------- Comment #2 from joseph at codesourcery dot com 2009-07-26 17:32 -------
Subject: Re: complex folding inexact
The example in this bug deals with excess overflow for division. For
infinities computing as NaN + iNaN, an example is (NaN + iInf) * (NaN
+iInf) (where NaN +iInf is obtained as (__builtin_inf () * (0.0 + 1.0i)))
- this works if computed at runtime but not constant folded. ("Works"
here means that at least one part of the result is an infinity - I do not
believe there is a specific requirement beyond that.)
For division producing NaN + iNaN, (1.0 + 0.0i) / (0.0 + 0.0i) should
produce an infinity, (__builtin_inf () * (0.0 + 1.0i)) / (1.0 + 0.0i)
should produce an infinity, (1.0 + 0.0i) / (__builtin_inf () * (0.0 +
1.0i)) should produce a zero (in which each part may have either sign).
All the above can usefully be tested for both constant folding and runtime
evaluation.
There are also cases for exact rounding where you'd expect MPC to produce
the right results but would *not* expect operations executed at runtime to
produce exactly those results. For example, (1.0 + DBL_EPSILON + 1.0i) *
(1.0 - DBL_EPSILON + 1.0i), which would only work at runtime if the code
happens to use exactly the right fused multiply-add operation.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30789
