[Clang][CoverageMapping] Fix switch counter codegen compile time explosion

C++ generated code with huge amount of switch cases chokes badly while emitting
coverage mapping, in our specific testcase (~72k cases), it won't stop after hours.
After this change, the frontend job now finishes in 4.5s and shrinks down `@__covrec_`
by 288k when compared to disabling simplification altogether.

There's probably no good way to create a testcase for this, but it's easy to
reproduce, just add thousands of cases in the below switch, and build with
`-fprofile-instr-generate -fcoverage-mapping`.

```
enum type : int {
 FEATURE_INVALID = 0,
 FEATURE_A = 1,
 ...
};

const char *to_string(type e) {
  switch (e) {
  case type::FEATURE_INVALID: return "FEATURE_INVALID";
  case type::FEATURE_A: return "FEATURE_A";}
  ...
  }

```

Differential Revision: https://reviews.llvm.org/D126345

Author: bcardosolopes

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -550,17 +550,18 @@ struct CounterCoverageMappingBuilder
   Counter GapRegionCounter;
 
   /// Return a counter for the subtraction of \c RHS from \c LHS
-  Counter subtractCounters(Counter LHS, Counter RHS) {
-    return Builder.subtract(LHS, RHS);
+  Counter subtractCounters(Counter LHS, Counter RHS, bool Simplify = true) {
+    return Builder.subtract(LHS, RHS, Simplify);
   }
 
   /// Return a counter for the sum of \c LHS and \c RHS.
-  Counter addCounters(Counter LHS, Counter RHS) {
-    return Builder.add(LHS, RHS);
+  Counter addCounters(Counter LHS, Counter RHS, bool Simplify = true) {
+    return Builder.add(LHS, RHS, Simplify);
   }
 
-  Counter addCounters(Counter C1, Counter C2, Counter C3) {
-    return addCounters(addCounters(C1, C2), C3);
+  Counter addCounters(Counter C1, Counter C2, Counter C3,
+                      bool Simplify = true) {
+    return addCounters(addCounters(C1, C2, Simplify), C3, Simplify);
   }
 
   /// Return the region counter for the given statement.
@@ -1317,11 +1318,16 @@ struct CounterCoverageMappingBuilder
     const SwitchCase *Case = S->getSwitchCaseList();
     for (; Case; Case = Case->getNextSwitchCase()) {
       HasDefaultCase = HasDefaultCase || isa<DefaultStmt>(Case);
-      CaseCountSum = addCounters(CaseCountSum, getRegionCounter(Case));
+      CaseCountSum =
+          addCounters(CaseCountSum, getRegionCounter(Case), /*Simplify=*/false);
       createSwitchCaseRegion(
           Case, getRegionCounter(Case),
           subtractCounters(ParentCount, getRegionCounter(Case)));
     }
+    // Simplify is skipped while building the counters above: it can get really
+    // slow on top of switches with thousands of cases. Instead, trigger
+    // simplification by adding zero to the last counter.
+    CaseCountSum = addCounters(CaseCountSum, Counter::getZero());
 
     // If no explicit default case exists, create a branch region to represent
     // the hidden branch, which will be added later by the CodeGen. This region

llvm/include/llvm/ProfileData/Coverage/CoverageMapping.h

@@ -195,11 +195,11 @@ class CounterExpressionBuilder {
   ArrayRef<CounterExpression> getExpressions() const { return Expressions; }
 
   /// Return a counter that represents the expression that adds LHS and RHS.
-  Counter add(Counter LHS, Counter RHS);
+  Counter add(Counter LHS, Counter RHS, bool Simplify = true);
 
   /// Return a counter that represents the expression that subtracts RHS from
   /// LHS.
-  Counter subtract(Counter LHS, Counter RHS);
+  Counter subtract(Counter LHS, Counter RHS, bool Simplify = true);
 };
 
 using LineColPair = std::pair<unsigned, unsigned>;

llvm/lib/ProfileData/Coverage/CoverageMapping.cpp

@@ -123,13 +123,15 @@ Counter CounterExpressionBuilder::simplify(Counter ExpressionTree) {
   return C;
 }
 
-Counter CounterExpressionBuilder::add(Counter LHS, Counter RHS) {
-  return simplify(get(CounterExpression(CounterExpression::Add, LHS, RHS)));
+Counter CounterExpressionBuilder::add(Counter LHS, Counter RHS, bool Simplify) {
+  auto Cnt = get(CounterExpression(CounterExpression::Add, LHS, RHS));
+  return Simplify ? simplify(Cnt) : Cnt;
 }
 
-Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS) {
-  return simplify(
-      get(CounterExpression(CounterExpression::Subtract, LHS, RHS)));
+Counter CounterExpressionBuilder::subtract(Counter LHS, Counter RHS,
+                                           bool Simplify) {
+  auto Cnt = get(CounterExpression(CounterExpression::Subtract, LHS, RHS));
+  return Simplify ? simplify(Cnt) : Cnt;
 }
 
 void CounterMappingContext::dump(const Counter &C, raw_ostream &OS) const {