[PATCH] D46324: [BranchFolding] Allow hoisting to block with a single conditional branch.

[Geoff Berry via Phabricator via llvm-commits]

gberry created this revision.
gberry added reviewers: MatzeB, rnk, kparzysz, twoh, aprantl.
Herald added a reviewer: javed.absar.
Herald added subscribers: JDevlieghere, kristof.beyls.

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.


Repository:
  rL LLVM

https://reviews.llvm.org/D46324

Files:
  lib/CodeGen/BranchFolding.cpp
  test/CodeGen/AArch64/branch-folder-oneinst.mir


Index: test/CodeGen/AArch64/branch-folder-oneinst.mir
===================================================================
--- /dev/null
+++ 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
+...
Index: lib/CodeGen/BranchFolding.cpp
===================================================================
--- lib/CodeGen/BranchFolding.cpp
+++ lib/CodeGen/BranchFolding.cpp
@@ -1867,7 +1867,7 @@
   if (Uses.empty())
     return Loc;
   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.


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