[llvm] [SelectionDAG] Resolve TODO: Use signedAddMayOverflow (PR #85586)

[via llvm-commits]

https://github.com/AtariDreams created https://github.com/llvm/llvm-project/pull/85586

This will result in more accurate results.

>From 8464e1422fd3416ab7532b6d12d218737cc72473 Mon Sep 17 00:00:00 2001
From: Rose <gfunni234 at gmail.com>
Date: Sun, 17 Mar 2024 18:04:03 -0400
Subject: [PATCH] [SelectionDAG] Resolve TODO: Use signedAddMayOverflow

This will result in more accurate results.
---
 llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 2670f48aebcff5..831fef27e5410c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4125,8 +4125,13 @@ SelectionDAG::computeOverflowForSignedAdd(SDValue N0, SDValue N1) const {
   if (ComputeNumSignBits(N0) > 1 && ComputeNumSignBits(N1) > 1)
     return OFK_Never;
 
-  // TODO: Add ConstantRange::signedAddMayOverflow handling.
-  return OFK_Sometime;
+  KnownBits N0Known = computeKnownBits(N0);
+  KnownBits N1Known = computeKnownBits(N1);
+
+  ConstantRange N0Range = ConstantRange::fromKnownBits(N0Known, false);
+  ConstantRange N1Range = ConstantRange::fromKnownBits(N1Known, false);
+
+  return mapOverflowResult(N0Range.signedAddMayOverflow(N1Range));
 }
 
 SelectionDAG::OverflowKind