[llvm] [SelectionDAG] Calculate Knownbits for SMIN (PR #85584)

[via llvm-commits]

https://github.com/AtariDreams created https://github.com/llvm/llvm-project/pull/85584

None

>From 1013f4a37785c1629234dbc9f1524eb686f0605c Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Sun, 17 Mar 2024 17:27:32 -0400
Subject: [PATCH] [SelectionDAG] Calculate Knownbits for SMIN

---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 2670f48aebcff5..f6ac5d98d1d402 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4007,8 +4007,6 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
 
     // For SMAX, if CstLow is non-negative we know the result will be
     // non-negative and thus all sign bits are 0.
-    // TODO: There's an equivalent of this for smin with negative constant for
-    // known ones.
     if (IsMax && CstLow) {
       const APInt &ValueLow = CstLow->getAPIntValue();
       if (ValueLow.isNonNegative()) {
@@ -4017,6 +4015,16 @@ KnownBits SelectionDAG::computeKnownBits(SDValue Op, const APInt &DemandedElts,
       }
     }
 
+    // For SMIN, if CstHigh is negative we know the result will be
+    // negative and thus all sign bits are 1.
+    if (!IsMax && CstHigh) {
+      const APInt &ValueHigh = CstHigh->getAPIntValue();
+      if (ValueHigh.isNegative()) {
+        unsigned SignBits = ComputeNumSignBits(Op.getOperand(0), Depth + 1);
+        Known.One.setHighBits(std::min(SignBits, ValueHigh.getNumSignBits()));
+      }
+    }
+
     break;
   }
   case ISD::UINT_TO_FP: {