On Sunday, 14 January 2018 at 12:35:43 UTC, rumbu wrote:
On Sunday, 14 January 2018 at 11:44:39 UTC, kdevel wrote:
On Sunday, 14 January 2018 at 07:20:26 UTC, rumbu wrote:
On Saturday, 13 January 2018 at 22:51:18 UTC, kdevel wrote:
[...]
Can you please tell me on your system what are the values for
real.sizeof and real.mant_dig?
They are 12 and 64 (-m32) and 16 and 64 (-m64).
This is curious. Because the only documented real type with a
64-bit mantissa is the 80-bit Intel Extended Precision format
with a unique sizeof of 10 (and not 12 or 16).
sizeof returns the in-memory size (at least in C and C++) which
is subject to alignment constraints. 32-bit mode typically
requires 4-byte aligned access (96 bit = 12 byte) while 64-bit
requires 8-byte alignment (128 bit = 16 bytes) for optimal
performance. Cf. e.g. p. 28 of "SYSTEM V APPLICATION BINARY
INTERFACE Intel386" (long double)
http://www.sco.com/developers/devspecs/abi386-4.pdf
and p. 12 of "System V Application Binary Interface AMD64
Architecture Processor Supplement"
https://www.uclibc.org/docs/psABI-x86_64.pdf
Actually, real80 is the only type I'm supporting in the
library, any other real type is converted to double. This will
explain some loss of precision.
Agreed.
Do you have more information about the real type on you system,
because on mine (Windows), real.sizeof is always 10,
irrespective of -m32 or -m64.
Mantissa will be easy to extract being 64-bit, but I don't know
the exact layout of your real type to extract the exponent.
Assumed that your machine has a X86_64 cpu real = long double is
the 80-bit extended precision format. I suppose you are running
windows and it may be that the Window ABI stores these reals
16-bit (2-byte) aligned. In this case there will be no padding.
But I have not found info on this topic.
