[clang][coverage] fixing "if constexpr" and "if consteval" coverage report

clang/docs/ReleaseNotes.rst

@@ -705,6 +705,9 @@ Bug Fixes in This Version
 - Fix assertion crash due to failed scope restoring caused by too-early VarDecl
   invalidation by invalid initializer Expr.
   Fixes (`#30908 <https://github.com/llvm/llvm-project/issues/30908>`_)
+- Clang now emits correct source location for code-coverage regions in `if constexpr`
+  and `if consteval` branches.
+  Fixes (`#54419 <https://github.com/llvm/llvm-project/issues/54419>`_)
 
 
 Bug Fixes to Compiler Builtins

clang/include/clang/AST/Stmt.h

@@ -1631,8 +1631,10 @@ class CompoundStmt final
                               SourceLocation RB);
 
   // Build an empty compound statement with a location.
-  explicit CompoundStmt(SourceLocation Loc)
-      : Stmt(CompoundStmtClass), LBraceLoc(Loc), RBraceLoc(Loc) {
+  explicit CompoundStmt(SourceLocation Loc) : CompoundStmt(Loc, Loc) {}
+
+  CompoundStmt(SourceLocation Loc, SourceLocation EndLoc)
+      : Stmt(CompoundStmtClass), LBraceLoc(Loc), RBraceLoc(EndLoc) {
     CompoundStmtBits.NumStmts = 0;
     CompoundStmtBits.HasFPFeatures = 0;
   }

clang/lib/CodeGen/CoverageMappingGen.cpp

@@ -1712,7 +1712,11 @@ struct CounterCoverageMappingBuilder
       extendRegion(S->getCond());
 
     Counter ParentCount = getRegion().getCounter();
-    Counter ThenCount = getRegionCounter(S);
+
+    // If this is "if !consteval" the then-branch will never be taken, we don't
+    // need to change counter
+    Counter ThenCount =
+        S->isNegatedConsteval() ? ParentCount : getRegionCounter(S);
 
     if (!S->isConsteval()) {
       // Emitting a counter for the condition makes it easier to interpret the
@@ -1729,7 +1733,12 @@ struct CounterCoverageMappingBuilder
     extendRegion(S->getThen());
     Counter OutCount = propagateCounts(ThenCount, S->getThen());
 
-    Counter ElseCount = subtractCounters(ParentCount, ThenCount);
+    // If this is "if consteval" the else-branch will never be taken, we don't
+    // need to change counter
+    Counter ElseCount = S->isNonNegatedConsteval()
+                            ? ParentCount
+                            : subtractCounters(ParentCount, ThenCount);
+
     if (const Stmt *Else = S->getElse()) {
       bool ThenHasTerminateStmt = HasTerminateStmt;
       HasTerminateStmt = false;

clang/lib/Sema/TreeTransform.h

@@ -7739,7 +7739,11 @@ TreeTransform<Derived>::TransformIfStmt(IfStmt *S) {
     if (Then.isInvalid())
       return StmtError();
   } else {
-    Then = new (getSema().Context) NullStmt(S->getThen()->getBeginLoc());
+    // Discarded branch is replaced with empty CompoundStmt so we can keep
+    // proper source location for start and end of original branch, so
+    // subsequent transformations like CoverageMapping work properly
+    Then = new (getSema().Context)
+        CompoundStmt(S->getThen()->getBeginLoc(), S->getThen()->getEndLoc());
   }
 
   // Transform the "else" branch.
@@ -7748,6 +7752,13 @@ TreeTransform<Derived>::TransformIfStmt(IfStmt *S) {
     Else = getDerived().TransformStmt(S->getElse());
     if (Else.isInvalid())
       return StmtError();
+  } else if (S->getElse() && ConstexprConditionValue &&
+             *ConstexprConditionValue) {
+    // Same thing here as with <then> branch, we are discarding it, we can't
+    // replace it with NULL nor NullStmt as we need to keep for source location
+    // range, for CoverageMapping
+    Else = new (getSema().Context)
+        CompoundStmt(S->getElse()->getBeginLoc(), S->getElse()->getEndLoc());
   }
 
   if (!getDerived().AlwaysRebuild() &&

clang/test/CoverageMapping/if.cpp

@@ -23,19 +23,49 @@ void foo() {                    // CHECK-NEXT: Gap,File 0, [[@LINE+1]]:21 -> [[@
 }                               // CHECK-NEXT: [[@LINE-2]]:9 -> [[@LINE-1]]:5 = #1
                                 // CHECK-NEXT: [[@LINE-2]]:5 -> [[@LINE-2]]:8 = #1
 
-// FIXME: Do not generate coverage for discarded branches in if consteval and if constexpr statements
-constexpr int check_consteval(int i) {
-    if consteval {
-      i++;
-    }
-    if !consteval {
-      i++;
-    }
-    if consteval {
-        return 42;
-    } else {
-        return i;
-    }
+// FIXME: Do not generate coverage for discarded branches in if constexpr
+// CHECK-LABEL: _Z30check_constexpr_true_with_elsei:
+int check_constexpr_true_with_else(int i) {   // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+                                // CHECK-NEXT: [[@LINE+2]]:16 -> [[@LINE+2]]:20 = #0
+                                // CHECK-NEXT: Branch,File 0, [[@LINE+1]]:16 -> [[@LINE+1]]:20 = 0, 0
+  if constexpr(true) {          // CHECK-NEXT: Gap,File 0, [[@LINE]]:21 -> [[@LINE]]:22 = #1
+    i *= 3;                     // CHECK-NEXT: [[@LINE-1]]:22 -> [[@LINE+1]]:4 = #1
+  } else {                      // CHECK-NEXT: Gap,File 0, [[@LINE]]:4 -> [[@LINE]]:10 = (#0 - #1)
+    i *= 5;                     // CHECK-NEXT: [[@LINE-1]]:10 -> [[@LINE+1]]:4 = (#0 - #1)
+  }
+  return i;
+}
+
+// CHECK-LABEL: _Z33check_constexpr_true_without_elsei:
+int check_constexpr_true_without_else(int i) {   // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+                                // CHECK-NEXT: [[@LINE+2]]:16 -> [[@LINE+2]]:20 = #0
+                                // CHECK-NEXT: Branch,File 0, [[@LINE+1]]:16 -> [[@LINE+1]]:20 = 0, 0
+  if constexpr(true) {          // CHECK-NEXT: Gap,File 0, [[@LINE]]:21 -> [[@LINE]]:22 = #1
+    i *= 3;                     // CHECK-NEXT: [[@LINE-1]]:22 -> [[@LINE+1]]:4 = #1
+  }
+  return i;
+}
+
+// CHECK-LABEL: _Z31check_constexpr_false_with_elsei:
+int check_constexpr_false_with_else(int i) {   // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+                                // CHECK-NEXT: [[@LINE+2]]:16 -> [[@LINE+2]]:21 = #0
+                                // CHECK-NEXT: Branch,File 0, [[@LINE+1]]:16 -> [[@LINE+1]]:21 = 0, 0
+  if constexpr(false) {         // CHECK-NEXT: Gap,File 0, [[@LINE]]:22 -> [[@LINE]]:23 = #1
+    i *= 3;                     // CHECK-NEXT: File 0, [[@LINE-1]]:23 -> [[@LINE+1]]:4 = #1
+  } else {                      // CHECK-NEXT: Gap,File 0, [[@LINE]]:4 -> [[@LINE]]:10 = (#0 - #1)
+    i *= 5;                     // CHECK-NEXT: File 0, [[@LINE-1]]:10 -> [[@LINE+1]]:4 = (#0 - #1)
+  }
+  return i;
+}
+
+// CHECK-LABEL: _Z34check_constexpr_false_without_elsei:
+int check_constexpr_false_without_else(int i) {   // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+                                // CHECK-NEXT: [[@LINE+2]]:16 -> [[@LINE+2]]:21 = #0
+                                // CHECK-NEXT: Branch,File 0, [[@LINE+1]]:16 -> [[@LINE+1]]:21 = 0, 0
+  if constexpr(false) {         // CHECK-NEXT: Gap,File 0, [[@LINE]]:22 -> [[@LINE]]:23 = #1
+    i *= 3;                     // CHECK-NEXT: File 0, [[@LINE-1]]:23 -> [[@LINE+1]]:4 = #1
+  }
+  return i;
 }
 
 // CHECK-LABEL: main:
@@ -75,10 +105,6 @@ int main() {                    // CHECK: File 0, [[@LINE]]:12 -> {{[0-9]+}}:2 =
                                 // CHECK-NEXT: File 0, [[@LINE+1]]:14 -> [[@LINE+1]]:20 = #6
   i = i == 0?i + 12:i + 10;     // CHECK-NEXT: File 0, [[@LINE]]:21 -> [[@LINE]]:27 = (#0 - #6)
 
-  // GH-57377
-  constexpr int c_i = check_consteval(0);
-  check_consteval(i);
-
   // GH-45481
   S s;                    
   s.the_prop = 0? 1 : 2;        // CHECK-NEXT: File 0, [[@LINE]]:16 -> [[@LINE]]:17 = #0
@@ -98,3 +124,51 @@ int main() {                    // CHECK: File 0, [[@LINE]]:12 -> {{[0-9]+}}:2 =
 void ternary() {
   true ? FOO : FOO; // CHECK-NOT: Gap,{{.*}}, [[@LINE]]:8 ->
 }
+
+// GH-57377
+// CHECK-LABEL: _Z40check_consteval_with_else_discarded_theni:
+// FIXME: Do not generate coverage for discarded <then> branch in if consteval
+constexpr int check_consteval_with_else_discarded_then(int i) { // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+  if consteval {                        
+    i *= 3;                             // CHECK-NEXT: [[@LINE-1]]:16 -> [[@LINE+1]]:4 = #1
+  } else {                              // CHECK-NEXT: Gap,File 0, [[@LINE]]:4 -> [[@LINE]]:10 = #0
+    i *= 5;                             // CHECK-NEXT: [[@LINE-1]]:10 -> [[@LINE+1]]:4 = #0
+  }
+  return i;                             // CHECK-NEXT: [[@LINE]]:3 -> [[@LINE]]:11 = (#0 + #1)
+}
+
+// CHECK-LABEL: _Z43check_notconsteval_with_else_discarded_elsei:
+// FIXME: Do not generate coverage for discarded <else> branch in if consteval
+constexpr int check_notconsteval_with_else_discarded_else(int i) { // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+  if !consteval {                        
+    i *= 3;                             // CHECK-NEXT: [[@LINE-1]]:17 -> [[@LINE+1]]:4 = #0
+  } else {                              // CHECK-NEXT: Gap,File 0, [[@LINE]]:4 -> [[@LINE]]:10 = 0
+    i *= 5;                             // CHECK-NEXT: [[@LINE-1]]:10 -> [[@LINE+1]]:4 = 0
+  }
+  return i;
+}
+
+// CHECK-LABEL: _Z32check_consteval_branch_discardedi:
+// FIXME: Do not generate coverage for discarded <then> branch in if consteval
+constexpr int check_consteval_branch_discarded(int i) { // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+  if consteval {                        
+    i *= 3;                             // CHECK-NEXT: [[@LINE-1]]:16 -> [[@LINE+1]]:4 = #1
+  }
+  return i;                             // CHECK-NEXT: [[@LINE]]:3 -> [[@LINE]]:11 = (#0 + #1)
+}
+
+// CHECK-LABEL: _Z30check_notconsteval_branch_kepti:
+constexpr int check_notconsteval_branch_kept(int i) { // CHECK-NEXT: [[@LINE]]:{{[0-9]+}} -> {{[0-9]+}}:2 = #0
+  if !consteval {                        
+    i *= 3;                             // CHECK-NEXT: [[@LINE-1]]:17 -> [[@LINE+1]]:4 = #0
+  }
+  return i;
+}
+
+int instantiate_consteval(int i) {
+  i *= check_consteval_with_else_discarded_then(i);
+  i *= check_notconsteval_with_else_discarded_else(i);
+  i *= check_consteval_branch_discarded(i);
+  i *= check_notconsteval_branch_kept(i);
+  return i;
+}