Re: Help with Hand-Optimized Assembly

[Bob Masta]

On Thu, 12 Jan 2012 13:38:44 -0800 (PST), Bill Woessner
<woessner@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
 
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