On Thu, 12 Jan 2012 13:38:44 -0800 (PST), Bill Woessner
<woess...@nospicedham.gmail.com> wrote:
>I'm a 100% total newbie at writing assembly. But I figured it would
>be a good exercise. And besides, this tiny chunk of code is
>definitely in the critical path of something I'm working on. Any and
>all advice would be appreciated.
>
>I'm trying to rewrite the following function in x86 assembly:
>
>inline double DiffAngle(double theta1, double theta2)
>{
> double delta(theta1 - theta2);
>
> return std::abs(delta) <= M_PI ? delta : delta - copysign(2 * M_PI,
>delta);
>}
>
>To my great surprise, I've actually been somewhat successful. Here's
>what I have so far:
>
>double DiffAngle(double theta1, double theta2)
>{
> asm(
> "fldl 4(%esp);"
> "fsubl 12(%esp);"
> "fxam;"
> "fnstsw %ax;"
> "fldl TWO_PI;"
> "testb $2, %ah;"
> "fldl NEG_TWO_PI;"
> "fcmovne %st(1), %st;"
> "fstp %st(1);"
> "fsubr %st(1), %st;"
> "fldpi;"
> "fld %st(2);"
> "fabs;"
> "fcomip %st(1), %st;"
> "fstp %st(0);"
> "fcmovbe %st(1), %st;"
> "fstp %st(1);"
> "rep;"
> "ret;"
> "NEG_TWO_PI:;"
> ".long 1413754136;"
> ".long 1075388923;"
> "TWO_PI:;"
> ".long 1413754136;"
> ".long -1072094725;"
> );
>}
>
>This compiles, runs and produces the correct answers. But I have a
>few issues with it:
>
>1) If I declare this function inline, it gives me garbage (like
>10^-304)
>2) If I compile with -Wall, I get a warning that the function doesn't
>return a value, which is absolutely true, but I don't know how to fix
>it.
>3) I don't like how TWO_PI and NEG_TWO_PI are defined. I had to steal
>it from some generated assembly. It would be nice to use M_PI,
>4*atan(1) or something like that.
>
Can't help with your compiler interface questions, but if
you are going to stick with the FPU, you can use FLDPI to
load the value of pi.
Best regards,
Bob Masta
DAQARTA v6.02
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
Frequency Counter, FREE Signal Generator
Pitch Track, Pitch-to-MIDI
Science with your sound card!
_______________________________________________
help-gplusplus mailing list
help-gplusplus@gnu.org
https://lists.gnu.org/mailman/listinfo/help-gplusplus
