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Switches to faster bit count logic.

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This commit is contained in:
Thomas Harte 2019-07-09 18:41:20 -04:00
parent b4a3f66773
commit 69b94719a1
1 changed files with 16 additions and 20 deletions
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36
Processors/68000/Implementation/68000Implementation.hpp
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@ -49,6 +49,12 @@
#define s_extend16(x) int32_t(int16_t(x))
#define s_extend8(x) int32_t(int8_t(x))
#define convert_to_bit_count_16(x) \
x = ((x & 0xaaaa) >> 1) + (x & 0x5555); \
x = ((x & 0xcccc) >> 2) + (x & 0x3333); \
x = ((x & 0xf0f0) >> 4) + (x & 0x0f0f); \
x = ((x & 0xff00) >> 8) + (x & 0x00ff);
// Sets the length of the next microcycle; if this is a debug build, also confirms
// that the microcycle being adjusted is the one that it's permissible to adjust.
#ifdef NDEBUG
@ -931,13 +937,8 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
zero_result_ = active_program_->destination->full;
negative_flag_ = zero_result_ & 0x80000000;
// TODO: optimise the below?
int number_of_ones = 0;
auto source = active_program_->source->halves.low.full;
while(source) {
number_of_ones += source&1;
source >>= 1;
}
int number_of_ones = active_program_->source->halves.low.full;
convert_to_bit_count_16(number_of_ones);
// Time taken = 38 cycles + 2 cycles per 1 in the source.
set_next_microcycle_length(HalfCycles(4 * number_of_ones + 34*2));
@ -952,14 +953,9 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
// Find the number of 01 or 10 pairs in the 17-bit number
// formed by the source value with a 0 suffix.
// TODO: optimise the below?
int number_of_pairs = 0;
int source = active_program_->source->halves.low.full;
source = (source ^ (source << 1)) & 0xffff;
while(source) {
number_of_pairs += source&1;
source >>= 1;
}
int number_of_pairs = active_program_->source->halves.low.full;
number_of_pairs = (number_of_pairs ^ (number_of_pairs << 1)) & 0xffff;
convert_to_bit_count_16(number_of_pairs);
// Time taken = 38 cycles + 2 cycles per 1 in the source.
set_next_microcycle_length(HalfCycles(4 * number_of_pairs + 34*2));
@ -1088,11 +1084,9 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
// in the unsigned quotient; there is an additional microcycle for
// every bit that is set. Also, since the possibility of overflow
// was already dealt with, it's now a smaller number.
int positive_quotient = int(abs(quotient));
for(int c = 0; c < 15; ++c) {
if(positive_quotient & 0x8000) cycles_expended += 2;
positive_quotient <<= 1;
}
int positive_quotient_bits = int(abs(quotient)) & 0xfffe;
convert_to_bit_count_16(positive_quotient_bits);
cycles_expended += 2 * positive_quotient_bits;
// There's then no way to terminate the loop that isn't at least six cycles long.
cycles_expended += 6;
@ -2205,3 +2199,5 @@ template <class T, bool dtack_is_implicit, bool signal_will_perform> void Proces
#undef s_extend16
#undef s_extend8
#undef set_next_microcycle_length
#undef convert_to_bit_count_16