[DebugInfo] Fix bug in LiveDebugVariables.

In lib/CodeGen/LiveDebugVariables.cpp, it uses std::prev(MBBI) to
get DebugValue's SlotIndex. However, the previous instruction may be
also a debug instruction. It could not use a debug instruction to query
SlotIndex in mi2iMap.

Scan all debug instructions and use the first debug instruction to query
SlotIndex for following debug instructions. Only handle DBG_VALUE in
handleDebugValue().

Differential Revision: https://reviews.llvm.org/D50621

llvm-svn: 340508

llvm/include/llvm/CodeGen/SlotIndexes.h

@@ -414,6 +414,8 @@ class raw_ostream;
     SlotIndex getInstructionIndex(const MachineInstr &MI) const {
       // Instructions inside a bundle have the same number as the bundle itself.
       const MachineInstr &BundleStart = *getBundleStart(MI.getIterator());
+      assert(!BundleStart.isDebugInstr() &&
+             "Could not use a debug instruction to query mi2iMap.");
       Mi2IndexMap::const_iterator itr = mi2iMap.find(&BundleStart);
       assert(itr != mi2iMap.end() && "Instruction not found in maps.");
       return itr->second;

llvm/lib/CodeGen/LiveDebugVariables.cpp

@@ -578,23 +578,28 @@ bool LDVImpl::collectDebugValues(MachineFunction &mf) {
     MachineBasicBlock *MBB = &*MFI;
     for (MachineBasicBlock::iterator MBBI = MBB->begin(), MBBE = MBB->end();
          MBBI != MBBE;) {
-      if (!MBBI->isDebugValue()) {
+      // Use the first debug instruction in the sequence to get a SlotIndex
+      // for following consecutive debug instructions.
+      if (!MBBI->isDebugInstr()) {
         ++MBBI;
         continue;
       }
-      // DBG_VALUE has no slot index, use the previous instruction instead.
+      // Debug instructions has no slot index. Use the previous
+      // non-debug instruction's SlotIndex as its SlotIndex.
       SlotIndex Idx =
           MBBI == MBB->begin()
               ? LIS->getMBBStartIdx(MBB)
               : LIS->getInstructionIndex(*std::prev(MBBI)).getRegSlot();
-      // Handle consecutive DBG_VALUE instructions with the same slot index.
+      // Handle consecutive debug instructions with the same slot index.
       do {
-        if (handleDebugValue(*MBBI, Idx)) {
+        // Only handle DBG_VALUE in handleDebugValue(). Skip all other
+        // kinds of debug instructions.
+        if (MBBI->isDebugValue() && handleDebugValue(*MBBI, Idx)) {
           MBBI = MBB->erase(MBBI);
           Changed = true;
         } else
           ++MBBI;
-      } while (MBBI != MBBE && MBBI->isDebugValue());
+      } while (MBBI != MBBE && MBBI->isDebugInstr());
     }
   }
   return Changed;

llvm/test/DebugInfo/Generic/debug-var-slot.ll

@@ -0,0 +1,59 @@
+; After adding new debug instruction for labels, it is possible to have
+; debug instructions before DBG_VALUE. When querying DBG_VALUE's slot
+; index using previous instruction and the previous instruction is debug
+; instruction, it will trigger an assertion as using debug instruction
+; to get slot index. This test is to emulate the case when DBG_VALUE's
+; previous instruction is DBG_LABEL in LiveDebugVariables pass.
+;
+; RUN: llc < %s -stop-after=livedebugvars -debug 2>&1 | FileCheck %s
+;
+; CHECK: COMPUTING LIVE DEBUG VARIABLES: foo
+; CHECK: DEBUG VARIABLES
+; CHECK-NEXT: "local_var,7"
+
+source_filename = "debug-var-slot.c"
+
+define dso_local i32 @foo(i32 %a, i32 %b) !dbg !6 {
+entry:
+  %a.addr = alloca i32, align 4
+  %b.addr = alloca i32, align 4
+  %sum = alloca i32, align 4
+  store i32 %a, i32* %a.addr, align 4
+  store i32 %b, i32* %b.addr, align 4
+  br label %top
+
+top:
+  call void @llvm.dbg.label(metadata !10), !dbg !13
+  call void @llvm.dbg.value(metadata i32 %0, metadata !12, metadata !DIExpression()), !dbg !14
+  %0 = load i32, i32* %a.addr, align 4
+  %1 = load i32, i32* %a.addr, align 4
+  %2 = load i32, i32* %b.addr, align 4
+  %add = add nsw i32 %1, %2
+  store i32 %add, i32* %sum, align 4
+  br label %done
+
+done:
+  %3 = load i32, i32* %sum, align 4
+  ret i32 %3, !dbg !15
+}
+
+declare void @llvm.dbg.label(metadata)
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.dbg.cu = !{!0}
+!llvm.module.flags = !{!4}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C99, file: !1, isOptimized: true, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "debug-var-slot.c", directory: "./")
+!2 = !{}
+!4 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "foo", scope: !1, file: !1, line: 1, type: !7, isLocal: false, isDefinition: true, scopeLine: 2, isOptimized: true, unit: !0, retainedNodes: !2)
+!7 = !DISubroutineType(types: !8)
+!8 = !{!9, !9, !9}
+!9 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!10 = !DILabel(scope: !6, name: "top", file: !1, line: 4)
+!11 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!12 = !DILocalVariable(name: "local_var", scope: !6, file: !1, line: 7, type: !11)
+!13 = !DILocation(line: 4, column: 1, scope: !6)
+!14 = !DILocation(line: 7, column: 1, scope: !6)
+!15 = !DILocation(line: 8, column: 3, scope: !6)