Re: ada/8606: GNAT floating point optimization bug

[Geert Bosch <bosch@gnat.com>]

The following reply was made to PR ada/8606; it has been noted by GNATS.

From: Geert Bosch <bosch@gnat.com>
To: gcc-prs@gcc.gnu.org, 166255@bugs.debian.org,
	kalmquist1@hotmail.com, kenner@gcc.gnu.org, gcc-bugs@gcc.gnu.org,
	gcc-gnats@gcc.gnu.org
Cc:  
Subject: Re: ada/8606: GNAT floating point optimization bug
Date: Tue, 13 May 2003 13:46:51 -0400

 Indeed the issue here is the standard one of double rounding for
 64-bit floating point types. There is nothing Ada-specific about
 this and these problems are identical in C. The -ffloat-store
 option may improve repeatability, but has two undesired effects:
 it degrades performance and it always causes double rounding,
 which can degrade accuracy.
 
 For numerical code that requires exact rounding, only use 32-bit
 or 80-bit floating point types. When you need an expression to
 be computed to exactly 32 bits without any excess precision,
 store it to a volatile variable.
 
 If you really need 64-bit types with exact rounding, you'll have
 to set the rounding mode accordingly, but this will need changes
 in the Ada run time to prevent it from using 80-bit types.
 
    -Geert