From 64a2ea41eaca32a6ba66db6b699ca33b2eee095b Mon Sep 17 00:00:00 2001
From: Geoff Berry <gberry@codeaurora.org>
Date: Mon, 14 May 2018 17:31:18 +0000
Subject: [PATCH] [BranchFolding] Allow hoisting to block with a single
 conditional branch.

Summary:
The BranchFolding pass is currently missing opportunities to hoist
common code if the hoisted-to block contains a single conditional branch
that has register uses.  This occurs somewhat frequently on AArch64 with
CBZ/TBZ opcodes.

This change also eliminates some code differences when debug info is
present since the presence of e.g. DBG_VALUE instructions in the
hoisted-to block can enable hoisting that wouldn't have occurred without
them.

Reviewers: MatzeB, rnk, kparzysz, twoh, aprantl, javed.absar

Subscribers: kristof.beyls, JDevlieghere, mcrosier, llvm-commits

Differential Revision: https://reviews.llvm.org/D46324

llvm-svn: 332265
---
 llvm/lib/CodeGen/BranchFolding.cpp                 |  6 ++++-
 .../test/CodeGen/AArch64/branch-folder-oneinst.mir | 29 ++++++++++++++++++++++
 2 files changed, 34 insertions(+), 1 deletion(-)
 create mode 100644 llvm/test/CodeGen/AArch64/branch-folder-oneinst.mir

diff --git a/llvm/lib/CodeGen/BranchFolding.cpp b/llvm/lib/CodeGen/BranchFolding.cpp
index 190dfc0..ef9b65d 100644
--- a/llvm/lib/CodeGen/BranchFolding.cpp
+++ b/llvm/lib/CodeGen/BranchFolding.cpp
@@ -1915,8 +1915,12 @@ MachineBasicBlock::iterator findHoistingInsertPosAndDeps(MachineBasicBlock *MBB,
 
   if (Uses.empty())
     return Loc;
+  // If the terminator is the only instruction in the block and Uses is not
+  // empty (or we would have returned above), we can still safely hoist
+  // instructions just before the terminator as long as the Defs/Uses are not
+  // violated (which is checked in HoistCommonCodeInSuccs).
   if (Loc == MBB->begin())
-    return MBB->end();
+    return Loc;
 
   // The terminator is probably a conditional branch, try not to separate the
   // branch from condition setting instruction.
diff --git a/llvm/test/CodeGen/AArch64/branch-folder-oneinst.mir b/llvm/test/CodeGen/AArch64/branch-folder-oneinst.mir
new file mode 100644
index 0000000..377b21d
--- /dev/null
+++ b/llvm/test/CodeGen/AArch64/branch-folder-oneinst.mir
@@ -0,0 +1,29 @@
+# RUN: llc -o - %s -mtriple=aarch64 -run-pass branch-folder | FileCheck %s
+# Check that BranchFolding pass is able to hoist a common instruction into a block with a single branch instruction.
+name: func
+tracksRegLiveness: true
+body: |
+  bb.0:
+    ; CHECK-LABEL: name: func
+    ; CHECK-LABEL: bb.0:
+    ; CHECK: $x0 = ADDXri $x0, 1, 0
+    ; CHECK: CBZX $x1, %bb.2
+    liveins: $x1
+    CBZX $x1, %bb.2
+
+  bb.1:
+    ; CHECK-LABEL: bb.1:
+    ; CHECK-NOT: $x0 = ADDXri $x0, 1, 0
+    liveins: $x0
+    $x0 = ADDXri $x0, 1, 0
+    $x0 = ADDXri $x0, 2, 0
+    RET_ReallyLR implicit $x0
+
+  bb.2:
+    ; CHECK-LABEL: bb.2:
+    ; CHECK-NOT: $x0 = ADDXri $x0, 1, 0
+    liveins: $x0
+    $x0 = ADDXri $x0, 1, 0
+    $x0 = ADDXri $x0, 3, 0
+    RET_ReallyLR implicit $x0
+...
-- 
2.7.4