From 38e76ba030cc11e0a3a68e39c19c0ec6de624a09 Mon Sep 17 00:00:00 2001
From: EgorBo <egorbo@gmail.com>
Date: Fri, 10 May 2024 19:28:45 +0200
Subject: [PATCH] Use BoundsCheckNoThrow assertion in
 optAssertionProp_RangeProperties

---
 src/coreclr/jit/assertionprop.cpp | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/coreclr/jit/assertionprop.cpp b/src/coreclr/jit/assertionprop.cpp
index 6e30d4874e447..7dd36e9b04e52 100644
--- a/src/coreclr/jit/assertionprop.cpp
+++ b/src/coreclr/jit/assertionprop.cpp
@@ -3930,6 +3930,18 @@ void Compiler::optAssertionProp_RangeProperties(ASSERT_VALARG_TP assertions,
     {
         AssertionDsc* curAssertion = optGetAssertion(GetAssertionIndex(index));
 
+        // if treeVN has a bound-check assertion where it's an index, then
+        // it means it's not negative, example:
+        //
+        //   array[idx] = 42; // creates 'BoundsCheckNoThrow' assertion
+        //   return idx % 8;  // idx is known to be never negative here, hence, MOD->UMOD
+        //
+        if (curAssertion->IsBoundsCheckNoThrow() && (curAssertion->op1.bnd.vnIdx == treeVN))
+        {
+            *isKnownNonNegative = true;
+            continue;
+        }
+
         // First, analyze possible X ==/!= CNS assertions.
         if (curAssertion->IsConstantInt32Assertion() && (curAssertion->op1.vn == treeVN))
         {