JIT: (x u>> cns) -> [0..(x's max value >> cns)] (dotnet#109900)

cshung · Nov 20, 2024 · 50cbcb3 · 50cbcb3
1 parent 1c54b48
commit 50cbcb3
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Showing 2 changed files with 27 additions and 11 deletions.
diff --git a/src/coreclr/jit/rangecheck.cpp b/src/coreclr/jit/rangecheck.cpp
@@ -1014,7 +1014,7 @@ void RangeCheck::MergeAssertion(BasicBlock* block, GenTree* op, Range* pRange DE
 // Compute the range for a binary operation.
 Range RangeCheck::ComputeRangeForBinOp(BasicBlock* block, GenTreeOp* binop, bool monIncreasing DEBUGARG(int indent))
 {
-    assert(binop->OperIs(GT_ADD, GT_AND, GT_RSH, GT_LSH, GT_UMOD, GT_MUL));
+    assert(binop->OperIs(GT_ADD, GT_AND, GT_RSH, GT_RSZ, GT_LSH, GT_UMOD, GT_MUL));
 
     GenTree* op1 = binop->gtGetOp1();
     GenTree* op2 = binop->gtGetOp2();
@@ -1036,7 +1036,7 @@ Range RangeCheck::ComputeRangeForBinOp(BasicBlock* block, GenTreeOp* binop, bool
     }
 
     // Special cases for binops where op2 is a constant
-    if (binop->OperIs(GT_AND, GT_RSH, GT_LSH, GT_UMOD))
+    if (binop->OperIs(GT_AND, GT_RSH, GT_RSZ, GT_LSH, GT_UMOD))
     {
         if (!op2IsCns)
         {
@@ -1073,6 +1073,25 @@ Range RangeCheck::ComputeRangeForBinOp(BasicBlock* block, GenTreeOp* binop, bool
                 icon = binop->OperIs(GT_RSH) ? (icon1 >> icon2) : (icon1 << icon2);
             }
         }
+        else if (binop->OperIs(GT_RSZ))
+        {
+            // (x u>> cns) -> [0..(x's max value >> cns)]
+            int shiftBy = static_cast<int>(op2->AsIntCon()->IconValue());
+            if (shiftBy < 0)
+            {
+                return Range(Limit::keUnknown);
+            }
+
+            int op1Width = (int)(genTypeSize(op1) * BITS_PER_BYTE);
+            if (shiftBy >= op1Width)
+            {
+                return Range(Limit(Limit::keConstant, 0));
+            }
+
+            // Calculate max possible value of op1, e.g. UINT_MAX for TYP_INT/TYP_UINT
+            uint64_t maxValue = (1ULL << op1Width) - 1;
+            icon              = (int)(maxValue >> static_cast<int>(op2->AsIntCon()->IconValue()));
+        }
 
         if (icon >= 0)
         {
@@ -1088,7 +1107,7 @@ Range RangeCheck::ComputeRangeForBinOp(BasicBlock* block, GenTreeOp* binop, bool
     }
 
     // other operators are expected to be handled above.
-    assert(binop->OperIs(GT_ADD, GT_MUL, GT_LSH, GT_RSH));
+    assert(binop->OperIs(GT_ADD, GT_MUL, GT_LSH, GT_RSH, GT_RSZ));
 
     Range* op1RangeCached = nullptr;
     Range  op1Range       = Limit(Limit::keUndef);
@@ -1453,7 +1472,7 @@ bool RangeCheck::ComputeDoesOverflow(BasicBlock* block, GenTree* expr, const Ran
     }
     // These operators don't overflow.
     // Actually, GT_LSH can overflow so it depends on the analysis done in ComputeRangeForBinOp
-    else if (expr->OperIs(GT_AND, GT_RSH, GT_LSH, GT_UMOD, GT_NEG))
+    else if (expr->OperIs(GT_AND, GT_RSH, GT_RSZ, GT_LSH, GT_UMOD, GT_NEG))
     {
         overflows = false;
     }
@@ -1547,7 +1566,7 @@ Range RangeCheck::ComputeRange(BasicBlock* block, GenTree* expr, bool monIncreas
         MergeAssertion(block, expr, &range DEBUGARG(indent + 1));
     }
     // compute the range for binary operation
-    else if (expr->OperIs(GT_ADD, GT_AND, GT_RSH, GT_LSH, GT_UMOD, GT_MUL))
+    else if (expr->OperIs(GT_ADD, GT_AND, GT_RSH, GT_RSZ, GT_LSH, GT_UMOD, GT_MUL))
     {
         range = ComputeRangeForBinOp(block, expr->AsOp(), monIncreasing DEBUGARG(indent + 1));
     }

diff --git a/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/HalfHelpers.netstandard.cs b/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/HalfHelpers.netstandard.cs
@@ -107,12 +107,9 @@ private static int Log2SoftwareFallback(uint value)
             value |= value >> 08;
             value |= value >> 16;
 
-            // uint.MaxValue >> 27 is always in range [0 - 31] so we use Unsafe.AddByteOffset to avoid bounds check
-            return Unsafe.AddByteOffset(
-                // Using deBruijn sequence, k=2, n=5 (2^5=32) : 0b_0000_0111_1100_0100_1010_1100_1101_1101u
-                ref MemoryMarshal.GetReference(Log2DeBruijn),
-                // uint|long -> IntPtr cast on 32-bit platforms does expensive overflow checks not needed here
-                (IntPtr)(int)((value * 0x07C4ACDDu) >> 27));
+            // Using deBruijn sequence, k=2, n=5 (2^5=32) : 0b_0000_0111_1100_0100_1010_1100_1101_1101u
+            // uint.MaxValue >> 27 is always in range [0 - 31] so no bounds check
+            return Log2DeBruijn[(int)((value * 0x07C4ACDDu) >> 27)];
         }
 
         private static float CreateSingleNaN(bool sign, ulong significand)