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Minor optimization for integer conversion to double #40557

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Minor optimization for integer conversion to double #40557

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17 changes: 8 additions & 9 deletions src/sage/rings/integer.pyx
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Original file line number Diff line number Diff line change
Expand Up @@ -7813,11 +7813,6 @@ cdef double mpz_get_d_nearest(mpz_t x) except? -648555075988944.5:
# to have 54 bits remaining.
cdef mp_bitcnt_t shift = sx - 54

# Compute q = trunc(x / 2^shift) and let remainder_is_zero be True
# if and only if no truncation occurred.
cdef int remainder_is_zero
remainder_is_zero = mpz_divisible_2exp_p(x, shift)

sig_on()

cdef mpz_t q
Expand All @@ -7842,12 +7837,16 @@ cdef double mpz_get_d_nearest(mpz_t x) except? -648555075988944.5:
# Round towards zero
pass
else:
if not remainder_is_zero:
# Remainder is nonzero: round away from zero
if (q64 & 2) == 2:
# round to even and round away from zero gives the same result, no need to check
q64 += 1
else:
# Halfway case: round to even
q64 += (q64 & 2) - 1
if mpz_divisible_2exp_p(x, shift):
# Halfway case: round to even
q64 -= 1
else:
# Remainder is nonzero: round away from zero
q64 += 1

# The conversion of q64 to double is *exact*.
# This is because q64 is even and satisfies 2^53 <= q64 <= 2^54.
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