David Kastrup wrote
> ArnoldTheresius <
> Arnold.Wendl@
> > writes:
>
> ...
> None of the following GCC options would help?
> ...
> -ffloat-store
> ...
> -fexcess-precision=style
> ...
> -ffast-math
> ...
> --
> David Kastrup
Only -ffloat-store did show the desired effect in a C program with typical
situations.
I did not check speed effect.
>float_opts_test-0.exe
Compilation configuration was default.
as compiled:
Var - (1.0 / 7.0) = -7.928228386e-018
Var - OneSeventh() = -7.928228386e-018
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = 6.938893904e-017
i.e. 0 step(s) of IEEE-double difference
after X87 config change:
Var - (1.0 / 7.0) = 0
Var - OneSeventh() = 0
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = -4.440892099e-015
i.e. -20 step(s) of IEEE-double difference
>float_opts_test-1.exe
Compilation configuration: -fexcess-precision=fast
as compiled:
Var - (1.0 / 7.0) = -7.928228386e-018
Var - OneSeventh() = -7.928228386e-018
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = 6.938893904e-017
i.e. 0 step(s) of IEEE-double difference
after X87 config change:
...
>float_opts_test-2.exe
Compilation configuration: -fexcess-precision=standard
as compiled:
Var - (1.0 / 7.0) = -7.928228386e-018
Var - OneSeventh() = 0
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = -4.440892099e-015
i.e. -20 step(s) of IEEE-double difference
after X87 config change:
...
>float_opts_test-3.exe
Compilation configuration: -ffast-math
as compiled:
Var - (1.0 / 7.0) = -7.928228386e-018
Var - OneSeventh() = -7.928228386e-018
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = 6.938893904e-017
i.e. 0 step(s) of IEEE-double difference
after X87 config change:
...
>float_opts_test-4.exe
Compilation configuration: -ffloat-store
as compiled:
Var - (1.0 / 7.0) = 0
Var - OneSeventh() = 0
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = -4.440892099e-015
i.e. -20 step(s) of IEEE-double difference
after X87 config change:
...
>float_opts_test-5.exe
Compilation configuration: -ffloat-store -O3
as compiled:
Var - (1.0 / 7.0) = 0
Var - OneSeventh() = 0
X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;
X - Y = -4.440892099e-015
i.e. -20 step(s) of IEEE-double difference
after X87 config change:
...
Source code:
/* FILE: float_opts_test.c
* compile for windows with several options
* with MINGW installed in LILYDEV:
*
* #!/bin/sh
* CNT=1
* CC=i686-w64-mingw32-gcc
* echo $CC $opt -ofloat_opts_test-0.exe float_opts_test.c
* $CC $opt -ofloat_opts_test-0.exe float_opts_test.c
*
* for opt in "-fexcess-precision=fast" "-fexcess-precision=standard"
"-ffast-math" \
* "-ffloat-store" "-ffloat-store -O3"
* do
* echo $CC $opt -DCOMPCONFIG='"'"$opt"'"' -ofloat_opts_test-$CNT.exe
float_opts_test.c
* $CC $opt -DCOMPCONFIG='"'"$opt"'"' -ofloat_opts_test-$CNT.exe
float_opts_test.c
* CNT=`expr $CNT + 1`
* done
*
*/
#include <stdio.h>
void X87mantissa53(void)
{
asm(
" push %ebp"
"\n mov $0x027f, %eax"
"\n push %eax"
"\n mov %esp, %ebp"
"\n fldcw (%ebp)"
"\n pop %eax"
"\n pop %ebp"
);
}
volatile double MyFraction;
volatile double X;
double OneSeventh(void)
{ double One = 1.0;
double Seven = 7.0;
return (One / Seven);
}
void TestSuite(void)
{
union {
double d;
long long int i;
} u1, u2;
double One = 1.0;
double Seven = 7.0;
// first typical situation: compare stored value with recalculated value
MyFraction = One / Seven;
printf(" Var - (1.0 / 7.0) = %.10lg\n", MyFraction - (One / Seven));
// second typical situation: compare stored value with recalculated
function return value
MyFraction = OneSeventh();
printf(" Var - OneSeventh() = %.10lg\n", MyFraction - OneSeventh());
{ // Don't use loops controlled by inexact floating point increment!
int cnt;
register double y;
X = ((double) 19999.0) / ((double) 10000.0);
y = 100.0;
MyFraction = X / y;
y = 0.0;
for (cnt = 0; cnt < 100; cnt++) y += MyFraction;
MyFraction = y;
printf(" X = 1.9999; z = X / 100.0; Y = 0.0; 100 times Y += z;\n
X - Y = %.10lg\n", X - y);
u1.d = X; u2.d = MyFraction;
printf(" i.e. %ld step(s) of IEEE-double difference\n", (long)
(u1.i - u2.i));
}
}
int main(int argc, char *argv[])
{
#ifdef COMPCONFIG
printf("Compilation configuration: %s\n", COMPCONFIG);
#else
printf("Compilation configuration was default.\n");
#endif
printf(" as compiled:\n");
TestSuite();
X87mantissa53();
printf(" after X87 config change:\n");
TestSuite();
return 0;
}
ArnoldTheresius
--
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