[BranchFolding] Fix missing predecessors of landing-pad (llvm#77608)

When removing an empty machine basic block, all of its successors should
be inherited by its fall through MBB. This keeps CFG as only have one
entry which is required by LiveDebugValues.

Reland llvm#77441 as LiveDebugValues test.

llvm/lib/CodeGen/BranchFolding.cpp

@@ -1363,6 +1363,14 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
         MachineBasicBlock *Pred = *(MBB->pred_end()-1);
         Pred->ReplaceUsesOfBlockWith(MBB, &*FallThrough);
       }
+      // Add rest successors of MBB to successors of FallThrough. Those
+      // successors are not directly reachable via MBB, so it should be
+      // landing-pad.
+      for (auto SI = MBB->succ_begin(), SE = MBB->succ_end(); SI != SE; ++SI)
+        if (*SI != &*FallThrough && !FallThrough->isSuccessor(*SI)) {
+          assert((*SI)->isEHPad() && "Bad CFG");
+          FallThrough->copySuccessor(MBB, SI);
+        }
       // If MBB was the target of a jump table, update jump tables to go to the
       // fallthrough instead.
       if (MachineJumpTableInfo *MJTI = MF.getJumpTableInfo())
@@ -1624,6 +1632,15 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
             } else {
               DidChange = true;
               PMBB->ReplaceUsesOfBlockWith(MBB, CurTBB);
+              // Add rest successors of MBB to successors of CurTBB. Those
+              // successors are not directly reachable via MBB, so it should be
+              // landing-pad.
+              for (auto SI = MBB->succ_begin(), SE = MBB->succ_end(); SI != SE;
+                   ++SI)
+                if (*SI != CurTBB && !CurTBB->isSuccessor(*SI)) {
+                  assert((*SI)->isEHPad() && "Bad CFG");
+                  CurTBB->copySuccessor(MBB, SI);
+                }
               // If this change resulted in PMBB ending in a conditional
               // branch where both conditions go to the same destination,
               // change this to an unconditional branch.

llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir

@@ -9,6 +9,8 @@ body:             |
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.2(0x00000800)
+  ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT:   RET 0
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.2 (machine-block-address-taken, landing-pad, ehfunclet-entry):

llvm/test/CodeGen/X86/windows-seh-EHa-PreserveCFG.ll

@@ -0,0 +1,80 @@
+; RUN: llc -mtriple=x86_64-pc-windows-msvc %s
+define dso_local void @main(ptr %addr, ptr %src, ptr %dst) personality ptr @__CxxFrameHandler3 !dbg !11 {
+entry:
+  %tmp0 = load float, ptr %src
+  %src1 = getelementptr inbounds float, ptr %src, i64 1
+  %tmp1 = load float, ptr %src1
+  %src2 = getelementptr inbounds float, ptr %src, i64 2
+  %tmp2 = load float, ptr %src2
+  %src3 = getelementptr inbounds float, ptr %src, i64 3
+  %tmp3 = load float, ptr %src3
+  %src4 = getelementptr inbounds float, ptr %src, i64 4
+  %tmp4 = load float, ptr %src4
+  %src5 = getelementptr inbounds float, ptr %src, i64 5
+  %tmp5 = load float, ptr %src5
+  %src6 = getelementptr inbounds float, ptr %src, i64 6
+  %tmp6 = load float, ptr %src6
+  invoke void @foo(ptr %addr)
+          to label %scope_begin unwind label %ehcleanup1, !dbg !13
+
+scope_begin:
+  invoke void @llvm.seh.scope.begin()
+          to label %scope_end unwind label %ehcleanup, !dbg !13
+
+scope_end:
+  invoke void @llvm.seh.scope.end()
+          to label %finish unwind label %ehcleanup, !dbg !13
+
+ehcleanup:
+  %0 = cleanuppad within none [], !dbg !13
+  call void @llvm.dbg.value(metadata ptr %addr, metadata !12, metadata !DIExpression()), !dbg !13
+  call void @foo(ptr %addr) [ "funclet"(token %0) ], !dbg !13
+  cleanupret from %0 unwind label %ehcleanup1, !dbg !13
+
+ehcleanup1:
+  %1 = cleanuppad within none [], !dbg !13
+  call void @foo(ptr %addr) [ "funclet"(token %1) ], !dbg !13
+  cleanupret from %1 unwind to caller, !dbg !13
+
+finish:
+  store float %tmp0, ptr %dst
+  %dst1 = getelementptr inbounds float, ptr %dst, i64 1
+  store float %tmp1, ptr %dst1
+  %dst2 = getelementptr inbounds float, ptr %dst, i64 2
+  store float %tmp2, ptr %dst2
+  %dst3 = getelementptr inbounds float, ptr %dst, i64 3
+  store float %tmp3, ptr %dst3
+  %dst4 = getelementptr inbounds float, ptr %dst, i64 4
+  store float %tmp4, ptr %dst4
+  %dst5 = getelementptr inbounds float, ptr %dst, i64 5
+  store float %tmp5, ptr %dst5
+  %dst6 = getelementptr inbounds float, ptr %dst, i64 6
+  store float %tmp6, ptr %dst6
+  ret void
+}
+
+declare dso_local void @llvm.seh.scope.begin()
+declare dso_local void @llvm.seh.scope.end()
+declare dso_local i32 @__CxxFrameHandler3(...)
+declare dso_local void @foo(ptr %addr)
+declare void @llvm.dbg.value(metadata, metadata, metadata)
+
+!llvm.module.flags = !{!0, !1, !2, !3}
+!llvm.dbg.cu = !{!14}
+
+!0 = !{i32 2, !"eh-asynch", i32 1}
+!1 = !{i32 2, !"CodeView", i32 1}
+!2 = !{i32 2, !"Debug Info Version", i32 3}
+!3 = !{i32 7, !"uwtable", i32 2}
+
+!4 = !DIBasicType(name: "float", size: 32, encoding: DW_ATE_float)
+!5 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !4, size: 64)
+!6 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
+!7 = !DIDerivedType(tag: DW_TAG_pointer_type, baseType: !6, size: 64)
+!8 = !DISubroutineType(types: !9)
+!9 = !{null, !7, !5, !5}
+!10 = !DIFile(filename: "c:/main.cpp", directory: "")
+!11 = distinct !DISubprogram(name: "main", scope: !10, file: !10, line: 5, type: !8, scopeLine: 11, unit: !14)
+!12 = !DILocalVariable(name: "addr", scope: !11, file: !10, line: 5, type: !7)
+!13 = !DILocation(line: 7, scope: !11)
+!14 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus_14, file: !10, isOptimized: true, emissionKind: FullDebug, splitDebugInlining: false, nameTableKind: None)