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

[via llvm-commits]

https://github.com/HaohaiWen created https://github.com/llvm/llvm-project/pull/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.


>From 80a650763278061630e6ff5635bbc3a2dacc093e Mon Sep 17 00:00:00 2001
From: Haohai Wen <haohai.wen at intel.com>
Date: Wed, 10 Jan 2024 16:09:00 +0800
Subject: [PATCH 1/2] [BranchFolding][SEH] Add test to track SEH CFG
 optimization

This test tracks BranchFolding pass which removes fall through jump and
leaves landing-pad to be machine basic block of no predecessors. It
would raise bug as introduced in #77441.
---
 .../X86/branchfolding-landingpad-cfg.mir      | 49 +++++++++++++++++++
 1 file changed, 49 insertions(+)
 create mode 100644 llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir

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..a494701c2a3997
--- /dev/null
+++ b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
@@ -0,0 +1,49 @@
+# 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:   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
+...

>From 34cce741218ed6f719214556b8e260c94b20deee Mon Sep 17 00:00:00 2001
From: Haohai Wen <haohai.wen at intel.com>
Date: Wed, 10 Jan 2024 20:50:24 +0800
Subject: [PATCH 2/2] [BranchFolding] Fix missing predecessors of landing-pad

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.
---
 llvm/lib/CodeGen/BranchFolding.cpp              | 17 +++++++++++++++++
 .../X86/branchfolding-landingpad-cfg.mir        |  2 ++
 2 files changed, 19 insertions(+)

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
index a494701c2a3997..8eef5450e252a6 100644
--- a/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
+++ b/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):