Fold Span.Slice(X) like bound checks

src/coreclr/jit/assertionprop.cpp

@@ -4461,6 +4461,36 @@ GenTree* Compiler::optAssertionPropGlobal_RelOp(ASSERT_VALARG_TP assertions,
                 return optAssertionProp_Update(newTree, tree, stmt);
             }
         }
+
+        // If op1VN is actually ADD(X, CNS), we can try peeling the constant offset and adjusting op2Cns accordingly.
+        // It's a bit more complicated for unsigned comparisons, so only do it for signed ones for now.
+        //
+        if (!tree->IsUnsigned())
+        {
+            ValueNum peeledOp1VN  = op1VN;
+            int      peeledOffset = 0;
+            vnStore->PeelOffsetsI32(&peeledOp1VN, &peeledOffset);
+
+            if (peeledOffset != 0)
+            {
+                Range peeledOffsetRng = Range(Limit(Limit::keConstant, peeledOffset));
+                Range peeledOp1Rng    = Range(Limit(Limit::keUnknown));
+                if (RangeCheck::TryGetRangeFromAssertions(this, peeledOp1VN, assertions, &peeledOp1Rng))
+                {
+                    // Subtract handles overflow internally.
+                    rng2 = RangeOps::Subtract(rng2, peeledOffsetRng);
+
+                    RangeOps::RelationKind kind =
+                        RangeOps::EvalRelop(tree->OperGet(), tree->IsUnsigned(), peeledOp1Rng, rng2);
+                    if ((kind != RangeOps::RelationKind::Unknown))
+                    {
+                        newTree = kind == RangeOps::RelationKind::AlwaysTrue ? gtNewTrue() : gtNewFalse();
+                        newTree = gtWrapWithSideEffects(newTree, tree, GTF_ALL_EFFECT);
+                        return optAssertionProp_Update(newTree, tree, stmt);
+                    }
+                }
+            }
+        }
     }
 
     // Else check if we have an equality check involving a local or an indir

src/coreclr/jit/rangecheck.h

@@ -328,6 +328,25 @@ struct RangeOps
         return Limit(Limit::keUnknown);
     }
 
+    // Perform 'value' - 'cns'
+    static Limit SubtractConstantLimit(const Limit& value, const Limit& cns)
+    {
+        assert(cns.IsConstant());
+
+        if (cns.GetConstant() == INT_MIN)
+        {
+            // Subtracting INT_MIN would overflow
+            return Limit(Limit::keUnknown);
+        }
+
+        Limit l = value;
+        if (l.AddConstant(-cns.GetConstant()))
+        {
+            return l;
+        }
+        return Limit(Limit::keUnknown);
+    }
+
     // Perform 'value' * 'cns'
     static Limit MultiplyConstantLimit(const Limit& value, const Limit& cns)
     {
@@ -352,9 +371,9 @@ struct RangeOps
         return Limit(Limit::keUnknown);
     }
 
-    // Given two ranges "r1" and "r2", perform an add operation on the
-    // ranges.
-    static Range Add(Range& r1, Range& r2)
+    // Given two ranges "r1" and "r2", perform a generic 'op' operation on the ranges.
+    template <typename Operation>
+    static Range ApplyRangeOp(Range& r1, Range& r2, Operation op)
     {
         Limit& r1lo = r1.LowerLimit();
         Limit& r1hi = r1.UpperLimit();
@@ -376,23 +395,45 @@ struct RangeOps
 
         if (r1lo.IsConstant())
         {
-            result.lLimit = AddConstantLimit(r2lo, r1lo);
+            result.lLimit = op(r2lo, r1lo);
         }
         if (r2lo.IsConstant())
         {
-            result.lLimit = AddConstantLimit(r1lo, r2lo);
+            result.lLimit = op(r1lo, r2lo);
         }
         if (r1hi.IsConstant())
         {
-            result.uLimit = AddConstantLimit(r2hi, r1hi);
+            result.uLimit = op(r2hi, r1hi);
         }
         if (r2hi.IsConstant())
         {
-            result.uLimit = AddConstantLimit(r1hi, r2hi);
+            result.uLimit = op(r1hi, r2hi);
         }
+
         return result;
     }
 
+    static Range Add(Range& r1, Range& r2)
+    {
+        return ApplyRangeOp(r1, r2, [](Limit& a, Limit& b) {
+            return AddConstantLimit(a, b);
+        });
+    }
+
+    static Range Subtract(Range& r1, Range& r2)
+    {
+        return ApplyRangeOp(r1, r2, [](Limit& a, Limit& b) {
+            return SubtractConstantLimit(a, b);
+        });
+    }
+
+    static Range Multiply(Range& r1, Range& r2)
+    {
+        return ApplyRangeOp(r1, r2, [](Limit& a, Limit& b) {
+            return MultiplyConstantLimit(a, b);
+        });
+    }
+
     static Range ShiftRight(Range& r1, Range& r2)
     {
         Limit& r1lo = r1.LowerLimit();
@@ -430,47 +471,6 @@ struct RangeOps
         return result;
     }
 
-    // Given two ranges "r1" and "r2", perform an multiply operation on the
-    // ranges.
-    static Range Multiply(Range& r1, Range& r2)
-    {
-        Limit& r1lo = r1.LowerLimit();
-        Limit& r1hi = r1.UpperLimit();
-        Limit& r2lo = r2.LowerLimit();
-        Limit& r2hi = r2.UpperLimit();
-
-        Range result = Limit(Limit::keUnknown);
-
-        // Check lo ranges if they are dependent and not unknown.
-        if ((r1lo.IsDependent() && !r1lo.IsUnknown()) || (r2lo.IsDependent() && !r2lo.IsUnknown()))
-        {
-            result.lLimit = Limit(Limit::keDependent);
-        }
-        // Check hi ranges if they are dependent and not unknown.
-        if ((r1hi.IsDependent() && !r1hi.IsUnknown()) || (r2hi.IsDependent() && !r2hi.IsUnknown()))
-        {
-            result.uLimit = Limit(Limit::keDependent);
-        }
-
-        if (r1lo.IsConstant())
-        {
-            result.lLimit = MultiplyConstantLimit(r2lo, r1lo);
-        }
-        if (r2lo.IsConstant())
-        {
-            result.lLimit = MultiplyConstantLimit(r1lo, r2lo);
-        }
-        if (r1hi.IsConstant())
-        {
-            result.uLimit = MultiplyConstantLimit(r2hi, r1hi);
-        }
-        if (r2hi.IsConstant())
-        {
-            result.uLimit = MultiplyConstantLimit(r1hi, r2hi);
-        }
-        return result;
-    }
-
     // 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)