[BranchFolding] Use isSuccessor to confirm fall through

[HaohaiWen]

When merging blocks, if the previous block has no any branch instruction
and has one successor, the successor may be SEH landing pad and the
block will always raise exception and nerver fall through to next block.
We can not merge them in such case. isSuccessor should be used to
confirm it can fall through to next block.

[llvmbot]

@llvm/pr-subscribers-backend-x86

Author: Haohai Wen (HaohaiWen)

Changes

When merging blocks, if the previous block has no any branch instruction
and has one successor, the successor may be SEH landing pad and the
block will always raise exception and nerver fall through to next block.
We can not merge them in such case. isSuccessor should be used to
confirm it can fall through to next block.

---

Full diff: https://github.com/llvm/llvm-project/pull/77923.diff

2 Files Affected:

• (modified) llvm/lib/CodeGen/BranchFolding.cpp (+1-1)
• (modified) llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir (+41)

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 599b7c72b2f5c6..a9f78358e57b92 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -1411,7 +1411,7 @@ bool BranchFolder::OptimizeBlock(MachineBasicBlock *MBB) {
     // This has to check PrevBB->succ_size() because EH edges are ignored by
     // analyzeBranch.
     if (PriorCond.empty() && !PriorTBB && MBB->pred_size() == 1 &&
-        PrevBB.succ_size() == 1 &&
+        PrevBB.succ_size() == 1 && PrevBB.isSuccessor(MBB) &&
         !MBB->hasAddressTaken() && !MBB->isEHPad()) {
       LLVM_DEBUG(dbgs() << "\nMerging into block: " << PrevBB
                         << "From MBB: " << *MBB);
diff --git a/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
index 8eef5450e252a6..98dadbfcc17bda 100644
--- a/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
+++ b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
@@ -49,3 +49,44 @@ body:             |
   bb.6:
     RET 0
 ...
+---
+name:            foo
+body:             |
+  ; CHECK-LABEL: name: foo
+  ; CHECK: bb.0:
+  ; CHECK-NEXT:   successors: %bb.1(0x7ffff800), %bb.2(0x00000800)
+  ; CHECK-NEXT:   liveins: $rcx
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   renamable $eax = MOV32rm renamable $rcx, 1, $noreg, 0, $noreg
+  ; CHECK-NEXT:   TEST32rr renamable $eax, renamable $eax, implicit-def $eflags
+  ; CHECK-NEXT:   JCC_1 %bb.2, 14, implicit killed $eflags
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT:   INT 3
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2:
+  ; CHECK-NEXT:   RET 0
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3 (machine-block-address-taken, landing-pad, ehfunclet-entry):
+  ; CHECK-NEXT:   CLEANUPRET
+  bb.0:
+    successors: %bb.1(0x7ffff800), %bb.2(0x00000800)
+    liveins: $rcx
+
+    renamable $eax = MOV32rm renamable $rcx, 1, $noreg, 0, $noreg
+    TEST32rr renamable $eax, renamable $eax, implicit-def $eflags
+    JCC_1 %bb.2, 14, implicit killed $eflags
+    JMP_1 %bb.1
+
+  bb.1:
+    successors: %bb.3(0x80000000)
+    INT 3
+
+  bb.2:
+    RET 0
+
+  bb.3 (machine-block-address-taken, landing-pad, ehfunclet-entry):
+    CLEANUPRET
+...

[HaohaiWen]

Test before this patch: 4832d77

See caab489 to see what test changed

[phoebewang]

LGTM.