ffi: support float type as variadic argument.

The float type promotes to double when passed as
a variadic argument.

This patch adds internal FFI types which models that
promotion. It uses double for its C type, while still
performing the range checks for float.

Also, the types be-float and le-float are rejected from
being variadic arguments.

* share/txr/stdlib/ffi.tl (analyze-argtypes): Rewrite
function, adding validation and substitution for the
variadic part of the argument type list.  Map float type to
double, and reject be-float and le-float.

* txr.1: Documented.

2 files changed, 69 insertions, 8 deletions

diff --git a/share/txr/stdlib/ffi.tl b/share/txr/stdlib/ffi.tl
index e171cb47..480faf5d 100644
--- a/share/txr/stdlib/ffi.tl
+++ b/share/txr/stdlib/ffi.tl
@@ -54,12 +54,18 @@
     (t exp)))
 
 (defun sys:analyze-argtypes (form argtypes)
-  (let ((p (posq : argtypes)))
-    (when p
-      (if (zerop p)
-        (compile-error form "variadic with zero fixed arguments not allowed")
-        (del [argtypes p])))
-    (list* (length argtypes) p argtypes)))
+  (tree-bind (: ftypes vtypes) (split* argtypes (op where (op eq :)))
+    (when vtypes
+      (when (null ftypes)
+        (compile-error form "variadic with zero fixed arguments not allowed"))
+      (set vtypes
+           (collect-each ((vt vtypes))
+             (caseq vt
+               ((float) 'double)
+               ((be-float le-float)
+                (compile-error form "variadic argument cannot be of type ~s"
+                               vt))))))
+    (list* (+ (len ftypes) (len vtypes)) (len ftypes) (append ftypes vtypes))))
 
 
 (defmacro deffi (:form f :env e name fun-expr rettype argtypes)
diff --git a/txr.1 b/txr.1
index 4da6bf8e..ad295cde 100644
--- a/txr.1
+++ b/txr.1
@@ -72113,6 +72113,26 @@ descriptor, and
 even if the return types and
 argument types match.
 
+Note: unlike the
+.code deffi
+and
+.code deffi-cb
+macros,
+the
+.code ffi-make-call-desc
+function doesn't perform any special treatment of variadic parameter types.
+When any of the types
+.codn float ,
+.code be-float
+or
+.code le-float
+occur in the variadic portion of
+.metn argtypes ,
+it is unspecified whether a descriptor is successfully produced and returned
+or whether an exception is thrown. If a descriptor is successfully produced,
+and then subsequently used for making or accepting calls, the behavior is
+undefined.
+
 .TP* Example:
 
 .verb
@@ -72689,8 +72709,9 @@ symbol appears as one of the elements of
 then the
 .code deffi
 form specifies a fixed call to a foreign function which is variadic. The
-argument types before the colon keyword are the fixed arguments. The types
-after the colon, if any, are the variadic arguments.
+argument types before the colon keyword are the types of the fixed arguments.
+The types after the colon, if any, are of the variadic arguments. Special
+considerations apply to some variadic argument types, described below.
 
 The following syntactic variants are permitted of the
 .meta fun-expr
@@ -72732,6 +72753,40 @@ macro.
 .RE
 
 .IP
+When the FFI type
+.code float
+is used as the type of a variadic parameter,
+.code deffi
+replaces it by the FFI type
+.codn double .
+This treatment is necessary because the C variadic argument mechanism promotes
+.code float
+values to
+.codn double .
+Note: due to this substitution, it is possible to pass floating-point values
+which are out of range of the
+.code float
+type, without any diagnosis. The behavior of is undefined in the
+Lisp-to-C direction, if the C function extracts an out-of-range
+.code double
+argument as if it were of type
+.codn float .
+
+The FFI types
+.code be-float
+and
+.code le-float
+cannot be used for specifying the types of a variadic argument. If any of
+these occur in that position,
+.code deffi
+throws an error.
+Rationale: these types are related to the C type
+.code float
+type, which requires promotion in variadic passing. Promotion cannot
+be performed on floating-point values whose byte order has been rearranged,
+because promotion is a value-preserving conversion.
+
+.IP
 The result value of a
 .code deffi
 form is