[PPL-devel] Troubles with rounding

[Abramo Bagnara]

Abramo Bagnara ha scritto:
> $ g++ -O2 -frounding-math bug.cc
> $ a.out
> 123456792 123456792
> 123456792 123456792
> 123456792 123456792
> 123456784 123456784
> $ g++ -O2 -fno-inline -frounding-math bug.cc
> $ a.out
> 123456784 123456792
> 123456784 123456792
> 123456784 123456792
> 123456784 123456792
> 
> Someone knows if this is the expected behaviour?
> 
> GCC bugs apart we definitely need a way to avoid this mess, someone
> knows a way?

The attached file show the trick I've found to solve that.

The magic asm force gcc to store in memory the variable and induce gcc
to believe that something unknown happens to the content then avoiding
common subexpressions elimination (CSE).

arg1 = unknown(arg1);
...
argn = unknown(argn);
to = func(arg1, ..., argn);
to = unknown(argn);

Please, please don't ask me why it's needed also to have

avoid_cse(to);

as I've no idea.

I've verified empirically that not only it's needed (although only in
int and long long case), but contributes in all cases to generate better
code too (?!?).

Unfortunately this idea is original and I've been unable to find similar
solutions and the exact reason why it works so well.

The sweet thing is that as far as I've read other libraries disables
optimization locally to avoid the problem ;-)

However take in account that rounding direction change is almost always
avoided using sign inversion trick. I've discovered the trouble
converting from big integer to float, where the unconditional sign
inversion cannot be done because the possible overflow.

Here:

$ gcc -v
Using built-in specs.
Target: i386-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-libgcj-multifile
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada
--enable-java-awt=gtk --disable-dssi
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre
--with-cpu=generic --host=i386-redhat-linux
Thread model: posix
gcc version 4.1.1 20060525 (Red Hat 4.1.1-1)

the generated code for the file attached is optimal, I like very much
you make further tests with other compilers and other architectures.

-- 
Abramo Bagnara                       mailto:abramobagnara at tin.it

Opera Unica                          Phone: +39.0546.656023
Via Emilia Interna, 140
48014 Castel Bolognese (RA) - Italy

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