I just noticed the following part of sbcl-1.0.10/src/compiler/x86/c-call.lisp
;;; While SBCL uses the FPU in 53-bit mode, most C libraries assume that
;;; the FPU is in 64-bit mode. So we change the FPU mode to 64-bit with
;;; the SET-FPU-WORD-FOR-C VOP before calling out to C and set it back
;;; to 53-bit mode after coming back using the SET-FPU-WORD-FOR-LISP VOP.
I reckon this refers to the internal mantissa length of the FPU. Why doesn't
SBCL just use 64-bit mode, as C does? It would make foreign-funcall a little
faster. What are the disadvantages?