Convert DIV/MOD to UDIV/UMOD using assertions (#94347)

Author: EgorBo

src/coreclr/jit/assertionprop.cpp

@@ -3541,6 +3541,84 @@ GenTree* Compiler::optAssertionProp_BlockStore(ASSERT_VALARG_TP assertions, GenT
     return nullptr;
 }
 
+//------------------------------------------------------------------------
+// optAssertionProp_ModDiv: Convert DIV/MOD to UDIV/UMOD if both operands
+//    can be proven to be non-negative, e.g.:
+//
+//    if (x > 0)         // creates "x > 0" assertion
+//        return x / 8;  // DIV can be converted to UDIV
+//
+// Arguments:
+//    assertions - set of live assertions
+//    tree       - the DIV/MOD node to optimize
+//    stmt       - statement containing DIV/MOD
+//
+// Returns:
+//    Updated UDIV/UMOD node, or "nullptr"
+//
+GenTree* Compiler::optAssertionProp_ModDiv(ASSERT_VALARG_TP assertions, GenTreeOp* tree, Statement* stmt)
+{
+    if (optLocalAssertionProp || !varTypeIsIntegral(tree) || BitVecOps::IsEmpty(apTraits, assertions))
+    {
+        return nullptr;
+    }
+
+    // For now, we're mainly interested in "X op CNS" pattern (where CNS > 0).
+    // Technically, we can check assertions for both operands, but it's not clear if it's worth it.
+    if (!tree->gtGetOp2()->IsNeverNegative(this))
+    {
+        return nullptr;
+    }
+
+    const ValueNum  dividendVN = vnStore->VNConservativeNormalValue(tree->gtGetOp1()->gtVNPair);
+    BitVecOps::Iter iter(apTraits, assertions);
+    unsigned        index = 0;
+    while (iter.NextElem(&index))
+    {
+        AssertionDsc* curAssertion = optGetAssertion(GetAssertionIndex(index));
+
+        // OAK_[NOT]_EQUAL assertion with op1 being O1K_CONSTANT_LOOP_BND
+        // representing "(X relop CNS) ==/!= 0" assertion.
+        if (!curAssertion->IsConstantBound())
+        {
+            continue;
+        }
+
+        ValueNumStore::ConstantBoundInfo info;
+        vnStore->GetConstantBoundInfo(curAssertion->op1.vn, &info);
+
+        if (info.cmpOpVN != dividendVN)
+        {
+            continue;
+        }
+
+        // Root assertion has to be either:
+        // (X relop CNS) == 0
+        // (X relop CNS) != 0
+        if ((curAssertion->op2.kind != O2K_CONST_INT) || (curAssertion->op2.u1.iconVal != 0))
+        {
+            continue;
+        }
+
+        genTreeOps cmpOper = static_cast<genTreeOps>(info.cmpOper);
+
+        // Normalize "(X relop CNS) == false" to "(X reversed_relop CNS) == true"
+        if (curAssertion->assertionKind == OAK_EQUAL)
+        {
+            cmpOper = GenTree::ReverseRelop(cmpOper);
+        }
+
+        // "X >= CNS" or "X > CNS" where CNS is >= 0
+        if ((info.constVal >= 0) && ((cmpOper == GT_GE) || (cmpOper == GT_GT)))
+        {
+            JITDUMP("Converting DIV/MOD to unsigned UDIV/UMOD since both operands are never negative...\n")
+            tree->SetOper(tree->OperIs(GT_DIV) ? GT_UDIV : GT_UMOD, GenTree::PRESERVE_VN);
+            return optAssertionProp_Update(tree, tree, stmt);
+        }
+    }
+    return nullptr;
+}
+
 //------------------------------------------------------------------------
 // optAssertionProp_Return: Try and optimize a GT_RETURN via assertions.
 //
@@ -4787,6 +4865,10 @@ GenTree* Compiler::optAssertionProp(ASSERT_VALARG_TP assertions, GenTree* tree,
         case GT_RETURN:
             return optAssertionProp_Return(assertions, tree->AsUnOp(), stmt);
 
+        case GT_MOD:
+        case GT_DIV:
+            return optAssertionProp_ModDiv(assertions, tree->AsOp(), stmt);
+
         case GT_BLK:
         case GT_IND:
         case GT_STOREIND:
@@ -4812,11 +4894,9 @@ GenTree* Compiler::optAssertionProp(ASSERT_VALARG_TP assertions, GenTree* tree,
         case GT_LE:
         case GT_GT:
         case GT_GE:
-
             return optAssertionProp_RelOp(assertions, tree, stmt);
 
         case GT_JTRUE:
-
             if (block != nullptr)
             {
                 return optVNConstantPropOnJTrue(block, tree);

src/coreclr/jit/compiler.h

@@ -7746,6 +7746,7 @@ class Compiler
     GenTree* optAssertionProp_LclFld(ASSERT_VALARG_TP assertions, GenTreeLclVarCommon* tree, Statement* stmt);
     GenTree* optAssertionProp_LocalStore(ASSERT_VALARG_TP assertions, GenTreeLclVarCommon* store, Statement* stmt);
     GenTree* optAssertionProp_BlockStore(ASSERT_VALARG_TP assertions, GenTreeBlk* store, Statement* stmt);
+    GenTree* optAssertionProp_ModDiv(ASSERT_VALARG_TP assertions, GenTreeOp* tree, Statement* stmt);
     GenTree* optAssertionProp_Return(ASSERT_VALARG_TP assertions, GenTreeUnOp* ret, Statement* stmt);
     GenTree* optAssertionProp_Ind(ASSERT_VALARG_TP assertions, GenTree* tree, Statement* stmt);
     GenTree* optAssertionProp_Cast(ASSERT_VALARG_TP assertions, GenTreeCast* cast, Statement* stmt);

src/coreclr/jit/gentree.cpp

@@ -8764,8 +8764,6 @@ void GenTreeOp::CheckDivideByConstOptimized(Compiler* comp)
 {
     if (UsesDivideByConstOptimized(comp))
     {
-        gtFlags |= GTF_DIV_BY_CNS_OPT;
-
         // Now set DONT_CSE on the GT_CNS_INT divisor, note that
         // with ValueNumbering we can have a non GT_CNS_INT divisor
         GenTree* divisor = gtGetOp2()->gtEffectiveVal(/*commaOnly*/ true);
@@ -11143,18 +11141,6 @@ void Compiler::gtDispNode(GenTree* tree, IndentStack* indentStack, _In_ _In_opt_
                 }
                 goto DASH;
 
-            case GT_DIV:
-            case GT_MOD:
-            case GT_UDIV:
-            case GT_UMOD:
-                if (tree->gtFlags & GTF_DIV_BY_CNS_OPT)
-                {
-                    printf("M"); // We will use a Multiply by reciprical
-                    --msgLength;
-                    break;
-                }
-                goto DASH;
-
             case GT_LCL_FLD:
             case GT_LCL_VAR:
             case GT_LCL_ADDR:

src/coreclr/jit/gentree.h

@@ -547,8 +547,6 @@ enum GenTreeFlags : unsigned int
 
     GTF_DIV_MOD_NO_OVERFLOW     = 0x40000000, // GT_DIV, GT_MOD -- Div or mod definitely does not overflow.
 
-    GTF_DIV_BY_CNS_OPT          = 0x80000000, // GT_DIV -- Uses the division by constant optimization to compute this division
-
     GTF_CHK_INDEX_INBND         = 0x80000000, // GT_BOUNDS_CHECK -- have proven this check is always in-bounds
 
     GTF_ARRLEN_NONFAULTING      = 0x20000000, // GT_ARR_LENGTH  -- An array length operation that cannot fault. Same as GT_IND_NONFAULTING.
@@ -3007,12 +3005,6 @@ struct GenTreeOp : public GenTreeUnOp
     // then sets the flag GTF_DIV_BY_CNS_OPT and GTF_DONT_CSE on the constant
     void CheckDivideByConstOptimized(Compiler* comp);
 
-    // True if this node is marked as using the division by constant optimization
-    bool MarkedDivideByConstOptimized() const
-    {
-        return (gtFlags & GTF_DIV_BY_CNS_OPT) != 0;
-    }
-
 #if !defined(TARGET_64BIT) || defined(TARGET_ARM64)
     bool IsValidLongMul();
 #endif

src/coreclr/jit/lower.cpp

@@ -6584,7 +6584,6 @@ bool Lowering::LowerUnsignedDivOrMod(GenTreeOp* divMod)
             unreached();
 #endif
         }
-        assert(divMod->MarkedDivideByConstOptimized());
 
         const bool     requiresDividendMultiuse = !isDiv;
         const weight_t curBBWeight              = m_block->getBBWeight(comp);