On 30/03/2017 11:33, Thierry wrote:
Hi,
On Thu, Mar 30, 2017 at 10:20:21AM +0200, Marc Mezzarobba wrote:
Thierry wrote:
I do not remember where i read that (correct me if i am wrong, or
provide a reference if you know where it is), but RDF is supposed to
round towards the nearest floating-point number.
RR is, but I think RDF may or may not provide correct rounding depending
what the underlying libm does.
I found the reference i was looking for :
http://doc.sagemath.org/html/en/reference/rings_numerical/sage/rings/real_double.html#sage.rings.real_double.RealDoubleElement.ulp
So, if this depends on libm, i guess this should be updated and we should
provide a way to know/modify the current rounding mode ?
You can obtain and modify the CPU rounding mode using the C functions
declared in fenv.h
int FE_TONEAREST
int FE_UPWARD
int FE_DOWNWARD
int FE_TOWARDZERO
int fegetround ()
int fesetround (int)
(there is an example in src/sage/graphs/base/static_sparse_graph.pyx)
However, this go beyond libm and will affect any library using floating
point numbers.
Vincent
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