Messages in this thread | | | Subject | Re: RFC: changing precision control setting in initial FPU context | From | (Kevin Buhr) | Date | 03 Mar 2001 17:29:53 -0600 |
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"Adam J. Richter" <adam@yggdrasil.com> writes: > > IEEE-754 floating point is available under glibc-based systems, > including most current GNU/Linux distributions, by linking with -lieee. > Your example program produces the "9 10" result you wanted when linked > this way, even when compiled with -O2
No, you've got it backwards. The "9 10" result is the *wrong* result. IEEE 64-bit arithmetic should give the result "10 10". Also, I can't duplicate your outcome. I see no difference linking with "-lieee" versus linking without it, at least under glibc-2.1.3:
$ gcc -v Reading specs from /usr/lib/gcc-lib/i386-linux/2.95.2/specs gcc version 2.95.2 20000220 (Debian GNU/Linux) $ cat modified.c #include <stdio.h> #include <fpu_control.h> int main() { int a = 10; fpu_control_t foo; _FPU_GETCW(foo); printf("%04x %d %d\n", foo, (int)( a*.3 + a*.7), /* first expression */ (int)(10*.3 + 10*.7)); /* second expression */ return 0; } $ gcc modified.c && ./a.out 037f 9 10 $ gcc -O2 modified.c && ./a.out 037f 10 10 $ gcc modified.c -lieee && ./a.out 037f 9 10 $ gcc -O2 modified.c -lieee && ./a.out 037f 10 10 $
As you can see, linking with "ieee" has no effect on the control word setting or the results. Perhaps this has changed post-glibc 2.1.3? Looking at the 2.1.3 code, it appears that all "ieee" does is set a variable that's referenced in the math library innards. It has no effect on startup code right now.
> When not linked with "-lieee", Linux personality ELF > x86 binaries start with Precision Control set to 3, just because that > is how the x86 fninit instruction sets it.
Yes. I know. In fact, the "fninit" instruction is executed in the kernel's "init_fpu()" when the first FPU instruction is executed by the program. I just think the hardware default happens to be a bad default on a system where most floating-point software is GCC-compiled with GCC's 64-bit doubles (and its 64-bit clean but 80-bit dirty floating point optimizations), so I'm proposing adding an instruction to "init_fpu()" to change the default hardware control word.
> In general, I think most real uses of floating point are for "fast and > sloppy" purposes, and programs that want to use floating point and > care about exact reproducibility will link with "-lieee".
However, this doesn't seem to work. Nor does "-ffloat-store".
Kevin <buhr@stat.wisc.edu> - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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