[llvm] [MIR2Vec] Handle machine functions with no basic blocks (PR #212294)

[Petr Kurapov via llvm-commits]

https://github.com/kurapov-peter created https://github.com/llvm/llvm-project/pull/212294

depth_first() asserts on an empty MachineFunction, since getEntryNode() dereferences the block list sentinel via front(). Return the zero vector instead, as IR2Vec does for declarations.

>From 71391ca2e0b0b94de02ac2fbb90d8ebdba0dd672 Mon Sep 17 00:00:00 2001
From: Petr Kurapov <petr.kurapov at amd.com>
Date: Mon, 27 Jul 2026 11:40:10 -0500
Subject: [PATCH] [MIR2Vec] Handle machine functions with no basic blocks

depth_first() asserts on an empty MachineFunction, since getEntryNode()
dereferences the block list sentinel via front(). Return the zero vector
instead, as IR2Vec does for declarations.
---
 llvm/lib/CodeGen/MIR2Vec.cpp                  |  3 ++
 .../MIR2Vec/empty-machine-function.mir        | 35 +++++++++++++++++++
 2 files changed, 38 insertions(+)
 create mode 100644 llvm/test/CodeGen/MIR2Vec/empty-machine-function.mir

diff --git a/llvm/lib/CodeGen/MIR2Vec.cpp b/llvm/lib/CodeGen/MIR2Vec.cpp
index 42e299834b77b..0b2d18203b8d5 100644
--- a/llvm/lib/CodeGen/MIR2Vec.cpp
+++ b/llvm/lib/CodeGen/MIR2Vec.cpp
@@ -554,6 +554,9 @@ Embedding MIREmbedder::computeEmbeddings(const MachineBasicBlock &MBB) const {
 Embedding MIREmbedder::computeEmbeddings() const {
   Embedding MFuncVector(Dimension, 0);
 
+  if (MF.empty())
+    return MFuncVector;
+
   // Consider all reachable machine basic blocks in the function
   for (const auto *MBB : depth_first(&MF))
     MFuncVector += computeEmbeddings(*MBB);
diff --git a/llvm/test/CodeGen/MIR2Vec/empty-machine-function.mir b/llvm/test/CodeGen/MIR2Vec/empty-machine-function.mir
new file mode 100644
index 0000000000000..6ef112bf3ac66
--- /dev/null
+++ b/llvm/test/CodeGen/MIR2Vec/empty-machine-function.mir
@@ -0,0 +1,35 @@
+# REQUIRES: x86-registered-target
+# RUN: llc -mtriple=x86_64-unknown-linux-gnu -run-pass=none -print-mir2vec -mir2vec-vocab-path=%S/Inputs/mir2vec_dummy_3D_vocab.json %s -o /dev/null 2>&1 | FileCheck %s
+# RUN: llvm-ir2vec triplets --mode=mir %s -o /dev/null
+
+# A function whose MIR body is absent embeds to
+# the zero vector instead.
+
+--- |
+  define void @no_body() { ret void }
+
+  define void @has_body() { ret void }
+...
+---
+name:            has_body
+tracksRegLiveness: true
+body:             |
+  bb.0:
+    RET 0
+...
+
+# The function vector is zero and there are no blocks to list.
+
+# CHECK: MIR2Vec embeddings for machine function no_body:
+# CHECK-NEXT: Machine Function vector:  [ 0.00  0.00  0.00 ]
+# CHECK-NEXT: Machine basic block vectors:
+# CHECK-NEXT: Machine instruction vectors:
+
+# A function in the same module still embeds normally: RET = [1.0 1.1 1.2]
+# plus the [0.1 0.1 0.1] immediate operand.
+
+# CHECK-NEXT: MIR2Vec embeddings for machine function has_body:
+# CHECK-NEXT: Machine Function vector:  [ 1.10  1.20  1.30 ]
+# CHECK-NEXT: Machine basic block vectors:
+# CHECK-NEXT: Machine basic block: has_body:BB0:
+# CHECK-NEXT:  [ 1.10  1.20  1.30 ]