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fp: Change FPUnpacked to a normalized representation

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Having a known position for the highest set bit makes writing algorithms easier
This commit is contained in:
MerryMage 2018-07-25 17:39:14 +01:00
parent 680395a803
commit 7a673a8a43
10 changed files with 71 additions and 56 deletions
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29
src/common/fp/fused.cpp
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@ -11,26 +11,15 @@
namespace Dynarmic::FP {
constexpr size_t normalized_point_position = 62;
constexpr size_t product_point_position = normalized_point_position * 2;
static FPUnpacked NormalizeUnpacked(FPUnpacked op) {
constexpr int desired_highest = static_cast<int>(normalized_point_position);
const int highest_bit = Common::HighestSetBit(op.mantissa);
DEBUG_ASSERT(highest_bit < desired_highest);
const int offset = desired_highest - highest_bit;
op.mantissa <<= offset;
op.exponent -= offset;
return op;
static FPUnpacked ReduceMantissa(bool sign, int exponent, const u128& mantissa) {
constexpr int point_position_correction = normalized_point_position - (product_point_position - 64);
// We round-to-odd here when reducing the bitwidth of the mantissa so that subsequent roundings are accurate.
return {sign, exponent + point_position_correction, mantissa.upper | static_cast<u64>(mantissa.lower != 0)};
}
FPUnpacked FusedMulAdd(FPUnpacked addend, FPUnpacked op1, FPUnpacked op2) {
addend = NormalizeUnpacked(addend);
op1 = NormalizeUnpacked(op1);
op2 = NormalizeUnpacked(op2);
const bool product_sign = op1.sign != op2.sign;
const auto [product_exponent, product_value] = [op1, op2]{
int exponent = op1.exponent + op2.exponent;
@ -47,10 +36,10 @@ FPUnpacked FusedMulAdd(FPUnpacked addend, FPUnpacked op1, FPUnpacked op2) {
}
if (addend.mantissa == 0) {
return FPUnpacked{product_sign, product_exponent + 64, product_value.upper | u64(product_value.lower != 0)};
return ReduceMantissa(product_sign, product_exponent, product_value);
}
const int exp_diff = product_exponent - (addend.exponent - normalized_point_position);
const int exp_diff = product_exponent - addend.exponent;
if (product_sign == addend.sign) {
// Addition
@ -63,7 +52,7 @@ FPUnpacked FusedMulAdd(FPUnpacked addend, FPUnpacked op1, FPUnpacked op2) {
// addend < product
const u128 result = product_value + StickyLogicalShiftRight(addend.mantissa, exp_diff - normalized_point_position);
return FPUnpacked{product_sign, product_exponent + 64, result.upper | u64(result.lower != 0)};
return ReduceMantissa(product_sign, product_exponent, result);
}
// Subtraction
@ -80,7 +69,7 @@ FPUnpacked FusedMulAdd(FPUnpacked addend, FPUnpacked op1, FPUnpacked op2) {
result = product_value - addend_long;
} else if (exp_diff <= 0) {
result_sign = !product_sign;
result_exponent = addend.exponent - normalized_point_position;
result_exponent = addend.exponent;
result = addend_long - StickyLogicalShiftRight(product_value, -exp_diff);
} else {
result_sign = product_sign;
@ -95,7 +84,7 @@ FPUnpacked FusedMulAdd(FPUnpacked addend, FPUnpacked op1, FPUnpacked op2) {
const int required_shift = normalized_point_position - Common::HighestSetBit(result.upper);
result = result << required_shift;
result_exponent -= required_shift;
return FPUnpacked{result_sign, result_exponent + 64, result.upper | u64(result.lower != 0)};
return ReduceMantissa(result_sign, result_exponent, result);
}
} // namespace Dynarmic::FP