[llvm] [SelectionDAG]: Have isKnownNeverZero treat SRL like division if all else fails (PR #89523)

[via llvm-commits]

https://github.com/AtariDreams updated https://github.com/llvm/llvm-project/pull/89523

>From 96a702a0017d02d1bf442b2322a30a61781ba6c6 Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Sat, 20 Apr 2024 21:31:50 -0400
Subject: [PATCH] [SelectionDAG]: Have isKnownNeverZero treat SRL like division
 if all else fails

---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 7dbf83b7adeef0..2bab57f95cbb7e 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5443,9 +5443,19 @@ bool SelectionDAG::isKnownNeverZero(SDValue Op, unsigned Depth) const {
     if (ValKnown.isNegative())
       return true;
     // If max shift cnt of known ones is non-zero, result is non-zero.
-    APInt MaxCnt = computeKnownBits(Op.getOperand(1), Depth + 1).getMaxValue();
-    if (MaxCnt.ult(ValKnown.getBitWidth()) &&
-        !ValKnown.One.lshr(MaxCnt).isZero())
+    const KnownBits Shift = computeKnownBits(Op.getOperand(1), Depth + 1);
+    APInt MaxCnt = Shift.getMaxValue();
+    if (MaxCnt.uge(ValKnown.getBitWidth()))
+      return false;
+    if (!ValKnown.One.lshr(MaxCnt).isZero())
+      return true;
+    // We try to see if we can turn it into a division
+    const KnownBits One =
+        KnownBits::makeConstant(APInt(ValKnown.getBitWidth(), 1));
+
+    std::optional<bool> uge =
+        KnownBits::uge(ValKnown, KnownBits::shl(One, Shift));
+    if (uge && *uge)
       return true;
     break;
   }