[llvm] [DAGCombiner] In mergeTruncStore, make sure we aren't storing shifted in bits. (PR #90939)

[Craig Topper via llvm-commits]

https://github.com/topperc updated https://github.com/llvm/llvm-project/pull/90939

>From 3abbe7889c815c45b3bc4fa12eaf593d1a8a00ec Mon Sep 17 00:00:00 2001
From: Craig Topper <craig.topper at sifive.com>
Date: Thu, 2 May 2024 22:16:50 -0700
Subject: [PATCH] [DAGCombiner] In mergeTruncStore, make sure we aren't storing
 shifted in bits.

When looking through a right shift, we need to make sure that all of
the bits we are using from the shift come from the shift input and
not the sign or zero bits that are shifted in.

Fixes #90936.
---
 llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp | 4 ++++
 llvm/test/CodeGen/AArch64/pr90936.ll          | 6 ++++--
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index c0bbea16a64262..fc6bbc119d3c1b 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -8840,6 +8840,10 @@ SDValue DAGCombiner::mergeTruncStores(StoreSDNode *N) {
       if (ShiftAmtC % NarrowNumBits != 0)
         return SDValue();
 
+      // Make sure we aren't reading bits that are shifted in.
+      if (ShiftAmtC > WideVal.getScalarValueSizeInBits() - NarrowNumBits)
+        return SDValue();
+
       Offset = ShiftAmtC / NarrowNumBits;
       WideVal = WideVal.getOperand(0);
     }
diff --git a/llvm/test/CodeGen/AArch64/pr90936.ll b/llvm/test/CodeGen/AArch64/pr90936.ll
index 62c18fa5218765..38cda8d388945f 100644
--- a/llvm/test/CodeGen/AArch64/pr90936.ll
+++ b/llvm/test/CodeGen/AArch64/pr90936.ll
@@ -3,8 +3,10 @@
 
 define void @f(i16 %arg, ptr %arg1) {
 ; CHECK-LABEL: f:
-; CHECK:       // %bb.0: // %bb
-; CHECK-NEXT:    strh w0, [x1]
+; CHECK:       // %bb.0:
+; CHECK-NEXT:    ubfx w8, w0, #8, #6
+; CHECK-NEXT:    strb w0, [x1]
+; CHECK-NEXT:    strb w8, [x1, #1]
 ; CHECK-NEXT:    ret
 bb:
   %i = trunc i16 %arg to i8