CODEWITHSUNDEEP
assemblyx86-64intelprecision
formateu
formateu

Reputation: 167

Intel x86_64 assembly, How to move between x87 and SSE2? (calculating arctangent of double)

I try to calculate arctangent of double precision float value, which I hold in xmm register. With normal float pointing it is possible to use old x87 instruction FPATAN, but how can I do this with double ?

Upvotes: 2

Views: 1316

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 365277

You can still copy data from xmm to x87 to use instructions like fpatan, but usually you should call a math library function. (fpatan is so slow that replacing it with many simple instructions is still good.) Wikipedia suggests looking at Netlib for a freely redistributable C implementation. (Obviously the easiest way is to just call the function in libm on whatever system you're using.)

If you are going to do it, don't use static storage for the memory you bounce through; use a temporary on the stack.

Also note that fpatan takes 2 inputs, because it implements the atan2 library function, giving a result in the appropriate quadrant depending on the sign of both inputs.

; assuming you did  sub  rsp, 24   or something earlier in your function

movsd   [rsp], xmm1
fld     qword [rsp]   ; st0 = xmm1
movsd   [rsp], xmm0
fld     qword [rsp]   ; st0 = xmm0,  st1 = xmm1

fpatan                ; st0 = arctan(xmm1/xmm0)

fstp    qword [rsp]     ; x87 stack is empty again
movsd   xmm0, [rsp]   ; xmm0 = arctan(xmm1/xmm0)

; and then   add rsp, 24   at some point before returning

Upvotes: 5

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