From 865fe131f87c68e27f64a1fe6f6351b6a6952cca Mon Sep 17 00:00:00 2001
From: "Haocong.Lu" <Haocong.Lu@streamcomputing.com>
Date: Fri, 25 Feb 2022 12:37:07 +0000
Subject: [PATCH] [RISCV] Fix a mistake in PostprocessISelDAG

With the condition N->use_empty(), the root node of DAG always
misses peephole optimization. So a dummy node is needed.

Reviewed By: craig.topper

Differential Revision: https://reviews.llvm.org/D119934
---
 llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp    |  3 +++
 llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll | 31 ++++++++++++++++++++++++++
 2 files changed, 34 insertions(+)

diff --git a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
index fbf5eb2..88eeca0 100644
--- a/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
+++ b/llvm/lib/Target/RISCV/RISCVISelDAGToDAG.cpp
@@ -132,6 +132,7 @@ void RISCVDAGToDAGISel::PreprocessISelDAG() {
 }
 
 void RISCVDAGToDAGISel::PostprocessISelDAG() {
+  HandleSDNode Dummy(CurDAG->getRoot());
   SelectionDAG::allnodes_iterator Position = CurDAG->allnodes_end();
 
   bool MadeChange = false;
@@ -146,6 +147,8 @@ void RISCVDAGToDAGISel::PostprocessISelDAG() {
     MadeChange |= doPeepholeMaskedRVV(N);
   }
 
+  CurDAG->setRoot(Dummy.getValue());
+
   if (MadeChange)
     CurDAG->RemoveDeadNodes();
 }
diff --git a/llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll b/llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll
index a297290..e23b29c 100644
--- a/llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll
+++ b/llvm/test/CodeGen/RISCV/fold-addi-loadstore.ll
@@ -166,6 +166,37 @@ entry:
    ret void
 }
 
+; Check if we can fold ADDI into the offset of store instructions,
+; when store instructions is the root node in DAG.
+
+@g_4_i32 = global i32 0, align 4
+
+define dso_local void @inc_g_i32() nounwind {
+; RV32I-LABEL: inc_g_i32:
+; RV32I:       # %bb.0: # %entry
+; RV32I-NEXT:    lui a0, %hi(g_4_i32)
+; RV32I-NEXT:    lw a1, %lo(g_4_i32)(a0)
+; RV32I-NEXT:    addi a1, a1, 1
+; RV32I-NEXT:    sw a1, %lo(g_4_i32)(a0)
+; RV32I-NEXT:    ret
+;
+; RV64I-LABEL: inc_g_i32:
+; RV64I:       # %bb.0: # %entry
+; RV64I-NEXT:    lui a0, %hi(g_4_i32)
+; RV64I-NEXT:    lw a1, %lo(g_4_i32)(a0)
+; RV64I-NEXT:    addiw a1, a1, 1
+; RV64I-NEXT:    sw a1, %lo(g_4_i32)(a0)
+; RV64I-NEXT:    ret
+entry:
+  %0 = load i32, i32* @g_4_i32
+  %inc = add i32 %0, 1
+  store i32 %inc, i32* @g_4_i32
+  br label %if.end
+
+if.end:
+  ret void
+}
+
 ; Check for folds in accesses to the second element of an i64 array.
 
 @ga_8 = dso_local local_unnamed_addr global [2 x i64] zeroinitializer, align 8
-- 
2.7.4