RE: negation of float on H8300

RE: negation of float on H8300

[Sanjiv Kumar Gupta, Noida]

>Not if it's a signalling NaN.  If the chip only supports QNaNs, then
>you might be able to get away with it.

>Flipping the sign bit is correct even for a signalling NaN.  See item 2 in
>the Appendix of IEEE Std 754-1985 (Binary Floating-Point Arithmetic).
>See also item 2 in the Appendix of IEEE Std 854-1987 (Radix-Independent
>Floating-Point Arithmetic).

>Jim

I'll try to write out a patch for this soon.

Regards
Sanjiv

Re: negation of float on H8300

[Jim Wilson]

>Not if it's a signalling NaN.  If the chip only supports QNaNs, then
>you might be able to get away with it.

Flipping the sign bit is correct even for a signalling NaN.  See item 2 in
the Appendix of IEEE Std 754-1985 (Binary Floating-Point Arithmetic).
See also item 2 in the Appendix of IEEE Std 854-1987 (Radix-Independent
Floating-Point Arithmetic).

Jim

Re: negation of float on H8300

[Alan Lehotsky]

At 9:19 AM +0200 8/8/02, Andreas Schwab wrote:

Re: negation of float on H8300

[Jeff Law]

In message < je3ctp7r7f.fsf@sykes.suse.de >, Andreas Schwab writes:
 >Alan Lehotsky <apl@alum.mit.edu> writes:
 >
 >|> At 12:48 PM +0530 8/7/02, Sanjiv Kumar Gupta, Noida wrote:
 >|> >Hi all,
 >|> >
 >|> >I see a libcall ___negsf2 is being used by gcc for negation of floats on
 >|> >H8300. Can't we just flip the first bit using bit insns provided in H8300
 >?
 >|> 
 >|> That wouldn't handle NaNs correctly (assuming the H8300 is IEEE FP
 >|> compliant...)
 >
 >But a negative NaN is still a NaN, isn't it?
Yes.  Bit twiddling to implement fneg has been recognized as IEEE FP compliant
for some time now.  I'm pretty sure we've got threads on this in our archives
somewhere.

In fact, the IEEE semantics for FP negation strongly encourage implementation
via bit twiddling.  You can't implement FP negation using subtraction and
get IEEE compliance.  Search the archives for a rather interesting discussion
on this topic and how it relates to the PA port ;-)

jeff

Re: negation of float on H8300

[Andreas Schwab]

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Alan Lehotsky <apl@alum.mit.edu> writes:

|> At 12:48 PM +0530 8/7/02, Sanjiv Kumar Gupta, Noida wrote:
|> >Hi all,
|> >
|> >I see a libcall ___negsf2 is being used by gcc for negation of floats on
|> >H8300. Can't we just flip the first bit using bit insns provided in H8300?
|> 
|> That wouldn't handle NaNs correctly (assuming the H8300 is IEEE FP
|> compliant...)

But a negative NaN is still a NaN, isn't it?

Andreas.

-- 
Andreas Schwab, SuSE Labs, schwab@suse.de
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Re: negation of float on H8300

[Alan Lehotsky]

At 12:48 PM +0530 8/7/02, Sanjiv Kumar Gupta, Noida wrote:

negation of float on H8300

[Sanjiv Kumar Gupta, Noida]

Hi all,

I see a libcall ___negsf2 is being used by gcc for negation of floats on
H8300. Can't we just flip the first bit using bit insns provided in H8300?

Regards
Sanjiv