[PATCH] D109940: Fixed bug with clang where a fallthrough switch statement wasn't getting proper debug information

[Shubham Sandeep Rastogi via Phabricator via llvm-commits]

rastogishubham created this revision.
rastogishubham added a reviewer: aprantl.
rastogishubham added a project: debug-info.
rastogishubham requested review of this revision.

When clang sees a switch with a fallthrough 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.

The IR output looks like this:
 Function Attrs: noinline nounwind optnone ssp uwtable
define i32 @main(i32 %0, i8** %1) #0 !dbg !9 {

  %3 = alloca i32, align 4
  %4 = alloca i32, align 4
  %5 = alloca i8**, align 8
  store i32 0, i32* %3, align 4
  store i32 %0, i32* %4, align 4
  call void @llvm.dbg.declare(metadata i32* %4, metadata !16, metadata !DIExpression()), !dbg !17
  store i8** %1, i8*** %5, align 8
  call void @llvm.dbg.declare(metadata i8*** %5, metadata !18, metadata !DIExpression()), !dbg !19
  %6 = load i32, i32* %4, align 4, !dbg !20
  switch i32 %6, label %9 [
    i32 0, label %7
    i32 10, label %8
  ], !dbg !21

7:                                                ; preds = %2

  call void @funcA(), !dbg !22
  br label %10, !dbg !24

8:                                                ; preds = %2

  br label %9, !dbg !24

9:                                                ; preds = %2, %8

  call void @funcB(), !dbg !25
  br label %10, !dbg !26

10:                                               ; preds = %9, %7

  %11 = load i32, i32* %3, align 4, !dbg !27
  ret i32 %11, !dbg !27

}

label 7 and label 8 both have branches but label 8's branch uses the same debug location as label 7's branch. This resolves to disassembly such as:

LBB0_5:
	.loc	1 0 5                           ## Documents/test.c:0:5
	movl	-20(%rbp), %eax                 ## 4-byte Reload
	.loc	1 5 5                           ## Documents/test.c:5:5
	subl	$10, %eax
	je	LBB0_2
	jmp	LBB0_3
LBB0_1:
Ltmp1:
	.loc	1 7 13 is_stmt 1                ## Documents/test.c:7:13
	callq	_funcA
	.loc	1 8 13                          ## Documents/test.c:8:13
	jmp	LBB0_4
LBB0_2:
	jmp	LBB0_3
LBB0_3:
	.loc	1 11 13                         ## Documents/test.c:11:13
	callq	_funcB

LBB0_2 does not get a .loc in this case


https://reviews.llvm.org/D109940

Files:
  clang/lib/CodeGen/CGStmt.cpp
  clang/test/CodeGen/switch-fallthrough.c


Index: clang/test/CodeGen/switch-fallthrough.c
===================================================================
--- /dev/null
+++ clang/test/CodeGen/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 -v
+// 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;
+  }
+}
\ No newline at end of file
Index: clang/lib/CodeGen/CGStmt.cpp
===================================================================
--- clang/lib/CodeGen/CGStmt.cpp
+++ clang/lib/CodeGen/CGStmt.cpp
@@ -1518,6 +1518,11 @@
     NextCase = dyn_cast<CaseStmt>(CurCase->getSubStmt());
   }
 
+  // Generate a stop point for debug info if the case statement is followed
+  // by a default statement.
+  if (CurCase->getSubStmt()->getStmtClass() == Stmt::DefaultStmtClass) {
+    EmitStopPoint(CurCase);
+  }
   // Normal default recursion for non-cases.
   EmitStmt(CurCase->getSubStmt());
 }


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