[llvm] [MacroFusion] Restricts pairs to have SDep::Data dependency only (PR #203793)

[Tomer Shafir via llvm-commits]

https://github.com/tomershafir created https://github.com/llvm/llvm-project/pull/203793

This patch aims to restrict target independent macro fusion to SDep::Data dependent paris only. The test demonstrates the case that has driven this patch - 2 instructions are being wrongly macro fused by an Artificial edge, without being RAW dependent. Currently macro fusion do not really require a more relaxed constraint ike it has today. If this is invalidated in the future, we can solve it later e.g. by adding a hook.

>From 9e6d35437e9e2e5a234f178a606b47c380071095 Mon Sep 17 00:00:00 2001
From: tomershafir <tomer.shafir8 at gmail.com>
Date: Sun, 14 Jun 2026 23:52:42 +0300
Subject: [PATCH] [MacroFusion] Restricts pairs to have SDep::Data dependency
 only

This patch aims to restrict target independent macro fusion to SDep::Data dependent paris only. The test demonstrates the case that has driven this patch - 2 instructions are being wrongly macro fused by an Artificial edge, without being RAW dependent. Currently macro fusion do not really require a more relaxed constraint ike it has today. If this is invalidated in the future, we can solve it later e.g. by adding a hook.
---
 llvm/lib/CodeGen/MacroFusion.cpp              |  4 +-
 .../misched-fusion-no-raw-dependency.mir      | 37 +++++++++++++++++++
 2 files changed, 39 insertions(+), 2 deletions(-)
 create mode 100644 llvm/test/CodeGen/AArch64/misched-fusion-no-raw-dependency.mir

diff --git a/llvm/lib/CodeGen/MacroFusion.cpp b/llvm/lib/CodeGen/MacroFusion.cpp
index 08f4c59b29d26..df37aa88b4567 100644
--- a/llvm/lib/CodeGen/MacroFusion.cpp
+++ b/llvm/lib/CodeGen/MacroFusion.cpp
@@ -204,8 +204,8 @@ bool MacroFusion::scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU)
 
   // Explorer for fusion candidates among the dependencies of the anchor instr.
   for (SDep &Dep : AnchorSU.Preds) {
-    // Ignore dependencies other than data or strong ordering.
-    if (Dep.isWeak() || isHazard(Dep))
+    // Ignore dependencies other than data
+    if (Dep.getKind() != SDep::Data)
       continue;
 
     SUnit &DepSU = *Dep.getSUnit();
diff --git a/llvm/test/CodeGen/AArch64/misched-fusion-no-raw-dependency.mir b/llvm/test/CodeGen/AArch64/misched-fusion-no-raw-dependency.mir
new file mode 100644
index 0000000000000..f2616a9f8a475
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/misched-fusion-no-raw-dependency.mir
@@ -0,0 +1,37 @@
+# REQUIRES: asserts
+
+# RUN: llc -o /dev/null %s -mtriple=aarch64-unknown -mattr=+arith-cbz-fusion -passes=machine-scheduler -misched-print-dags -print-before=machine-scheduler 2>&1 | FileCheck %s
+# RUN: llc -o /dev/null %s -mtriple=aarch64-unknown -mcpu=cyclone -passes=machine-scheduler -misched-print-dags -print-before=machine-scheduler 2>&1 | FileCheck %s
+# RUN: llc -o /dev/null %s -mtriple=aarch64-unknown -mcpu=apple-m5 -passes=machine-scheduler -misched-print-dags -print-before=machine-scheduler 2>&1 | FileCheck %s
+
+## Check that we dont fuse non-RAW dependent pairs.
+
+## The arith op's def ($w0) is live-out via bb.1's livein,
+## so an Artificial edge from SU(ORRWri) to ExitSU is created by
+## addSchedBarrierDeps. The CBZ tests $w1, an unrelated register,
+## so there is no RAW between the candidate pair. MacroFusion must 
+## not cluster them.
+
+# CHECK-LABEL: live_out_exit_su_no_raw
+# CHECK: SU({{[0-9]+}}): $w0 = ORRWri $w0, 4096
+# CHECK: Successors:
+# CHECK-NOT: Cluster
+# CHECK: ExitSU: CBZW
+---
+name: live_out_exit_su_no_raw
+tracksRegLiveness: true
+isSSA: false
+noVRegs: true
+body: |
+  bb.0:
+    successors: %bb.1, %bb.2
+    liveins: $w0, $w1
+    $w0 = ADDWri $w0, 13, 0
+    $w3 = ORRWri $w1, 4096
+    CBZW $w1, %bb.2
+  bb.1:
+    liveins: $w0
+    RET undef $lr, implicit $w0
+  bb.2:
+    RET undef $lr
+...