[llvm] [BranchFolding][SEH] Use isSuccessor to confirm fall through (PR #77923)

[Haohai Wen via llvm-commits]

https://github.com/HaohaiWen created https://github.com/llvm/llvm-project/pull/77923

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.

>From 4832d772f08fb6f574d2d212fdaeb974a9fa4eab Mon Sep 17 00:00:00 2001
From: Haohai Wen <haohai.wen at intel.com>
Date: Fri, 12 Jan 2024 20:30:42 +0800
Subject: [PATCH 1/2] [BranchFolding][SEH] Add test to check block merge bug

---
 .../X86/branchfolding-landingpad-cfg.mir      | 41 +++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
index 8eef5450e252a6..f1f97a7836e525 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.3(0x7ffff800), %bb.1(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.3, 15, implicit $eflags
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.1:
+  ; CHECK-NEXT:   successors:
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.2 (machine-block-address-taken, landing-pad, ehfunclet-entry):
+  ; CHECK-NEXT:   CLEANUPRET
+  ; CHECK-NEXT: {{  $}}
+  ; CHECK-NEXT: bb.3:
+  ; CHECK-NEXT:   INT 3
+  ; CHECK-NEXT:   RET 0
+  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
+...

>From caab48946775ead91511a8b4aeb221393dbb2162 Mon Sep 17 00:00:00 2001
From: Haohai Wen <haohai.wen at intel.com>
Date: Fri, 12 Jan 2024 20:46:18 +0800
Subject: [PATCH 2/2] [BranchFolding][SEH] Use isSuccessor to confirm fall
 through

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.
---
 llvm/lib/CodeGen/BranchFolding.cpp               |  2 +-
 .../CodeGen/X86/branchfolding-landingpad-cfg.mir | 16 ++++++++--------
 2 files changed, 9 insertions(+), 9 deletions(-)

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 f1f97a7836e525..98dadbfcc17bda 100644
--- a/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
+++ b/llvm/test/CodeGen/X86/branchfolding-landingpad-cfg.mir
@@ -54,23 +54,23 @@ name:            foo
 body:             |
   ; CHECK-LABEL: name: foo
   ; CHECK: bb.0:
-  ; CHECK-NEXT:   successors: %bb.3(0x7ffff800), %bb.1(0x00000800)
+  ; 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.3, 15, implicit $eflags
+  ; CHECK-NEXT:   JCC_1 %bb.2, 14, implicit killed $eflags
   ; CHECK-NEXT: {{  $}}
   ; CHECK-NEXT: bb.1:
-  ; CHECK-NEXT:   successors:
-  ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT: bb.2 (machine-block-address-taken, landing-pad, ehfunclet-entry):
-  ; CHECK-NEXT:   CLEANUPRET
+  ; CHECK-NEXT:   successors: %bb.3(0x80000000)
   ; CHECK-NEXT: {{  $}}
-  ; CHECK-NEXT: bb.3:
   ; 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