-;;;
+;;;; a bunch of handy macros for the PPC
-(in-package "SB!VM")
+;;;; This software is part of the SBCL system. See the README file for
+;;;; more information.
+;;;;
+;;;; This software is derived from the CMU CL system, which was
+;;;; written at Carnegie Mellon University and released into the
+;;;; public domain. The software is in the public domain and is
+;;;; provided with absolutely no warranty. See the COPYING and CREDITS
+;;;; files for more information.
+(in-package "SB!VM")
\f
;;; Instruction-like macros.
(defmacro move (dst src)
"Move SRC into DST unless they are location=."
(once-only ((n-dst dst)
- (n-src src))
+ (n-src src))
`(unless (location= ,n-dst ,n-src)
(inst mr ,n-dst ,n-src))))
(macrolet
- ((frob (op inst shift)
+ ((def (op inst shift)
`(defmacro ,op (object base &optional (offset 0) (lowtag 0))
- `(inst ,',inst ,object ,base (- (ash ,offset ,,shift) ,lowtag)))))
- (frob loadw lwz word-shift)
- (frob storew stw word-shift))
+ `(inst ,',inst ,object ,base (- (ash ,offset ,,shift) ,lowtag)))))
+ (def loadw lwz word-shift)
+ (def storew stw word-shift))
(defmacro load-symbol (reg symbol)
`(inst addi ,reg null-tn (static-symbol-offset ,symbol)))
(macrolet
((frob (slot)
(let ((loader (intern (concatenate 'simple-string
- "LOAD-SYMBOL-"
- (string slot))))
- (storer (intern (concatenate 'simple-string
- "STORE-SYMBOL-"
- (string slot))))
- (offset (intern (concatenate 'simple-string
- "SYMBOL-"
- (string slot)
- "-SLOT")
- (find-package "SB!VM"))))
- `(progn
- (defmacro ,loader (reg symbol)
- `(inst lwz ,reg null-tn
- (+ (static-symbol-offset ',symbol)
- (ash ,',offset word-shift)
- (- other-pointer-lowtag))))
- (defmacro ,storer (reg symbol)
- `(inst stw ,reg null-tn
- (+ (static-symbol-offset ',symbol)
- (ash ,',offset word-shift)
- (- other-pointer-lowtag))))))))
+ "LOAD-SYMBOL-"
+ (string slot))))
+ (storer (intern (concatenate 'simple-string
+ "STORE-SYMBOL-"
+ (string slot))))
+ (offset (intern (concatenate 'simple-string
+ "SYMBOL-"
+ (string slot)
+ "-SLOT")
+ (find-package "SB!VM"))))
+ `(progn
+ (defmacro ,loader (reg symbol)
+ `(inst lwz ,reg null-tn
+ (+ (static-symbol-offset ',symbol)
+ (ash ,',offset word-shift)
+ (- other-pointer-lowtag))))
+ (defmacro ,storer (reg symbol)
+ `(inst stw ,reg null-tn
+ (+ (static-symbol-offset ',symbol)
+ (ash ,',offset word-shift)
+ (- other-pointer-lowtag))))))))
(frob value)
(frob function))
"Loads the type bits of a pointer into target independent of
byte-ordering issues."
(once-only ((n-target target)
- (n-source source)
- (n-offset offset))
+ (n-source source)
+ (n-offset offset))
(ecase *backend-byte-order*
(:little-endian
`(inst lbz ,n-target ,n-source ,n-offset))
(:big-endian
- `(inst lbz ,n-target ,n-source (+ ,n-offset 3))))))
+ `(inst lbz ,n-target ,n-source (+ ,n-offset (1- n-word-bytes)))))))
;;; Macros to handle the fact that we cannot use the machine native call and
-;;; return instructions.
+;;; return instructions.
(defmacro lisp-jump (function lip)
"Jump to the lisp function FUNCTION. LIP is an interior-reg temporary."
`(progn
;; something is deeply bogus. look at this
- ;; (loadw ,lip ,function sb!vm:function-code-offset sb!vm:function-pointer-type)
- (inst addi ,lip ,function (- (* n-word-bytes sb!vm:simple-fun-code-offset) sb!vm:fun-pointer-lowtag))
+ ;; (loadw ,lip ,function function-code-offset function-pointer-type)
+ (inst addi ,lip ,function (- (* n-word-bytes simple-fun-code-offset) fun-pointer-lowtag))
(inst mtctr ,lip)
(move code-tn ,function)
(inst bctr)))
\f
;;;; Stack TN's
-;;; Load-Stack-TN, Store-Stack-TN -- Interface
-;;;
-;;; Move a stack TN to a register and vice-versa.
-;;;
+;;; Move a stack TN to a register and vice-versa.
(defmacro load-stack-tn (reg stack)
`(let ((reg ,reg)
- (stack ,stack))
+ (stack ,stack))
(let ((offset (tn-offset stack)))
(sc-case stack
- ((control-stack)
- (loadw reg cfp-tn offset))))))
-
+ ((control-stack)
+ (loadw reg cfp-tn offset))))))
(defmacro store-stack-tn (stack reg)
`(let ((stack ,stack)
- (reg ,reg))
+ (reg ,reg))
(let ((offset (tn-offset stack)))
(sc-case stack
- ((control-stack)
- (storew reg cfp-tn offset))))))
-
+ ((control-stack)
+ (storew reg cfp-tn offset))))))
-;;; MAYBE-LOAD-STACK-TN -- Interface
-;;;
(defmacro maybe-load-stack-tn (reg reg-or-stack)
"Move the TN Reg-Or-Stack into Reg if it isn't already there."
(once-only ((n-reg reg)
- (n-stack reg-or-stack))
+ (n-stack reg-or-stack))
`(sc-case ,n-reg
((any-reg descriptor-reg)
- (sc-case ,n-stack
- ((any-reg descriptor-reg)
- (move ,n-reg ,n-stack))
- ((control-stack)
- (loadw ,n-reg cfp-tn (tn-offset ,n-stack))))))))
+ (sc-case ,n-stack
+ ((any-reg descriptor-reg)
+ (move ,n-reg ,n-stack))
+ ((control-stack)
+ (loadw ,n-reg cfp-tn (tn-offset ,n-stack))))))))
\f
;;;; Storage allocation:
-(defmacro with-fixed-allocation ((result-tn flag-tn temp-tn type-code size)
- &body body)
+;;; This is the main mechanism for allocating memory in the lisp heap.
+;;;
+;;; The allocated space is stored in RESULT-TN with the lowtag LOWTAG
+;;; applied. The amount of space to be allocated is SIZE bytes (which
+;;; must be a multiple of the lisp object size).
+;;;
+;;; On other platforms (Non-PPC), if STACK-P is given, then allocation
+;;; occurs on the control stack (for dynamic-extent). In this case,
+;;; you MUST also specify NODE, so that the appropriate compiler
+;;; policy can be used, and TEMP-TN, which is needed for work-space.
+;;; TEMP-TN MUST be a non-descriptor reg. FIXME: This is not yet
+;;; implemented on PPC. We should implement this and replace the
+;;; inline stack-based allocation that presently occurs in the
+;;; VOPs. The stack-p argument is ignored on PPC.
+;;;
+;;; If generational GC is enabled, you MUST supply a value for TEMP-TN
+;;; because a temp register is needed to do inline allocation.
+;;; TEMP-TN, in this case, can be any register, since it holds a
+;;; double-word aligned address (essentially a fixnum).
+(defmacro allocation (result-tn size lowtag &key stack-p node temp-tn flag-tn)
+ ;; We assume we're in a pseudo-atomic so the pseudo-atomic bit is
+ ;; set. If the lowtag also has a 1 bit in the same position, we're all
+ ;; set. Otherwise, we need to zap out the lowtag from alloc-tn, and
+ ;; then or in the lowtag.
+ ;; Normal allocation to the heap.
+ (declare (ignore stack-p node)
+ #!-gencgc
+ (ignore temp-tn flag-tn))
+ #!-gencgc
+ (let ((alloc-size (gensym)))
+ `(let ((,alloc-size ,size))
+ (if (logbitp (1- n-lowtag-bits) ,lowtag)
+ (progn
+ (inst ori ,result-tn alloc-tn ,lowtag))
+ (progn
+ (inst clrrwi ,result-tn alloc-tn n-lowtag-bits)
+ (inst ori ,result-tn ,result-tn ,lowtag)))
+ (if (numberp ,alloc-size)
+ (inst addi alloc-tn alloc-tn ,alloc-size)
+ (inst add alloc-tn alloc-tn ,alloc-size))))
+ #!+gencgc
+ (let ((fix-addr (gensym))
+ (inline-alloc (gensym)))
+ `(let ((,fix-addr (gen-label))
+ (,inline-alloc (gen-label)))
+ ;; Make temp-tn be the size
+ (cond ((numberp ,size)
+ (inst lr ,temp-tn ,size))
+ (t
+ (move ,temp-tn ,size)))
+
+ (inst lr ,flag-tn (make-fixup "boxed_region" :foreign))
+ (inst lwz ,result-tn ,flag-tn 0)
+
+ ;; we can optimize this to only use one fixup here, once we get
+ ;; it working
+ ;; (inst lr ,flag-tn (make-fixup "boxed_region" :foreign 4))
+ ;; (inst lwz ,flag-tn ,flag-tn 0)
+ (inst lwz ,flag-tn ,flag-tn 4)
+
+ (without-scheduling ()
+ ;; CAUTION: The C code depends on the exact order of
+ ;; instructions here. In particular, three instructions before
+ ;; the TW instruction must be an ADD or ADDI instruction, so it
+ ;; can figure out the size of the desired allocation.
+ ;; Now make result-tn point at the end of the object, to
+ ;; figure out if we overflowed the current region.
+ (inst add ,result-tn ,result-tn ,temp-tn)
+ ;; result-tn points to the new end of the region. Did we go past
+ ;; the actual end of the region? If so, we need a full alloc.
+ ;; The C code depends on this exact form of instruction. If
+ ;; either changes, you have to change the other appropriately!
+ (inst cmpw ,result-tn ,flag-tn)
+
+ (inst bng ,inline-alloc)
+ (inst tw :lge ,result-tn ,flag-tn))
+ (inst b ,fix-addr)
+
+ (emit-label ,inline-alloc)
+ (inst lr ,flag-tn (make-fixup "boxed_region" :foreign))
+ (inst stw ,result-tn ,flag-tn 0)
+
+ (emit-label ,fix-addr)
+ ;; At this point, result-tn points at the end of the object.
+ ;; Adjust to point to the beginning.
+ (inst sub ,result-tn ,result-tn ,temp-tn)
+ ;; Set the lowtag appropriately
+ (inst ori ,result-tn ,result-tn ,lowtag))))
+
+(defmacro with-fixed-allocation ((result-tn flag-tn temp-tn type-code size
+ &key (lowtag other-pointer-lowtag))
+ &body body)
"Do stuff to allocate an other-pointer object of fixed Size with a single
word header having the specified Type-Code. The result is placed in
Result-TN, and Temp-TN is a non-descriptor temp (which may be randomly used
by the body.) The body is placed inside the PSEUDO-ATOMIC, and presumably
initializes the object."
(once-only ((result-tn result-tn) (temp-tn temp-tn) (flag-tn flag-tn)
- (type-code type-code) (size size))
- `(pseudo-atomic (,flag-tn :extra (pad-data-block ,size))
- (inst ori ,result-tn alloc-tn other-pointer-lowtag)
- (inst lr ,temp-tn (logior (ash (1- ,size) n-widetag-bits) ,type-code))
- (storew ,temp-tn ,result-tn 0 other-pointer-lowtag)
+ (type-code type-code) (size size) (lowtag lowtag))
+ `(pseudo-atomic (,flag-tn)
+ (allocation ,result-tn (pad-data-block ,size) ,lowtag
+ :temp-tn ,temp-tn
+ :flag-tn ,flag-tn)
+ (when ,type-code
+ (inst lr ,temp-tn (logior (ash (1- ,size) n-widetag-bits) ,type-code))
+ (storew ,temp-tn ,result-tn 0 ,lowtag))
,@body)))
-\f
-;;;; Type testing noise.
-
-;;; GEN-RANGE-TEST -- internal
-;;;
-;;; Generate code that branches to TARGET iff REG contains one of VALUES.
-;;; If NOT-P is true, invert the test. Jumping to NOT-TARGET is the same
-;;; as falling out the bottom.
-;;;
-(defun gen-range-test (reg target not-target not-p min seperation max values)
- (let ((tests nil)
- (start nil)
- (end nil)
- (insts nil))
- (multiple-value-bind (equal less-or-equal greater-or-equal label)
- (if not-p
- (values :ne :gt :lt not-target)
- (values :eq :le :ge target))
- (flet ((emit-test ()
- (if (= start end)
- (push start tests)
- (push (cons start end) tests))))
- (dolist (value values)
- (cond ((< value min)
- (error "~S is less than the specified minimum of ~S"
- value min))
- ((> value max)
- (error "~S is greater than the specified maximum of ~S"
- value max))
- ((not (zerop (rem (- value min) seperation)))
- (error "~S isn't an even multiple of ~S from ~S"
- value seperation min))
- ((null start)
- (setf start value))
- ((> value (+ end seperation))
- (emit-test)
- (setf start value)))
- (setf end value))
- (emit-test))
- (macrolet ((inst (name &rest args)
- `(push (list 'inst ',name ,@args) insts)))
- (do ((remaining (nreverse tests) (cdr remaining)))
- ((null remaining))
- (let ((test (car remaining))
- (last (null (cdr remaining))))
- (if (atom test)
- (progn
- (inst cmpwi reg test)
- (if last
- (inst b? equal target)
- (inst beq label)))
- (let ((start (car test))
- (end (cdr test)))
- (cond ((and (= start min) (= end max))
- (warn "The values ~S cover the entire range from ~
- ~S to ~S [step ~S]."
- values min max seperation)
- (push `(unless ,not-p (inst b ,target)) insts))
- ((= start min)
- (inst cmpwi reg end)
- (if last
- (inst b? less-or-equal target)
- (inst ble label)))
- ((= end max)
- (inst cmpwi reg start)
- (if last
- (inst b? greater-or-equal target)
- (inst bge label)))
- (t
- (inst cmpwi reg start)
- (inst blt (if not-p target not-target))
- (inst cmpwi reg end)
- (if last
- (inst b? less-or-equal target)
- (inst ble label))))))))))
- (nreverse insts)))
-
-(defun gen-other-immediate-test (reg target not-target not-p values)
- (gen-range-test reg target not-target not-p
- (+ other-immediate-0-lowtag lowtag-limit)
- (- other-immediate-1-lowtag other-immediate-0-lowtag)
- (ash 1 n-widetag-bits)
- values))
-
-
-(defun test-type-aux (reg temp target not-target not-p lowtags immed hdrs
- function-p)
- (let* ((fixnump (and (member even-fixnum-lowtag lowtags :test #'eql)
- (member odd-fixnum-lowtag lowtags :test #'eql)))
- (lowtags (sort (if fixnump
- (delete even-fixnum-lowtag
- (remove odd-fixnum-lowtag lowtags
- :test #'eql)
- :test #'eql)
- (copy-list lowtags))
- #'<))
- (lowtag (if function-p
- sb!vm:fun-pointer-lowtag
- sb!vm:other-pointer-lowtag))
- (hdrs (sort (copy-list hdrs) #'<))
- (immed (sort (copy-list immed) #'<)))
- (append
- (when immed
- `((inst andi. ,temp ,reg widetag-mask)
- ,@(if (or fixnump lowtags hdrs)
- (let ((fall-through (gensym)))
- `((let (,fall-through (gen-label))
- ,@(gen-other-immediate-test
- temp (if not-p not-target target)
- fall-through nil immed)
- (emit-label ,fall-through))))
- (gen-other-immediate-test temp target not-target not-p immed))))
- (when fixnump
- `((inst andi. ,temp ,reg 3)
- ,(if (or lowtags hdrs)
- `(inst beq ,(if not-p not-target target))
- `(inst b? ,(if not-p :ne :eq) ,target))))
- (when (or lowtags hdrs)
- `((inst andi. ,temp ,reg lowtag-mask)))
- (when lowtags
- (if hdrs
- (let ((fall-through (gensym)))
- `((let ((,fall-through (gen-label)))
- ,@(gen-range-test temp (if not-p not-target target)
- fall-through nil
- 0 1 (1- lowtag-limit) lowtags)
- (emit-label ,fall-through))))
- (gen-range-test temp target not-target not-p 0 1
- (1- lowtag-limit) lowtags)))
- (when hdrs
- `((inst cmpwi ,temp ,lowtag)
- (inst bne ,(if not-p target not-target))
- (load-type ,temp ,reg (- ,lowtag))
- ,@(gen-other-immediate-test temp target not-target not-p hdrs))))))
-
-(defparameter immediate-types
- (list base-char-widetag unbound-marker-widetag))
-
-(defparameter function-subtypes
- (list funcallable-instance-header-widetag
- simple-fun-header-widetag closure-fun-header-widetag
- closure-header-widetag))
-
-(defmacro test-type (register temp target not-p &rest type-codes)
- (let* ((type-codes (mapcar #'eval type-codes))
- (lowtags (remove lowtag-limit type-codes :test #'<))
- (extended (remove lowtag-limit type-codes :test #'>))
- (immediates (intersection extended immediate-types :test #'eql))
- (headers (set-difference extended immediate-types :test #'eql))
- (function-p nil))
- (unless type-codes
- (error "Must supply at least on type for test-type."))
- (when (and headers (member other-pointer-lowtag lowtags))
- (warn "OTHER-POINTER-LOWTAG supersedes the use of ~S" headers)
- (setf headers nil))
- (when (and immediates
- (or (member other-immediate-0-lowtag lowtags)
- (member other-immediate-1-lowtag lowtags)))
- (warn "OTHER-IMMEDIATE-n-LOWTAG supersedes the use of ~S" immediates)
- (setf immediates nil))
- (when (intersection headers function-subtypes)
- (unless (subsetp headers function-subtypes)
- (error "Can't test for mix of function subtypes and normal ~
- header types."))
- (setq function-p t))
-
- (let ((n-reg (gensym))
- (n-temp (gensym))
- (n-target (gensym))
- (not-target (gensym)))
- `(let ((,n-reg ,register)
- (,n-temp ,temp)
- (,n-target ,target)
- (,not-target (gen-label)))
- (declare (ignorable ,n-temp))
- ,@(if (constantp not-p)
- (test-type-aux n-reg n-temp n-target not-target
- (eval not-p) lowtags immediates headers
- function-p)
- `((cond (,not-p
- ,@(test-type-aux n-reg n-temp n-target not-target t
- lowtags immediates headers
- function-p))
- (t
- ,@(test-type-aux n-reg n-temp n-target not-target nil
- lowtags immediates headers
- function-p)))))
- (emit-label ,not-target)))))
+(defun align-csp (temp)
+ ;; is used for stack allocation of dynamic-extent objects
+ (let ((aligned (gen-label)))
+ (inst andi. temp csp-tn lowtag-mask)
+ (inst beq aligned)
+ (inst addi csp-tn csp-tn n-word-bytes)
+ (storew zero-tn csp-tn -1)
+ (emit-label aligned)))
\f
;;;; Error Code
-
-(defvar *adjustable-vectors* nil)
-
-(defmacro with-adjustable-vector ((var) &rest body)
- `(let ((,var (or (pop *adjustable-vectors*)
- (make-array 16
- :element-type '(unsigned-byte 8)
- :fill-pointer 0
- :adjustable t))))
- (setf (fill-pointer ,var) 0)
- (unwind-protect
- (progn
- ,@body)
- (push ,var *adjustable-vectors*))))
-
(eval-when (:compile-toplevel :load-toplevel :execute)
(defun emit-error-break (vop kind code values)
(let ((vector (gensym)))
`((let ((vop ,vop))
- (when vop
- (note-this-location vop :internal-error)))
- (inst unimp ,kind)
- (with-adjustable-vector (,vector)
- (write-var-integer (error-number-or-lose ',code) ,vector)
- ,@(mapcar #'(lambda (tn)
- `(let ((tn ,tn))
- (write-var-integer (make-sc-offset (sc-number
- (tn-sc tn))
- (tn-offset tn))
- ,vector)))
- values)
- (inst byte (length ,vector))
- (dotimes (i (length ,vector))
- (inst byte (aref ,vector i))))
- (align word-shift)))))
+ (when vop
+ (note-this-location vop :internal-error)))
+ (inst unimp ,kind)
+ (with-adjustable-vector (,vector)
+ (write-var-integer (error-number-or-lose ',code) ,vector)
+ ,@(mapcar #'(lambda (tn)
+ `(let ((tn ,tn))
+ (write-var-integer (make-sc-offset (sc-number
+ (tn-sc tn))
+ (tn-offset tn))
+ ,vector)))
+ values)
+ (inst byte (length ,vector))
+ (dotimes (i (length ,vector))
+ (inst byte (aref ,vector i))))
+ (align word-shift)))))
(defmacro error-call (vop error-code &rest values)
"Cause an error. ERROR-CODE is the error to cause."
(cons 'progn
- (emit-error-break vop error-trap error-code values)))
+ (emit-error-break vop error-trap error-code values)))
(defmacro cerror-call (vop label error-code &rest values)
Emit code for a continuable error with the specified Error-Code and
context Values. If the error is continued, execution resumes after
the GENERATE-CERROR-CODE form."
- (let ((continue (gensym "CONTINUE-LABEL-"))
- (error (gensym "ERROR-LABEL-")))
+ (with-unique-names (continue error)
`(let ((,continue (gen-label)))
(emit-label ,continue)
(assemble (*elsewhere*)
- (let ((,error (gen-label)))
- (emit-label ,error)
- (cerror-call ,vop ,continue ,error-code ,@values)
- ,error)))))
-
-
+ (let ((,error (gen-label)))
+ (emit-label ,error)
+ (cerror-call ,vop ,continue ,error-code ,@values)
+ ,error)))))
\f
-;;; PSEUDO-ATOMIC -- Handy macro for making sequences look atomic.
-;;;
-;;; flag-tn must be wired to NL3. If a deferred interrupt happens
-;;; while we have the low bits of alloc-tn set, we add a "large"
-;;; constant to flag-tn. On exit, we add flag-tn to alloc-tn
-;;; which (a) aligns alloc-tn again and (b) makes alloc-tn go
-;;; negative. We then trap if alloc-tn's negative (handling the
-;;; deferred interrupt) and using flag-tn - minus the large constant -
-;;; to correct alloc-tn.
-(defmacro pseudo-atomic ((flag-tn &key (extra 0)) &rest forms)
- (let ((n-extra (gensym)))
- `(let ((,n-extra ,extra))
- (without-scheduling ()
- ;; Extra debugging stuff:
- #+debug
- (progn
- (inst andi. ,flag-tn alloc-tn 7)
- (inst twi :ne ,flag-tn 0))
- (inst lr ,flag-tn (- ,n-extra 4))
- (inst addi alloc-tn alloc-tn 4))
- ,@forms
- (without-scheduling ()
- (inst add alloc-tn alloc-tn ,flag-tn)
- (inst twi :lt alloc-tn 0))
- #+debug
- (progn
- (inst andi. ,flag-tn alloc-tn 7)
- (inst twi :ne ,flag-tn 0)))))
-
-
+;;;; PSEUDO-ATOMIC
+;;; handy macro for making sequences look atomic
+;;;
+;;; FLAG-TN must be wired to NL3. If a deferred interrupt happens
+;;; while we have the low bits of ALLOC-TN set, we add a "large"
+;;; constant to FLAG-TN. On exit, we add FLAG-TN to ALLOC-TN which (a)
+;;; aligns ALLOC-TN again and (b) makes ALLOC-TN go negative. We then
+;;; trap if ALLOC-TN's negative (handling the deferred interrupt) and
+;;; using FLAG-TN - minus the large constant - to correct ALLOC-TN.
+(defmacro pseudo-atomic ((flag-tn) &body forms)
+ `(progn
+ (without-scheduling ()
+ ;; Extra debugging stuff:
+ #+debug
+ (progn
+ (inst andi. ,flag-tn alloc-tn 7)
+ (inst twi :ne ,flag-tn 0))
+ (inst ori alloc-tn alloc-tn 4))
+ ,@forms
+ (without-scheduling ()
+ (inst li ,flag-tn -5)
+ (inst and alloc-tn alloc-tn ,flag-tn)
+ ;; Now test to see if the pseudo-atomic interrupted bit is set.
+ (inst andi. ,flag-tn alloc-tn 1)
+ (inst twi :ne ,flag-tn 0))
+ #+debug
+ (progn
+ (inst andi. ,flag-tn alloc-tn 7)
+ (inst twi :ne ,flag-tn 0))))
+
+(def!macro with-pinned-objects ((&rest objects) &body body)
+ "Arrange with the garbage collector that the pages occupied by
+OBJECTS will not be moved in memory for the duration of BODY.
+Useful for e.g. foreign calls where another thread may trigger
+garbage collection. This is currently implemented by disabling GC"
+ (declare (ignore objects)) ; should we eval these for side-effect?
+ `(without-gcing
+ ,@body))