[Bug middle-end/26374] Compile failure on long double

[Bug middle-end/26374] Compile failure on long double

[jackie.rosen at hushmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

Jackie Rosen <jackie.rosen at hushmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |jackie.rosen at hushmail dot com

--- Comment #13 from Jackie Rosen <jackie.rosen at hushmail dot com> ---
*** Bug 260998 has been marked as a duplicate of this bug. ***
Seen from the domain http://volichat.com
Page where seen: http://volichat.com/adult-chat-rooms
Marked for reference. Resolved as fixed @bugzilla.

[Bug middle-end/26374] Compile failure on long double

[beebe at math dot utah.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #14 from beebe at math dot utah.edu ---
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

That is a really old one, from early 2006; I would have hoped that it
had long since been repaired in gcc.

However, I just checked my VM for CentOS 7 on PowerPC, and found this:

        % cat foo.c
        static const double x = 1.0L / 42 ;

        % /usr/bin/gcc -c foo.c
        foo.c:1:1: error: initializer element is not computable at load time
         static const double x = 1.0L / 42 ;

That is gcc-4.8.5 (20150623 (Red Hat 4.8.5-44)).

I recently built gcc-8.5.0 on that system, and it raises the same
error.

Changing 42 to 42.0L to have a pure long-double expression does not
eliminate the error with either compiler.

The assignment is perfectly legal in all ISO Standards for C from 1989
to date, and the expression must be evaluated to IEEE 754 requirements
by the compiler, producing a correctly rounded result.  That latter
would not be possible if it were expressed as a fractional value,
because the C Standards do not require correct conversion of decimal
fractions to binary fractions.

The build of gcc-8.5.0 took five days on this qemu-emulated CPU, so it
really is not practical for me to try the test file with gcc-9 through
gcc-12 versions.  I needed 8.5.0 to be able to build TeX Live 2022:
see

        http://www.math.utah.edu/pub/texlive-utah/#centos-7-ppc64be

for my report (and many others elsewhere in that Web page).

-------------------------------------------------------------------------------
- Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
- University of Utah                    FAX: +1 801 581 4148                  -
- Department of Mathematics, 110 LCB    Internet e-mail: beebe@math.utah.edu  -
- 155 S 1400 E RM 233                       beebe@acm.org  beebe@computer.org -
- Salt Lake City, UT 84112-0090, USA    URL: http://www.math.utah.edu/~beebe/ -
-------------------------------------------------------------------------------

[Bug middle-end/26374] Compile failure on long double

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #15 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to beebe from comment #14)
> >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374
> 
> That is a really old one, from early 2006; I would have hoped that it
> had long since been repaired in gcc.
> 
> However, I just checked my VM for CentOS 7 on PowerPC, and found this:

Well powerpc currently uses what is called double double for 128bit long
double. GCC does not know how to constant fold that nicely. But powerpc linux
targets are moving over to use standard 128bit IEEE long double and IIRC for
GCC 12 (or is it 13) will default to IEEE long double so this will be less of
an issue.

[Bug middle-end/26374] Compile failure on long double

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #16 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #15)
> (In reply to beebe from comment #14)
> > >> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374
> > 
> > That is a really old one, from early 2006; I would have hoped that it
> > had long since been repaired in gcc.
> > 
> > However, I just checked my VM for CentOS 7 on PowerPC, and found this:
> 
> Well powerpc currently uses what is called double double for 128bit long
> double. GCC does not know how to constant fold that nicely. But powerpc
> linux targets are moving over to use standard 128bit IEEE long double and
> IIRC for GCC 12 (or is it 13) will default to IEEE long double so this will
> be less of an issue.

Will not default to unless configured so, but e.g. in Fedora 36 GCC 12 is
configured to default to long double being IEEE quad on powerpc64 little
endian.
As for constant folding, even with double double gcc is able to fold some
constant arithmetics in that format, but because the emulation is only
approximate (it pretends it is 106-bit precision format while in reality it is
variable precision up to some thousands depending on the exact values).
As has been said elsewhere, the emulation would be implementable if gcc handled
double double in all the arithmetics as a pair of doubles with all the rules
for it.  But e.g. mpfr/libmpc isn't able to do something like that, so we'd
need to compute everything with much bigger precision etc.
In any case, I think nobody is going to work on it now that ppc64le is
switching to IEEE quad.

[Bug middle-end/26374] Compile failure on long double

[beebe at math dot utah.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #17 from beebe at math dot utah.edu ---
>> ... powerpc currently uses what is called double double for 128bit
>> long double. GCC does not know how to constant fold that nicely.

Yes, I know about the double double format that IBM, SGI, and possibly
others introduced in the 1990s for implementing long double.  I
strongly disapprove of that decision, and wrote about it extensively
in my big book

        The Mathematical-Function Computation Handbook
        https://doi.org/10.1007/978-3-319-64110-2
        http://www.math.utah.edu/pub/tex/bib/master.html#Beebe:2017:MFC

that addresses accurate computation in binary and decimal arithmetic,
for IEEE 754, and also for some important historical floating-point
designs.

>> GCC does not know how to constant fold that nicely.

Because gcc has used gmp for several years now for correctly rounded
conversions of decimal numbers to binary IEEE 754 formats, I would
have thought that handling a constant expression like "1.0L / 42.0L"
would have been relatively trivial.

>> powerpc linux targets are moving over to use standard 128bit IEEE
>> long double and IIRC for GCC 12 (or is it 13) will default to IEEE
>> long double so this will be less of an issue.

Thanks for that interesting news!  It is a big change, because it
affects both -lc and -lm libraries, plus lots of user code.  

The IBM POWER architecture was introduced to the world in February
1990, and we here in Utah had some of the earliest models available,
so now that we are 2**5 years later, it is high time to conform to
IEEE 754.

POWER made a big impact for those who learned how to exploit its
interesting features.  I wrote about it here:

        High-Performance Matrix Multiplication
        http://www.math.utah.edu/pub/benchmarks/usirep.pdf

That document showed there that a 44x performance boost was possible
in some cases, without leaving standard programming languages for
assembly code fragments.

I wrote a lot more about the importance of understanding architecture
and memory designs it here:

        The Impact of Memory and Architecture on Computer Performance
        http://www.math.utah.edu/~beebe/memperf.pdf

-------------------------------------------------------------------------------
- Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
- University of Utah                    FAX: +1 801 581 4148                  -
- Department of Mathematics, 110 LCB    Internet e-mail: beebe@math.utah.edu  -
- 155 S 1400 E RM 233                       beebe@acm.org  beebe@computer.org -
- Salt Lake City, UT 84112-0090, USA    URL: http://www.math.utah.edu/~beebe/ -
-------------------------------------------------------------------------------

[Bug middle-end/26374] Compile failure on long double

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #18 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Note, for the time being only little endian powerpc64 can switch (I think only
power9 and later have hw support for IEEE quad and in the ABI it is passed in
Altivec/VSX registers.  ppc64le only supports power8 and later CPUs.

[Bug middle-end/26374] Compile failure on long double

[beebe at math dot utah.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #19 from beebe at math dot utah.edu ---
Thanks for the further comments on PowerPC, double double as fake long
double, and the news of gcc-12 on Fedora 36 for that CPU.  I'll soon
build a VM to get access to that combination.  I have VMs already for
PowerPC on CentOS 7 (big endian), CentOS 8 (little endian), and Debian
10 (little endian).

Let's consider the initializer bug an "unfixed (mis)feature" of gcc
history, and move on to better arithmetic!

-------------------------------------------------------------------------------
- Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
- University of Utah                    FAX: +1 801 581 4148                  -
- Department of Mathematics, 110 LCB    Internet e-mail: beebe@math.utah.edu  -
- 155 S 1400 E RM 233                       beebe@acm.org  beebe@computer.org -
- Salt Lake City, UT 84112-0090, USA    URL: http://www.math.utah.edu/~beebe/ -
-------------------------------------------------------------------------------

[Bug middle-end/26374] Compile failure on long double

[dje at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #20 from David Edelsohn <dje at gcc dot gnu.org> ---
Double-double has advantages and disadvantages. You're welcome to debate
William Kahan about the choice instead of making snarky comments here.

[Bug middle-end/26374] Compile failure on long double

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #21 from Vincent Lefèvre <vincent-gcc at vinc17 dot net> ---
(In reply to Jakub Jelinek from comment #16)
> As for constant folding, even with double double gcc is able to fold some
> constant arithmetics in that format, but because the emulation is only
> approximate (it pretends it is 106-bit precision format while in reality it
> is variable precision up to some thousands depending on the exact values).
> As has been said elsewhere, the emulation would be implementable if gcc
> handled double double in all the arithmetics as a pair of doubles with all
> the rules for it.  But e.g. mpfr/libmpc isn't able to do something like
> that, so we'd need to compute everything with much bigger precision etc.

Well, the C standard does not require correct rounding, and while correct
rounding is important for the IEEE formats, it is rather useless for the
double-double format, whose goal was just to provide more precision than
double, but still be rather fast (compared to quad emulation). The main
drawback would be that results could be different whether a FP expression is
evaluated at run time or at compile time, but unless users seek to control
everything (e.g. with IEEE formats), they should get use to that (FYI, you can
have the same kind of issues with the contraction of FP expressions, such as
FMA generation from mul-add, which GCC enable by default). So, in short, doing
the compile-time evaluation at a 106-bit precision or more would be acceptable
IMHO, at least better than a compiler error.

Note: Even though double-double can be very interesting as a compromise between
performance and accuracy, there exist various algorithms and which algorithm
should be chosen depends on the context, which only the author of the program
can know in general. Thus it was a bad idea to implement double-double as a
native FP type (here, long double); instead, the selection of the algorithms
should be left to the developer. So the switch to IEEE quad is a good thing.
But for how long will old ABIs be around?

[Bug middle-end/26374] Compile failure on long double

[beebe at math dot utah.edu]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

--- Comment #22 from beebe at math dot utah.edu ---
Yesterday, I got a Fedora 36 PPC64LE VM up, and left it installing
hundreds of packages overnight.  QEMU 4.2.1 on Ubuntu 20.04 picks a
default CPU type of POWER9.  Alas, the ISO installation gets nicely
into a normal boot, then dies with a kernel panic.  I changed the
default CPU type to POWER8, and that led to a successful installation.

This morning, I built my feature test package

        http://www.math.utah.edu/pub/features/

and got this nice report back:

        LDBL_DECIMAL_DIG = 36
        LDBL_DIG         = 33
        LDBL_EPSILON     = 0x1p-112
        LDBL_HAS_SUBNORM = 1
        LDBL_MANT_DIG    = 113
        LDBL_MAX         = 0x1.ffffffffffffffffffffffffffffp+16383
        LDBL_MAX_10_EXP  = 4932
        LDBL_MAX_EXP     = 16384
        LDBL_MIN         = 0x1p-16382
        LDBL_MIN_10_EXP  = -4931
        LDBL_MIN_EXP     = -16381
        LDBL_NORM_MAX    = [not defined]
        LDBL_TRUE_MIN    = 0x0.0000000000000000000000000001p-16382
        HUGE_VALL        = inf
        ...

        Computed long double limits:
                smallest floating-point number: 0.00000000e+00 == 2^(-16494)   
[IEEE 754 smallest 128-bit subnormal]
                machine epsilon:                1.92592994e-34 == 2^(-112)     
[IEEE 754 128-bit conformant]
                long double appears to be 128-bit value stored in 128-bit
(16-byte) field

That indicates that long double implemented as doubled double is now
history on this platform, and that is good news for me.

Thanks for telling me about this change for Fedora on PowePC.



-------------------------------------------------------------------------------
- Nelson H. F. Beebe                    Tel: +1 801 581 5254                  -
- University of Utah                    FAX: +1 801 581 4148                  -
- Department of Mathematics, 110 LCB    Internet e-mail: beebe@math.utah.edu  -
- 155 S 1400 E RM 233                       beebe@acm.org  beebe@computer.org -
- Salt Lake City, UT 84112-0090, USA    URL: http://www.math.utah.edu/~beebe/ -
-------------------------------------------------------------------------------

[Bug middle-end/26374] Compile failure on long double

[pinskia at gcc dot gnu dot org]

------- Comment #12 from pinskia at gcc dot gnu dot org  2007-03-22 23:49 -------
*** Bug 31321 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |beebe at math dot utah dot
                   |                            |edu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

[Bug middle-end/26374] Compile failure on long double

[pinskia at gcc dot gnu dot org]

------- Comment #11 from pinskia at gcc dot gnu dot org  2006-12-28 14:48 -------
(In reply to comment #10)
> Any progress on this?
There are two way of fixing this as far as I can see:
teach real.c about how to fold IBM 128bit long double format
use MPFR instead

I would use the latter if I got any time but I don't have any time to do
either.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

[Bug middle-end/26374] Compile failure on long double

[dwmw2 at infradead dot org]

------- Comment #10 from dwmw2 at infradead dot org  2006-12-28 14:32 -------
Any progress on this?


-- 

dwmw2 at infradead dot org changed:

           What    |Removed                     |Added
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                 CC|                            |dwmw2 at infradead dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

[Bug middle-end/26374] Compile failure on long double

[pinskia at gcc dot gnu dot org]

------- Comment #9 from pinskia at gcc dot gnu dot org  2006-04-06 16:13 -------
*** Bug 27054 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |jakub at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

[Bug middle-end/26374] Compile failure on long double

[pinskia at gcc dot gnu dot org]

------- Comment #8 from pinskia at gcc dot gnu dot org  2006-02-24 21:04 -------
*** Bug 26462 has been marked as a duplicate of this bug. ***


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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                 CC|                            |pluto at agmk dot net


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374

[Bug middle-end/26374] Compile failure on long double

[pinskia at gcc dot gnu dot org]

------- Comment #7 from pinskia at gcc dot gnu dot org  2006-02-20 19:33 -------
Confirmed.


-- 

pinskia at gcc dot gnu dot org changed:

           What    |Removed                     |Added
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             Status|UNCONFIRMED                 |NEW
          Component|target                      |middle-end
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-02-20 19:33:44
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26374