Apply proper source location to fallthrough switch cases.

This fixes a bug in clang where, when clang sees a switch with a
fallthrough to a default like this:

static void funcA(void) {}
static void funcB(void) {}

int main(int argc, char **argv) {

switch (argc) {
    case 0:
        funcA();
        break;
    case 10:
    default:
        funcB();
        break;
}
}

It does not add a proper debug location for that switch case, such as
case 10: above.

Patch by Shubham Rastogi!

Differential Revision: https://reviews.llvm.org/D109940

Author: adrian-prantl

clang/lib/CodeGen/CGStmt.cpp

@@ -1521,6 +1521,12 @@ void CodeGenFunction::EmitCaseStmt(const CaseStmt &S,
     NextCase = dyn_cast<CaseStmt>(CurCase->getSubStmt());
   }
 
+  // Generate a stop point for debug info if the case statement is
+  // followed by a default statement. A fallthrough case before a
+  // default case gets its own branch target.
+  if (CurCase->getSubStmt()->getStmtClass() == Stmt::DefaultStmtClass)
+    EmitStopPoint(CurCase);
+
   // Normal default recursion for non-cases.
   EmitStmt(CurCase->getSubStmt());
 }

clang/test/CodeGen/debug-info-switch-fallthrough.c

@@ -0,0 +1,17 @@
+// RUN:  %clang_cc1 -triple x86_64-apple-macosx11.0.0 -debug-info-kind=standalone -emit-llvm %s -o - | FileCheck %s
+// CHECK: ], !dbg !{{[0-9]+}}
+// CHECK-EMPTY:
+// CHECK-NEXT: {{.+}}
+// CHECK-NEXT: br {{.+}}, !dbg !{{[0-9+]}}
+// CHECK-EMPTY:
+// CHECK-NEXT: {{.+}}
+// CHECK-NEXT: br {{.+}}, !dbg ![[LOC:[0-9]+]]
+void test(int num) {
+  switch (num) {
+  case 0:
+    break;
+  case 10: // CHECK: ![[LOC]] = !DILocation(line: [[@LINE]], column:{{.+}}, scope: {{.+}})
+  default:
+    break;
+  }
+}