it) = EssentiallySignedType() and
-      typeLowerBound(it) <= c and
-      typeUpperBound(it) >= c
-    |
-      result.getSize() <= it.getSize()
-    )
+  result = stlr_c(const.getValue().toFloat())
+}
+
+/**
+ * Given an integer constant that appears in the program, gets the signed type of lowest rank
+ * that can hold it.
+ */
+pragma[nomagic]
+private IntegralType stlr_c(IntegerConstantAsFloat c) {
+  isCandidateIntegralType(EssentiallySignedType(), result, c) and
+  forall(IntegralType it | isCandidateIntegralType(EssentiallySignedType(), it, c) |
+    result.getSize() <= it.getSize()
   )
 }