stillwater-sc / universal / 26350396300

85%

master: 84%

Build Type

push

github

Committed by web-flow

Commit Message

fix(ereal): IEEE 754 division-by-zero + special values (#968) (#970)

ereal division routed straight into expansion_quotient (= e *
reciprocal(f)), which has no special-value handling: reciprocal(0) returns
Inf, then a * Inf renormalized to NaN, and any zero operand collapsed to a
positive +0 limb. So finite/0 gave NaN (not signed Inf), 0/0 gave +0 (not
NaN), signed-zero quotients lost their sign, and EREAL_THROW_ARITHMETIC_
EXCEPTION was never consulted.

Add apply_ieee754_div_special_values(), a sibling to the add/mul handlers,
and apply it in operator/= before the Newton-reciprocal quotient. The sign
of a quotient is signbit(a) XOR signbit(b), including for zero and infinite
results:

  NaN / x, x / NaN       = NaN
  x / 0 (x != 0)         = Inf, sign = a ^ b   (or throw, see below)
  0 / 0                  = NaN
  Inf / Inf              = NaN
  Inf / finite-nonzero   = Inf, sign = a ^ b
  finite / Inf           = 0,   sign = a ^ b
  0 / finite-nonzero     = 0,   sign = a ^ b

When EREAL_THROW_ARITHMETIC_EXCEPTION is enabled, a zero divisor throws
ereal_divide_by_zero (the exception type already existed but was unwired in
operator/=). operator/=(double) now delegates to the ereal overload so the
table applies uniformly (matches operator*=/-=).

division.cpp: the previously gated divide-by-zero block is un-gated and
extended -- NaN propagation, the full IEEE divide-by-zero table, infinity
operands, and signed-zero quotients, plus a throw-mode block guarded by
EREAL_THROW_ARITHMETIC_EXCEPTION.

Verified on gcc-13 and clang-18: division foundational + fuzz pass in both
default and throw modes; addition/subtraction/multiplication unchanged.

Resolves #968

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Coverage Stats

23 of 23 new or added lines in 1 file covered. (100.0%)

7 existing lines in 3 files now uncovered.

47314 of 56349 relevant lines covered (83.97%)

5359054.17 hits per line