Misc improvements for rangecheck (#129101)

- Fix unsound bounds in `RangeOps::Or` (could exclude reachable values);
use `max`-based lower/upper bounds
- existing bug: `[3..5] | [4..7] -> lo = 3|4 = 7, hi = 5|7 = 7 (claims
[7..7])`
- Add `UDIV` constant-range support to `GetRangeFromAssertionsWorker`
- Populate `m_isVNNeverNegative` on checked-bound / VN-relop assertions 
- Restrict the never-negative `O2K_VN_ADD_CNS` range deduction to LT/LE
relops (GT/GE were unsound)


[Diffs](https://dev.azure.com/dnceng-public/public/_build/results?buildId=1453272&view=ms.vss-build-web.run-extensions-tab)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: EgorBo

src/coreclr/jit/compiler.h

@@ -8976,13 +8976,14 @@ class Compiler
             assert(op1VN != ValueNumStore::NoVN);
             assert(checkedBndVN != ValueNumStore::NoVN);
 
-            AssertionDsc dsc           = CreateEmptyAssertion(comp);
-            dsc.m_assertionKind        = FromVNFunc(relop);
-            dsc.m_op1.m_kind           = O1K_VN;
-            dsc.m_op1.m_vn             = op1VN;
-            dsc.m_op2.m_kind           = O2K_VN_ADD_CNS;
-            dsc.m_op2.m_vn             = checkedBndVN;
-            dsc.m_op2.m_icon.m_iconVal = cns;
+            AssertionDsc dsc              = CreateEmptyAssertion(comp);
+            dsc.m_assertionKind           = FromVNFunc(relop);
+            dsc.m_op1.m_kind              = O1K_VN;
+            dsc.m_op1.m_vn                = op1VN;
+            dsc.m_op2.m_kind              = O2K_VN_ADD_CNS;
+            dsc.m_op2.m_vn                = checkedBndVN;
+            dsc.m_op2.m_icon.m_iconVal    = cns;
+            dsc.m_op2.m_isVNNeverNegative = comp->vnStore->IsVNNeverNegative(checkedBndVN);
             return dsc;
         }
 
@@ -9021,7 +9022,7 @@ class Compiler
             dsc.m_op2.m_kind              = O2K_VN_ADD_CNS;
             dsc.m_op2.m_vn                = op2VN;
             dsc.m_op2.m_icon.m_iconVal    = 0; // TODO-CQ: consider decomposing VN to VN* + CNS if beneficial.
-            dsc.m_op2.m_isVNNeverNegative = false;
+            dsc.m_op2.m_isVNNeverNegative = comp->vnStore->IsVNNeverNegative(op2VN);
             return dsc;
         }
     };

src/coreclr/jit/rangecheck.cpp

@@ -791,6 +791,7 @@ Range RangeCheck::GetRangeFromAssertionsWorker(
             case VNF_RSH:
             case VNF_RSZ:
             case VNF_UMOD:
+            case VNF_UDIV:
             {
                 // Get ranges of both operands and perform the same operation on the ranges.
                 Range r1 = GetRangeFromAssertionsWorker(comp, funcApp.GetArg(0), assertions, --budget, visited);
@@ -825,6 +826,9 @@ Range RangeCheck::GetRangeFromAssertionsWorker(
                     case VNF_UMOD:
                         binOpResult = RangeOps::UnsignedMod(r1, r2);
                         break;
+                    case VNF_UDIV:
+                        binOpResult = RangeOps::UnsignedDivide(r1, r2);
+                        break;
                     default:
                         unreached();
                 }
@@ -1230,7 +1234,13 @@ void RangeCheck::MergeEdgeAssertionsWorker(Compiler*                        comp
         //  Example: "(uint)normalLclVN < span.Length"   means normalLclVN's range is [0..INT32_MAX-1]
         //  Example: "(uint)normalLclVN <= array.Length" means normalLclVN's range is [0..Array.MaxLength]
         //
-        else if (!canUseCheckedBounds && curAssertion.IsRelop() && (curAssertion.GetOp1().GetVN() == normalLclVN) &&
+        // This is restricted to upper-bound (LT/LE) relops: we substitute op2 with its upper bound
+        // (maxValue), which is only valid when op2 bounds normalLclVN from above. GT/GE relops (where
+        // op2 is a lower bound) would need op2's lower bound instead, so they fall through to the
+        // general "X <relop> Y" branch below.
+        else if (!canUseCheckedBounds &&
+                 curAssertion.KindIs(Compiler::OAK_LT, Compiler::OAK_LE, Compiler::OAK_LT_UN, Compiler::OAK_LE_UN) &&
+                 (curAssertion.GetOp1().GetVN() == normalLclVN) &&
                  (curAssertion.GetOp2().KindIs(Compiler::O2K_VN_ADD_CNS)) &&
                  (curAssertion.GetOp2().IsVNNeverNegative()) && (curAssertion.GetOp2().GetCns() == 0))
         {

src/coreclr/jit/rangecheck.h

@@ -443,19 +443,39 @@ struct RangeOps
 
     static Range Or(const Range& r1, const Range& r2)
     {
-        // For OR we require both operands to be constant to produce a constant result.
-        // No useful information can be derived if only one operand is constant.
+        // For OR we require both operands to be non-negative constant ranges.
         //
         // Example: [0..3] | [1..255] = [1..255]
         //          [X..Y] | [1..255] = [unknown..unknown]
         //
-        return ApplyRangeOp(r1, r2, [](const Limit& a, const Limit& b) {
-            if (a.IsConstant() && b.IsConstant() && (a.GetConstant() >= 0) && (b.GetConstant() >= 0))
-            {
-                return Limit(Limit::keConstant, a.GetConstant() | b.GetConstant());
-            }
-            return Limit(Limit::keUnknown);
-        });
+        if (!r1.IsConstantRange() || !r2.IsConstantRange())
+        {
+            return Range(Limit(Limit::keUnknown));
+        }
+
+        const int r1lo = r1.LowerLimit().GetConstant();
+        const int r1hi = r1.UpperLimit().GetConstant();
+        const int r2lo = r2.LowerLimit().GetConstant();
+        const int r2hi = r2.UpperLimit().GetConstant();
+
+        if ((r1lo < 0) || (r2lo < 0))
+        {
+            return Range(Limit(Limit::keUnknown));
+        }
+
+        // a | b >= max(a, b), so max(r1lo, r2lo) is a sound lower bound.
+        const int lo = max(r1lo, r2lo);
+
+        // a | b cannot set any bit above the most-significant set bit of max(r1hi, r2hi), so it is
+        // bounded above by that value with all lower bits filled in (smallest 2^k - 1 >= the max).
+        int hi = max(r1hi, r2hi);
+        hi |= hi >> 1;
+        hi |= hi >> 2;
+        hi |= hi >> 4;
+        hi |= hi >> 8;
+        hi |= hi >> 16;
+
+        return Range(Limit(Limit::keConstant, lo), Limit(Limit::keConstant, hi));
     }
 
     static Range And(const Range& r1, const Range& r2)
@@ -501,6 +521,31 @@ struct RangeOps
         return Range(Limit(Limit::keUnknown));
     }
 
+    static Range UnsignedDivide(const Range& r1, const Range& r2)
+    {
+        // We only handle constant ranges for both operands.
+        if (!r1.IsConstantRange() || !r2.IsConstantRange())
+        {
+            return Range(Limit(Limit::keUnknown));
+        }
+
+        const int numLo = r1.LowerLimit().GetConstant();
+        const int numHi = r1.UpperLimit().GetConstant();
+        const int divLo = r2.LowerLimit().GetConstant();
+        const int divHi = r2.UpperLimit().GetConstant();
+
+        // Require a non-negative dividend and a strictly-positive divisor so that the signed
+        // [lo..hi] bounds coincide with their unsigned interpretation (and no division by zero).
+        if ((numLo < 0) || (divLo <= 0))
+        {
+            return Range(Limit(Limit::keUnknown));
+        }
+
+        // (uint)x / (uint)y is maximized by the largest dividend over the smallest divisor and
+        // minimized by the smallest dividend over the largest divisor.
+        return Range(Limit(Limit::keConstant, numLo / divHi), Limit(Limit::keConstant, numHi / divLo));
+    }
+
     // Given two ranges "r1" and "r2", do a Phi merge. If "monIncreasing" is true,
     // then ignore the dependent variables for the lower bound but not for the upper bound.
     static Range Merge(const Range& r1, const Range& r2, bool monIncreasing)