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LibM: Count fractions when exponent < -1 with mantissa == 0
Without this change, floor(-0.125) returned 0. This is because the number has a fraction part even if mantissa is zero while the unbiased exponent is negative. The names of some variables were also made clearer. Co-Authored-By: Daniel Bertalan <dani@danielbertalan.dev>
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@ -113,49 +113,56 @@ static FloatType internal_to_integer(FloatType x, RoundingMode rounding_mode)
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{
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if (!isfinite(x))
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return x;
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using Extractor = FloatExtractor<decltype(x)>;
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Extractor extractor;
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extractor.d = x;
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auto unbiased_exponent = extractor.exponent - Extractor::exponent_bias;
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bool round = false;
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bool guard = false;
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bool has_half_fraction = false;
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bool has_nonhalf_fraction = false;
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if (unbiased_exponent < 0) {
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// it was easier to special case [0..1) as it saves us from
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// handling subnormals, underflows, etc
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if (unbiased_exponent == -1) {
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round = true;
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has_half_fraction = true;
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}
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guard = extractor.mantissa != 0;
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has_nonhalf_fraction = unbiased_exponent < -1 || extractor.mantissa != 0;
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extractor.mantissa = 0;
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extractor.exponent = 0;
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} else {
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if (unbiased_exponent >= Extractor::mantissa_bits)
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return x;
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auto dead_bitcount = Extractor::mantissa_bits - unbiased_exponent;
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auto dead_mask = (1ull << dead_bitcount) - 1;
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auto dead_bits = extractor.mantissa & dead_mask;
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extractor.mantissa &= ~dead_mask;
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auto guard_mask = dead_mask >> 1;
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guard = (dead_bits & guard_mask) != 0;
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round = (dead_bits & ~guard_mask) != 0;
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auto nonhalf_fraction_mask = dead_mask >> 1;
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has_nonhalf_fraction = (dead_bits & nonhalf_fraction_mask) != 0;
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has_half_fraction = (dead_bits & ~nonhalf_fraction_mask) != 0;
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}
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bool should_round = false;
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switch (rounding_mode) {
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case RoundingMode::ToEven:
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should_round = round;
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should_round = has_half_fraction;
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break;
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case RoundingMode::Up:
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if (!extractor.sign)
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should_round = guard || round;
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should_round = has_nonhalf_fraction || has_half_fraction;
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break;
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case RoundingMode::Down:
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if (extractor.sign)
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should_round = guard || round;
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should_round = has_nonhalf_fraction || has_half_fraction;
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break;
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case RoundingMode::ToZero:
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break;
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}
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if (should_round) {
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// We could do this ourselves, but this saves us from manually
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// handling overflow.
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