;; As of SBCL 0.8, this is only supposed to work in x86 Linux, on which
;; system it's implemented using clone(2) and the %fs segment register.
;; Note that no consistent effort to audit the SBCL library code for
- ;; thread safety has been performed, so caveat executor
+ ;; thread safety has been performed, so caveat executor.
; :sb-thread
+
+ ;; Kernel support for futexes (so-called "fast userspace mutexes") is
+ ;; available in Linux 2.6 and some versions of 2.4 (Red Hat vendor
+ ;; kernels, possibly other vendors too). We can take advantage of
+ ;; these to do faster and probably more reliable mutex and condition
+ ;; variable support. An SBCL built with this feature will fall back
+ ;; to the old system if the futex() syscall is not available at
+ ;; runtime
+ ; :sb-futex
;; This affects the definition of a lot of things in bignum.lisp. It
;; doesn't seem to be documented anywhere what systems it might apply
;; to. It doesn't seem to be needed for X86 systems anyway.
; :32x16-divide
- ;; This is probably true for some processor types, but not X86. It
- ;; affects a lot of floating point code.
- ; :negative-zero-is-not-zero
-
;; This is set in classic CMU CL, and presumably there it means
;; that the floating point arithmetic implementation
;; conforms to IEEE's standard. Here it definitely means that the
;; in the *FEATURES* list
;;
+ ;; Any target feature which affects binary compatibility of fasl files
+ ;; needs to be recorded in *FEATURES-POTENTIALLY-AFFECTING-FASL-FORMAT*
+ ;; (elsewhere).
+
;; notes on the :NIL and :IGNORE features:
;;
;; #+NIL is used to comment out forms. Occasionally #+IGNORE is used