[llvm] e1e4bf1 - [DAGCombine] Prevent the transform of combine for multi-use operand

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2021-09-06T15:30:32-04:00
New Revision: e1e4bf174b09bcd4b25cd624f177537890bff785

URL: https://github.com/llvm/llvm-project/commit/e1e4bf174b09bcd4b25cd624f177537890bff785
DIFF: https://github.com/llvm/llvm-project/commit/e1e4bf174b09bcd4b25cd624f177537890bff785.diff

LOG: [DAGCombine] Prevent the transform of combine for multi-use operand

The test is based on a miscompile example in:
https://llvm.org/PR51321

Differential Revision: https://reviews.llvm.org/D107692

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
    llvm/test/CodeGen/AArch64/arm64-srl-and.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d1663b99d4c73..7cd6d2b7f4a8b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -5149,8 +5149,9 @@ SDValue DAGCombiner::visitANDLike(SDValue N0, SDValue N1, SDNode *N) {
   if (SDValue V = foldLogicOfSetCCs(true, N0, N1, DL))
     return V;
 
+  // TODO: Rewrite this to return a new 'AND' instead of using CombineTo.
   if (N0.getOpcode() == ISD::ADD && N1.getOpcode() == ISD::SRL &&
-      VT.getSizeInBits() <= 64) {
+      VT.getSizeInBits() <= 64 && N0->hasOneUse()) {
     if (ConstantSDNode *ADDI = dyn_cast<ConstantSDNode>(N0.getOperand(1))) {
       if (ConstantSDNode *SRLI = dyn_cast<ConstantSDNode>(N1.getOperand(1))) {
         // Look for (and (add x, c1), (lshr y, c2)). If C1 wasn't a legal

diff  --git a/llvm/test/CodeGen/AArch64/arm64-srl-and.ll b/llvm/test/CodeGen/AArch64/arm64-srl-and.ll
index 2f024e444d25f..c6b301b8e80ef 100644
--- a/llvm/test/CodeGen/AArch64/arm64-srl-and.ll
+++ b/llvm/test/CodeGen/AArch64/arm64-srl-and.ll
@@ -1,7 +1,8 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=aarch64-linux-gnu -O3 < %s | FileCheck %s
 
-; Disable the dagcombine if operand has multi use
+; This used to miscompile:
+; The 16-bit -1 should not become 32-bit -1 (sub w8, w8, #1).
 
 @g = global i16 0, align 4
 define i32 @srl_and()  {
@@ -12,7 +13,8 @@ define i32 @srl_and()  {
 ; CHECK-NEXT:    mov w9, #50
 ; CHECK-NEXT:    ldrh w8, [x8]
 ; CHECK-NEXT:    eor w8, w8, w9
-; CHECK-NEXT:    sub w8, w8, #1
+; CHECK-NEXT:    mov w9, #65535
+; CHECK-NEXT:    add w8, w8, w9
 ; CHECK-NEXT:    and w0, w8, w8, lsr #16
 ; CHECK-NEXT:    ret
 entry: