Use LowBit encoding for BitRange

Author: xoofx

src/AsmArm64.CodeGen/InstructionProcessor.cs

@@ -907,7 +907,7 @@ private ImmediateOperandDescriptor ProcessImmediate(Instruction instruction, Ope
             if (symbol.BitValues.Count != 0)
             {
                 var kind = $"[{string.Join(",", symbol.BitValues.Select(bitValue => $"{bitValue.BitFields[0]} = {bitValue.Value}"))}]";
-                if (immediate.Size == 7 && immediate.Encoding[0] == new BitRange(22, 7) && symbol.BitValues.Count == 4 && symbol.BitValueNames.Count == 1 && symbol.BitValueNames[0] == "immh")
+                if (immediate.Size == 7 && immediate.Encoding[0] == new BitRange(16, 7) && symbol.BitValues.Count == 4 && symbol.BitValueNames.Count == 1 && symbol.BitValueNames[0] == "immh")
                 {
                     switch (kind)
                     {
@@ -946,7 +946,7 @@ private ImmediateOperandDescriptor ProcessImmediate(Instruction instruction, Ope
                 }
                 else if (symbol.BitValueNames.Count == 1 && symbol.BitValueNames[0] == "cmode<0>" && immediate.Encoding.Count == 1)
                 {
-                    Debug.Assert(immediate.Encoding[0] == new BitRange(15,4));
+                    Debug.Assert(immediate.Encoding[0] == new BitRange(12,4));
                     immediate.ImmediateKind = Arm64ImmediateEncodingKind.EnumAmount8Or16;
                     immediate.Size = 1;
                     immediate.Encoding[0] = new BitRange(12, 1);
@@ -1213,10 +1213,10 @@ private static int CollectEncodingForSymbol(Instruction instruction, List<string
             if (encoding.Count > 0)
             {
                 var previousEncoding = encoding[^1];
-                if (previousEncoding.HiBit - previousEncoding.Width == bitRangeInfo.HiBit)
+                if (previousEncoding.LowBit - 1 == bitRangeInfo.HiBit)
                 {
                     // Merge with previous encoding
-                    encoding[^1] = new(previousEncoding.HiBit, previousEncoding.Width + bitRangeInfo.Width);
+                    encoding[^1] = new(bitRangeInfo.HiBit - bitRangeInfo.Width + 1, previousEncoding.Width + bitRangeInfo.Width);
                     size += bitRangeInfo.Width;
                     continue;
                 }
@@ -1245,10 +1245,10 @@ private static int CollectEncodingForSymbol(Instruction instruction, EncodingSym
             if (encoding.Count > 0)
             {
                 var previousEncoding = encoding[^1];
-                if (previousEncoding.HiBit - previousEncoding.Width == bitRangeInfo.HiBit)
+                if (previousEncoding.LowBit - 1 == bitRangeInfo.HiBit)
                 {
                     // Merge with previous encoding
-                    encoding[^1] = new(previousEncoding.HiBit, previousEncoding.Width + bitRangeInfo.Width);
+                    encoding[^1] = new(bitRangeInfo.HiBit - bitRangeInfo.Width + 1, previousEncoding.Width + bitRangeInfo.Width);
                     size += bitRangeInfo.Width;
                     continue;
                 }

src/AsmArm64.CodeGen/Model/Operands.cs

@@ -407,13 +407,8 @@ sealed class ShiftOperandDescriptor : OperandDescriptor
     public int AmountSize { get; set; }
 
     public BitRange AmountEncoding { get; set; }
-
-
 }
 
-
-
-
 sealed class DynamicRegisterSelector
 {
     public int SelectorSize { get; set; }
@@ -504,9 +499,9 @@ public override string ToString()
     }
 }
 
-readonly record struct BitRange(int HiBit, int Width)
+readonly record struct BitRange(int LowBit, int Width)
 {
-    public override string ToString() => $"({HiBit}:{Width})";
+    public override string ToString() => $"({LowBit}:{Width})";
 }
 
 readonly record struct BitValueToRegister(string BitValues, Arm64RegisterEncodingKind RegisterKind)