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OpcodeDispatcher: optimize bzhi

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Trickery to save an instruction :')

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
This commit is contained in:
Alyssa Rosenzweig committed 2023-11-12 17:32:24 -04:00
1 parent 224a1f19a3
commit b2a9785959
1 file changed
+10 -10
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FEXCore/Source/Interface/Core/OpcodeDispatcher.cpp
+10 -10
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@@ -2229,24 +2229,24 @@ void OpDispatchBuilder::BZHI(OpcodeArgs) {
auto* Src = LoadSource(GPRClass, Op, Op->Src[0], Op->Flags);
auto* Index = LoadSource(GPRClass, Op, Op->Src[1], Op->Flags);
// Mask off the index so we only consider the lower byte.
auto MaskedIndex = _And(OpSize::i64Bit, Index, _Constant(0xFF));
// Now clear the high bits specified by the index.
// Clear the high bits specified by the index. A64 only considers bottom bits
// of the shift, so we don't need to mask bottom 8-bits ourselves.
// Out-of-bounds results ignored after.
auto NegOne = _Constant(OperandSize, -1);
auto Mask = _Lshl(IR::SizeToOpSize(Size), NegOne, MaskedIndex);
auto Mask = _Lshl(IR::SizeToOpSize(Size), NegOne, Index);
auto MaskResult = _Andn(IR::SizeToOpSize(Size), Src, Mask);
// If the index is above OperandSize, we don't clear anything.
auto Bounds = _Constant(OperandSize - 1);
_SubNZCV(OpSize::i64Bit, MaskedIndex, Bounds);
auto Result = _NZCVSelect(IR::SizeToOpSize(Size), CondClassType{COND_UGT},
// If the index is above OperandSize, we don't clear anything. BZHI only
// considers the bottom 8-bits, so we really want to know if the bottom 8-bits
// have their top bits set. Test exactly that.
_TestNZ(OpSize::i64Bit, Index, _Constant(0xFF & ~(OperandSize - 1)));
auto Result = _NZCVSelect(IR::SizeToOpSize(Size), CondClassType{COND_NEQ},
Src, MaskResult);
StoreResult(GPRClass, Op, Result, -1);
auto Zero = _Constant(0);
auto One = _Constant(1);
auto CF = _NZCVSelect(OpSize::i32Bit, CondClassType{COND_UGT}, One, Zero);
auto CF = _NZCVSelect(OpSize::i32Bit, CondClassType{COND_NEQ}, One, Zero);
GenerateFlags_BZHI(Op, Result, CF);
}