github.com/afumu/libc@v0.0.6/musl/src/math/i386/acos.s (about)

     1  # use acos(x) = atan2(fabs(sqrt((1-x)*(1+x))), x)
     2  
     3  .global acos
     4  .type acos,@function
     5  acos:
     6  	fldl 4(%esp)
     7  	fld %st(0)
     8  	fld1
     9  	fsub %st(0),%st(1)
    10  	fadd %st(2)
    11  	fmulp
    12  	fsqrt
    13  	fabs         # fix sign of zero (matters in downward rounding mode)
    14  	fxch %st(1)
    15  	fpatan
    16  	fstpl 4(%esp)
    17  	fldl 4(%esp)
    18  	ret