[fpc-devel] sse2 registers

[Vincent Snijders]

Vincent Snijders schreef:
> I am trying out the sse2 support in fpc 2.1.1 and compile my code with 
> -Cfsse2.
> 
> The program is the mandelbrot benchmark:
> http://shootout.alioth.debian.org/gp4/benchmark.php?test=mandelbrot&lang=fpascal 
> 
> 
> The innerloop is like this:
>       Zr := 0;  Zi := 0; Tr := 0; Ti := 0;
>       i := 0;
>       while (i<50) and (Tr + Ti< limit) do begin
>         inc(i);
>         Zi := 2*Zr*Zi + Ci;
>         Zr := Tr - Ti + Cr;
>         Ti := Zi * Zi;
>         Tr := Zr * Zr;
>       end;
> 
> If I look at the generated assembler, I noticed that only %xmm0 is used 
> and all each time the variable needs to be retrieved from the stack.
> 
> # Var Zr located in register mreg0md
> # Var Zi located in register mreg0md
> # Var Ti located in register mreg0md
> # Var Tr located in register mreg0md
> # Var Cr located in register mreg0md
> # Var Ci located in register mreg0md
> 
> ...
> 
> # [33] Zr := Tr - Ti + Cr;
>     movsd    -88(%ebp),%xmm0
>     subsd    -80(%ebp),%xmm0
>     addsd    -96(%ebp),%xmm0
>     movsd    %xmm0,-112(%ebp)
> # [34] Ti := Zi * Zi;
>     movsd    -72(%ebp),%xmm0
>     mulsd    -72(%ebp),%xmm0
>     movsd    %xmm0,-80(%ebp)
> 
> I thought there were more than one xmm registers. Why aren't they used?
> 

They are not used, because the same body contains a procedure call and the %xmm0 
registers are volatile. See also the notes with issue 7533:
http://www.freepascal.org/mantis/view.php?id=7533

Vincent