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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-backend-x86

Author: None (HaohaiWen)

<details>
<summary>Changes</summary>

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.


---
Full diff: https://github.com/llvm/llvm-project/pull/77608.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/BranchFolding.cpp (+17) 
- (added) llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir (+51) 


``````````diff
diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 0801296cab49f8..599b7c72b2f5c6 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/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.
diff --git a/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
new file mode 100644
index 00000000000000..8eef5450e252a6
--- /dev/null
+++ b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
@@ -0,0 +1,51 @@
+# NOTE: Assertions have been autogenerated by utils/update_mir_test_checks.py UTC_ARGS: --version 4
+# RUN: llc -mtriple=x86_64-pc-windows-msvc -run-pass=branch-folder -o - %s | FileCheck %s
+---
+name:            main
+body:             |
+  ; CHECK-LABEL: name: main
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x7ffff800), %bb.3(0x00000800)
+  ; 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):
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   CLEANUPRET
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3 (landing-pad, ehfunclet-entry):
+  ; CHECK-NEXT:   CLEANUPRET
+  bb.0:
+    successors: %bb.1(0x7ffff800), %bb.5(0x00000800)
+    JMP_1 %bb.1
+
+  bb.1:
+    successors: %bb.2(0x7ffff800), %bb.4(0x00000800)
+
+    JMP_1 %bb.2
+
+  bb.2:
+    successors: %bb.3(0x7ffff800), %bb.4(0x00000800)
+
+    JMP_1 %bb.3
+
+  bb.3:
+    successors: %bb.6(0x7ffff800)
+
+    JMP_1 %bb.6
+
+  bb.4 (machine-block-address-taken, landing-pad, ehfunclet-entry):
+    successors: %bb.5(0x80000000)
+    CLEANUPRET
+
+  bb.5 (landing-pad, ehfunclet-entry):
+    CLEANUPRET
+
+  bb.6:
+    RET 0
+...

``````````

</details>


https://github.com/llvm/llvm-project/pull/77608