[llvm] [BranchRelaxation] Fix invalid branch generation in branch-relaxation (PR #162065)

[via llvm-commits]

https://github.com/sc-clulzze created https://github.com/llvm/llvm-project/pull/162065

If we have MBB with only one successor which is accessable through both conditional and unconditional branches (TBB == FBB), in `fixupConditionalBranch` we will first replace FBB with NewMBB in successors list - `MBB->replaceSuccessor(FBB, NewBB);`, and then create branch to TBB - `insertBranch(MBB, &NextBB, TBB, Cond);`, ending up with two branches to different blocks, but only one successor.

Fixes: https://github.com/llvm/llvm-project/issues/162063

>From 33b74f5d6bef49667f37ef6b41605eb34dd072f2 Mon Sep 17 00:00:00 2001
From: sc-cluzze <d.marakulin at syntacore.com>
Date: Fri, 3 Oct 2025 10:28:48 +0000
Subject: [PATCH] [BranchRelaxation] Fix invalid branch generation in
 branch-relaxation

---
 llvm/lib/CodeGen/BranchRelaxation.cpp  | 14 ++++++++++++++
 llvm/test/CodeGen/RISCV/branch-rel.mir | 24 ++++++++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 llvm/test/CodeGen/RISCV/branch-rel.mir

diff --git a/llvm/lib/CodeGen/BranchRelaxation.cpp b/llvm/lib/CodeGen/BranchRelaxation.cpp
index 2d50167faa085..bb75c638d1bcc 100644
--- a/llvm/lib/CodeGen/BranchRelaxation.cpp
+++ b/llvm/lib/CodeGen/BranchRelaxation.cpp
@@ -491,6 +491,20 @@ bool BranchRelaxation::fixupConditionalBranch(MachineInstr &MI) {
       return true;
     }
     if (FBB) {
+      // If we get here with a MBB which ends like this:
+      //
+      // bb.1:
+      // successors: %bb.2;
+      // ...
+      // BNE $x1, $x0, %bb.2
+      // PseudoBR %bb.2
+      //
+      // Just remove conditional branch.
+      if (TBB == FBB) {
+        BlockInfo[MBB->getNumber()].Size -= TII->getInstSizeInBytes(MI);
+        MI.eraseFromParent();
+        return true;
+      }
       // We need to split the basic block here to obtain two long-range
       // unconditional branches.
       NewBB = createNewBlockAfter(*MBB);
diff --git a/llvm/test/CodeGen/RISCV/branch-rel.mir b/llvm/test/CodeGen/RISCV/branch-rel.mir
new file mode 100644
index 0000000000000..218ebe1d09f44
--- /dev/null
+++ b/llvm/test/CodeGen/RISCV/branch-rel.mir
@@ -0,0 +1,24 @@
+# NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py UTC_ARGS: --version 5
+# RUN: llc %s -mtriple=riscv64 -run-pass=branch-relaxation -o - -verify-machineinstrs | FileCheck %s
+
+--- |
+  define void @foo() {
+    ret void
+  }
+...
+---
+name:            foo
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    liveins: $x1
+    BNE $x1, $x0, %bb.3
+    PseudoBR %bb.3
+  bb.1:
+    liveins: $x1
+    INLINEASM &".space 4096", 1
+    BGE $x1, $x0, %bb.3
+  bb.3:
+    PseudoRET
+## NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
+# CHECK: {{.*}}