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2 level inline table (armeilleure)

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TODOS:
- Lightning JIT
- Sparse page using signal handler
- Fallback page? Would save a few instructions
This commit is contained in:
riperiperi
2024-06-15 20:34:50 +01:00
parent d86249cb0a
commit 4e381009a9
4 changed files with 110 additions and 2 deletions
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41
src/ARMeilleure/Instructions/InstEmitFlowHelper.cs
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@@ -193,6 +193,8 @@ namespace ARMeilleure.Instructions
Operand hostAddress;
var table = context.FunctionTable;
// If address is mapped onto the function table, we can skip the table walk. Otherwise we fallback
// onto the dispatch stub.
if (guestAddress.Kind == OperandKind.Constant && context.FunctionTable.IsValid(guestAddress.Value))
@@ -203,6 +205,45 @@ namespace ARMeilleure.Instructions
hostAddress = context.Load(OperandType.I64, hostAddressAddr);
}
else if (table.Levels.Length == 2)
{
// Inline table lookup. Only enabled when the sparse function table is enabled with 2 levels.
// Deliberately attempts to avoid branches.
var level0 = table.Levels[0];
// Currently no bounds check. Maybe conditionally do this for unsafe host mapped.
Operand index = context.ShiftLeft(context.ShiftRightUI(guestAddress, Const(level0.Index)), Const(3));
Operand tableBase = !context.HasPtc ?
Const(table.Base) :
Const(table.Base, Ptc.FunctionTableSymbol);
Operand page = context.Load(OperandType.I64, context.Add(tableBase, index));
// Second level
var level1 = table.Levels[1];
int clearBits = 64 - (level1.Index + level1.Length);
Operand index2 = context.ShiftLeft(
context.ShiftRightUI(context.ShiftLeft(guestAddress, Const(clearBits)), Const(clearBits + level1.Index)),
Const(3)
);
// TODO: could possibly make a fallback page that level 1 is filled with that contains dispatch stub on all pages
// Would save this load and the comparisons
// 16MB of the same value is a bit wasteful so it could replicate with remapping.
Operand fallback = !context.HasPtc ?
Const((long)context.FunctionTable.Fallback) :
Const((long)context.FunctionTable.Fallback, Ptc.DispatchFallbackSymbol);
Operand pageIsZero = context.ICompareEqual(page, Const(0L));
// Small trick to keep this branchless - if the page is zero, load a fallback table entry that always contains the dispatch stub.
hostAddress = context.Load(OperandType.I64, context.ConditionalSelect(pageIsZero, fallback, context.Add(page, index2)));
}
else
{
hostAddress = !context.HasPtc ?