[llvm] [Mips][AsmPrinter] Fix for assembler not printing a label (PR #80666)

[via llvm-commits]

https://github.com/urosbericsyrmia created https://github.com/llvm/llvm-project/pull/80666

This PR is a fix for a bug that manifests in [rust-lang/rust#108835](https://github.com/rust-lang/rust/issues/108835).
rustc-demangle couldn't compile due to an undefined temporary symbol error which arises when the assembly code is branching to a basic block whose label is not emitted.
One of the reasons not to emit the label of a basic block is when that block is only reachable by fallthrough. If a block has only one predecessor and that predecessor is immediately before it, the block is reachable only by fallthrough. The problem is that the MipsAsmPrinter didn't take it into account what if that predecessor also branches to this basic block, which in this case happens. In that case, we need a label for the block.
The solution is to check if the predecessor branches to this basic block.

>From bcf5396c0684c9c573ee48ff189adfbcb7dc85aa Mon Sep 17 00:00:00 2001
From: Uros Beric <Uros.Beric at Syrmia.com>
Date: Mon, 29 Jan 2024 09:21:53 +0100
Subject: [PATCH] [Mips] Fix for assembler not printing a label

---
 llvm/lib/Target/Mips/MipsAsmPrinter.cpp     | 17 +++++++++++++++++
 llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll |  2 +-
 llvm/test/CodeGen/Mips/br_equal_labels.ll   | 14 ++++++++++++++
 3 files changed, 32 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/Mips/br_equal_labels.ll

diff --git a/llvm/lib/Target/Mips/MipsAsmPrinter.cpp b/llvm/lib/Target/Mips/MipsAsmPrinter.cpp
index 718844bc36ff9..1fd3ae9cd36ca 100644
--- a/llvm/lib/Target/Mips/MipsAsmPrinter.cpp
+++ b/llvm/lib/Target/Mips/MipsAsmPrinter.cpp
@@ -502,6 +502,23 @@ bool MipsAsmPrinter::isBlockOnlyReachableByFallthrough(const MachineBasicBlock*
   if (Pred->empty())
     return true;
 
+  // Check the terminators in the previous block
+  for (const auto &MI : Pred->terminators()) {
+    // If it is not a simple branch, we are in a table somewhere.
+    if (!MI.isBranch() || MI.isIndirectBranch())
+      return false;
+
+    // If we are the operands of one of the branches, this is not a fall
+    // through. Note that targets with delay slots will usually bundle
+    // terminators with the delay slot instruction.
+    for (ConstMIBundleOperands OP(MI); OP.isValid(); ++OP) {
+      if (OP->isJTI())
+        return false;
+      if (OP->isMBB() && OP->getMBB() == MBB)
+        return false;
+    }
+  }
+
   // Otherwise, check the last instruction.
   // Check if the last terminator is an unconditional branch.
   MachineBasicBlock::const_iterator I = Pred->end();
diff --git a/llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll b/llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll
index 9e64d7b2fa039..c276515920d52 100644
--- a/llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll
+++ b/llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll
@@ -11,7 +11,7 @@ define void @test(i32 %x, ptr %p) nounwind {
 ; CHECK-NEXT:    andi $1, $4, 1
 ; CHECK-NEXT:    bgtz $1, $BB0_1
 ; CHECK-NEXT:    nop
-; CHECK-NEXT:  # %bb.1: # %foo
+; CHECK-NEXT:  $BB0_1: # %foo
 ; CHECK-NEXT:    jr $ra
 ; CHECK-NEXT:    nop
   %y = and i32 %x, 1
diff --git a/llvm/test/CodeGen/Mips/br_equal_labels.ll b/llvm/test/CodeGen/Mips/br_equal_labels.ll
new file mode 100644
index 0000000000000..6ba2a713e5e2a
--- /dev/null
+++ b/llvm/test/CodeGen/Mips/br_equal_labels.ll
@@ -0,0 +1,14 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc --filetype=asm -march=mips --relocation-model=pic -O0  < %s | FileCheck %s
+
+define void @test(i32 %x) nounwind {
+  %y = icmp eq i32 %x, 1
+  br i1 %y, label %bb1, label %bb1
+
+bb1:
+  ret void
+}
+
+; CHECK:      bgtz	${{[0-9]+}}, $[[BB1:BB[0-9]+_[0-9]+]]
+; CHECK-NEXT: nop
+; CHECK-NEXT: $[[BB1]]:
\ No newline at end of file