[SimplifyCFG] Update debug location when folding branch to common destination

Sometimes a dead block gets folded and the debug information is still
retained. This manifests as jumpy stepping in lldb, see the bugzilla PR
for an end-to-end C testcase.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46008

Differential Revision:  https://reviews.llvm.org/D82062

lib/Transforms/Utils/SimplifyCFG.cpp

@@ -2776,6 +2776,12 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, MemorySSAUpdater *MSSAU,
       if (isa<DbgInfoIntrinsic>(BonusInst))
         continue;
       Instruction *NewBonusInst = BonusInst->clone();
+
+      // When we fold the bonus instructions we want to make sure we
+      // reset their debug locations in order to avoid stepping on dead
+      // code caused by folding dead branches.
+      NewBonusInst->setDebugLoc(DebugLoc());
+
       RemapInstruction(NewBonusInst, VMap,
                        RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
       VMap[&*BonusInst] = NewBonusInst;
@@ -2795,6 +2801,11 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, MemorySSAUpdater *MSSAU,
     // Clone Cond into the predecessor basic block, and or/and the
     // two conditions together.
     Instruction *CondInPred = Cond->clone();
+
+    // Reset the condition debug location to avoid jumping on dead code
+    // as the result of folding dead branches.
+    CondInPred->setDebugLoc(DebugLoc());
+
     RemapInstruction(CondInPred, VMap,
                      RF_NoModuleLevelChanges | RF_IgnoreMissingLocals);
     PredBlock->getInstList().insert(PBI->getIterator(), CondInPred);

test/Transforms/SimplifyCFG/basictest.ll

@@ -98,7 +98,7 @@ define i8 @test6f() {
 ; CHECK: alloca i8, align 1
 ; CHECK-NEXT: call i8 @test6g
 ; CHECK-NEXT: icmp eq i8 %tmp, 0
-; CHECK-NEXT: load i8, i8* %r, align 1, !dbg !{{[0-9]+$}}
+; CHECK-NEXT: load i8, i8* %r, align 1{{$}}
 
 bb0:
   %r = alloca i8, align 1

test/Transforms/SimplifyCFG/fold-debug-location.ll

@@ -0,0 +1,47 @@
+; RUN: opt -S -simplifycfg < %s | FileCheck %s --match-full-lines
+
+; Make sure we reset the debug location when folding instructions.
+; CHECK: [[VAL:%.*]] = and i32 %c2, %k
+; CHECK-NEXT: [[VAL2:%.*]] icmp eq i32 [[VAL]], 0
+
+declare i32 @bar(...)
+
+define i32 @patatino(i32 %k, i32 %c1, i32 %c2) !dbg !6 {
+  %1 = and i32 %c1, %k, !dbg !8
+  %2 = icmp eq i32 %1, 0, !dbg !9
+  br i1 %2, label %8, label %3, !dbg !10
+
+3:
+  %4 = and i32 %c2, %k, !dbg !11
+  %5 = icmp eq i32 %4, 0, !dbg !12
+  br i1 %5, label %8, label %6, !dbg !13
+
+6:
+  %7 = tail call i32 (...) @bar(), !dbg !14
+  br label %8, !dbg !15
+
+8:
+  ret i32 undef, !dbg !16
+}
+
+!llvm.dbg.cu = !{!0}
+!llvm.debugify = !{!3, !4}
+!llvm.module.flags = !{!5}
+
+!0 = distinct !DICompileUnit(language: DW_LANG_C, file: !1, producer: "debugify", isOptimized: true, runtimeVersion: 0, emissionKind: FullDebug, enums: !2)
+!1 = !DIFile(filename: "a.ll", directory: "/")
+!2 = !{}
+!3 = !{i32 9}
+!4 = !{i32 0}
+!5 = !{i32 2, !"Debug Info Version", i32 3}
+!6 = distinct !DISubprogram(name: "patatino", linkageName: "patatino", scope: null, file: !1, line: 1, type: !7, scopeLine: 1, spFlags: DISPFlagDefinition | DISPFlagOptimized, unit: !0, retainedNodes: !2)
+!7 = !DISubroutineType(types: !2)
+!8 = !DILocation(line: 1, column: 1, scope: !6)
+!9 = !DILocation(line: 2, column: 1, scope: !6)
+!10 = !DILocation(line: 3, column: 1, scope: !6)
+!11 = !DILocation(line: 4, column: 1, scope: !6)
+!12 = !DILocation(line: 5, column: 1, scope: !6)
+!13 = !DILocation(line: 6, column: 1, scope: !6)
+!14 = !DILocation(line: 7, column: 1, scope: !6)
+!15 = !DILocation(line: 8, column: 1, scope: !6)
+!16 = !DILocation(line: 9, column: 1, scope: !6)