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OpcodeDispatcher: optimize X87FTWTag

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using bit twiddling tricks :-)

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
This commit is contained in:
Alyssa Rosenzweig committed 2025-05-28 13:13:16 -04:00
1 parent 8e079c1965
commit b8dd5d95b0
2 files changed
+18 -16
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FEXCore/Source/Interface/Core/OpcodeDispatcher.h
-1
View File
@@ -2334,7 +2334,6 @@ private:
/** @} */
Ref GetX87Top();
Ref GetX87Tag(Ref Value, Ref AbridgedFTW);
void SetX87FTW(Ref FTW);
Ref GetX87FTW_Helper();
void SetX87Top(Ref Value);
FEXCore/Source/Interface/Core/OpcodeDispatcher/X87.cpp
+18 -15
View File
@@ -31,14 +31,6 @@ Ref OpDispatchBuilder::GetX87Top() {
return _LoadContext(OpSize::i8Bit, GPRClass, offsetof(FEXCore::Core::CPUState, flags) + FEXCore::X86State::X87FLAG_TOP_LOC);
}
Ref OpDispatchBuilder::GetX87Tag(Ref Value, Ref AbridgedFTW) {
Ref RegValid = _And(OpSize::i32Bit, _Lshr(OpSize::i32Bit, AbridgedFTW, Value), _Constant(1));
Ref X87Empty = _Constant(static_cast<uint8_t>(FPState::X87Tag::Empty));
Ref X87Valid = _Constant(static_cast<uint8_t>(FPState::X87Tag::Valid));
return _Select(FEXCore::IR::COND_EQ, RegValid, _Constant(0), X87Empty, X87Valid);
}
void OpDispatchBuilder::SetX87FTW(Ref FTW) {
Ref X87Empty = _Constant(static_cast<uint8_t>(FPState::X87Tag::Empty));
Ref NewAbridgedFTW {};
@@ -312,14 +304,25 @@ void OpDispatchBuilder::FSUB(OpcodeArgs, IR::OpSize Width, bool Integer, bool Re
}
Ref OpDispatchBuilder::GetX87FTW_Helper() {
Ref FTW = _Constant(0);
// AbridgedFTWIndex has 1-bit per slot (8 slots). Duplicate each bit to get
// 2-bits per slot (16-bit result). Duplicating bits is equivalent to
// Morton interleaving a number with itself. To interleave efficiently two
// bytes, we use the well-known bit twiddling algorithm:
//
// https://graphics.stanford.edu/~seander/bithacks.html#InterleaveBMN
Ref X = LoadContext(AbridgedFTWIndex);
X = _Orlshl(OpSize::i32Bit, X, X, 4);
X = _And(OpSize::i32Bit, X, _Constant(0x0f0f0f0f));
X = _Orlshl(OpSize::i32Bit, X, X, 2);
X = _And(OpSize::i32Bit, X, _Constant(0x33333333));
X = _Orlshl(OpSize::i32Bit, X, X, 1);
X = _And(OpSize::i32Bit, X, _Constant(0x55555555));
X = _Orlshl(OpSize::i32Bit, X, X, 1);
for (int i = 0; i < 8; i++) {
Ref RegTag = GetX87Tag(_Constant(i), LoadContext(AbridgedFTWIndex));
FTW = _Orlshl(OpSize::i32Bit, FTW, RegTag, i * 2);
}
return FTW;
// The above sequence sets valid to 11 and empty to 00, so invert to finalize.
static_assert(static_cast<uint8_t>(FPState::X87Tag::Valid) == 0b00);
static_assert(static_cast<uint8_t>(FPState::X87Tag::Empty) == 0b11);
return _Xor(OpSize::i32Bit, X, _Constant(0xffff));
}
void OpDispatchBuilder::X87FNSTENV(OpcodeArgs) {