Re: c++/6914: -O2 and -O give different results for the same valid FP code

["Michael Veksler" <VEKSLER@il.ibm.com>]

The following reply was made to PR c++/6914; it has been noted by GNATS.

From: "Michael Veksler" <VEKSLER@il.ibm.com>
To: Franz Sirl <Franz.Sirl-kernel@lauterbach.com>
Cc: tprince@computer.org, gcc-gnats@gcc.gnu.org, gcc-prs@gcc.gnu.org,
   gcc-bugs@gcc.gnu.org
Subject: Re: c++/6914: -O2 and -O give different results for the same valid FP code
Date: Tue, 4 Jun 2002 09:48:07 +0300

 Franz Sirl <Franz.Sirl-kernel@lauterbach.com> wrote:
 > At 17:49 03.06.2002, Michael Veksler wrote:
 > >If the decision will be to keep the current behavior, documentation
 > > should be updated. It should be more clear that -float-store is
 > > critical for IEEE
 > > conformance on targets like x86. Also, the bugs section should
 > > contain this as a known bug (and bug it is -- gcc does not
 > > conform to the IEEE standard on x86).
 >
 > Because GCC does not mess with the settings of the CPU, which is
 > certainly _not_ GCC's job. Checkout glibc's documentation and look
 > at stuff like fpu_control.h and fenv.h.
 
 I totaly agree that it is not GCC's job to fiddle with the settings of the 
 CPU.
 fpu_control.h took me by surprise (especially the _FPU_EXTENDED vs.
 _FPU_DOUBLE macros).
 
 Do you think this is a glibc bug?
 In "fpu_control.h" (at least 2.2-9) it is written:
 /* The fdlibm code requires strict IEEE double precision arithmetic,
    and no interrupts for exceptions, rounding to nearest.  */
 
 And then they go on and define
 #define _FPU_DEFAULT  0x037f
 
 Which effectively sets rounding to _FPU_EXTENDED (=0x300), instead of 
 double.
 This does seem like a glibc bug (documentation does not match reality).
 
 
 But, there are several open issues:
 1. This should be more clearly stated in the documentation of GCC.
    - In the bugs section (or non-bugs, actually).
      Recommend to use either -float-store, or the following code:
 #include <fpu_control.h>
 void set_rounding_to_double()
 {
 fpu_control_t fpu_bits;
 _FPU_GETCW(fpu_bits);
 fpu_bits &= ~_FPU_EXTENDED;
 fpu_bits |= _FPU_DOUBLE;
 _FPU_SETCW(fpu_bits);
 }
    - In gcc --help (on -float-store)
 2. The _FPU_* macros are not documented in the "man" pages, nor in the 
 info 
     (the only reference to them is in g77.info)
 
 I still get the feeling that GCC douesthe wrong thing here. Are there no
 opcodes for "divide and round to double", and for "compare as double" ?
 I think that GCC should minimize its dependency on the "correctness" of
 CPU settings. This of course, only if that does not change code size, and
 does not hurt performance.