diff --git a/LOG b/LOG
index 8a40deae00..60d8fbb286 100644
--- a/LOG
+++ b/LOG
@@ -345,3 +345,8 @@
      fasl.c, gc.c, globals.h, prim.c, prim5.c, scheme.c, schsig.c,
      misc.ms, root-experr*,
      objects.stex
+- for non-win32 systems, now setting al register to a count of the
+  floating-point register arguments as required for varargs functions
+  by the System V ABI.
+     x86_64.ss,
+     foreign.ms
diff --git a/mats/foreign.ms b/mats/foreign.ms
index ac9b4913ba..3aed34e020 100644
--- a/mats/foreign.ms
+++ b/mats/foreign.ms
@@ -2577,3 +2577,59 @@
               (equal? s3 "#<procedure abort>")
               (eqv? pid^ pid)))))))
 )
+
+(mat varargs
+  (begin
+    (define load-libc
+      (machine-case
+        [(i3ob ti3ob a6ob ta6ob i3fb ti3fb a6fb ta6fb a6s2 ta6s2 i3s2 ti3s2 i3qnx ti3qnx i3nb ti3nb a6nb ta6nb)
+         '(load-shared-object "libc.so")]
+        [(i3le ti3le a6le ta6le arm32le tarm32le ppc32le tppc32le)
+         '(load-shared-object "libc.so.6")]
+        [(i3nt ti3nt a6nt ta6nt)
+         (load-shared-object "msvcrt.dll")]
+        [(i3osx ti3osx a6osx ta6osx)
+         '(load-shared-object "libc.dylib")]
+        [else (error 'load-libc "unrecognized machine type ~s" (machine-type))]))
+    #t)
+  (equal?
+    (with-input-from-string
+      (separate-eval
+        `(begin
+           ,load-libc
+           (define f (foreign-procedure "printf" (string double) int))
+           (f "(%g)" 3.5)
+           (void)))
+      read)
+    '(3.5))
+  (equal?
+    (with-input-from-string
+      (separate-eval
+        `(begin
+           ,load-libc
+           (define f (foreign-procedure "printf" (string double double double double double double) int))
+           (f "(%g %g %g %g %g %g)" 3.5 2.5 -1.5 6.75 8.25 -9.5)
+           (void)))
+      read)
+    '(3.5 2.5 -1.5 6.75 8.25 -9.5))
+  (equal?
+    (with-input-from-string
+      (separate-eval
+        `(begin
+           ,load-libc
+           (define f (foreign-procedure "printf" (string double double double double double double double double) int))
+           (f "(%g %g %g %g %g %g %g %g)" 3.5 2.5 -1.5 6.75 8.25 -9.5 1e32 -4.5)
+           (void)))
+      read)
+    '(3.5 2.5 -1.5 6.75 8.25 -9.5 1e32 -4.5))
+  (equal?
+    (with-input-from-string
+      (separate-eval
+        `(begin
+           ,load-libc
+           (define f (foreign-procedure "printf" (string double double double double double double double double double double) int))
+           (f "(%g %g %g %g %g %g %g %g %g %g)" 3.5 2.5 -1.5 6.75 8.25 -9.5 1e32 -4.5 7.25 -0.5)
+           (void)))
+      read)
+    '(3.5 2.5 -1.5 6.75 8.25 -9.5 1e32 -4.5 7.25 -0.5))
+)
diff --git a/s/x86_64.ss b/s/x86_64.ss
index 071556dc16..7aa144241e 100644
--- a/s/x86_64.ss
+++ b/s/x86_64.ss
@@ -2441,7 +2441,7 @@
                     (if-feature windows
                       (let loop ([types types] [locs '()] [regs '()] [i 0] [isp 0])
                         (if (null? types)
-                            (values isp locs regs)
+                            (values isp 0 locs regs)
                             (nanopass-case (Ltype Type) (car types)
                               [(fp-double-float)
                                (if (< i 4)
@@ -2471,7 +2471,7 @@
                                      regs i (fx+ isp 8)))])))
                       (let loop ([types types] [locs '()] [regs '()] [iint 0] [ifp 0] [isp 0])
                         (if (null? types)
-                            (values isp locs regs)
+                            (values isp ifp locs regs)
                             (nanopass-case (Ltype Type) (car types)
                               [(fp-double-float)
                                (if (< ifp 8)
@@ -2522,7 +2522,7 @@
                   [arg-type* (info-foreign-arg-type* info)]
                   [result-type (info-foreign-result-type info)])
               (with-values (do-args arg-type* (make-vint) (make-vfp))
-                (lambda (frame-size locs live*)
+                (lambda (frame-size nfp locs live*)
                   (returnem frame-size locs
                     (lambda (t0)
                       (if-feature windows
@@ -2530,7 +2530,11 @@
                           (set! ,%sp ,(%inline - ,%sp (immediate 32)))
                           (inline ,(make-info-kill*-live* (reg-list %rax) live*) ,%c-call ,t0)
                           (set! ,%sp ,(%inline + ,%sp (immediate 32))))
-                        `(inline ,(make-info-kill*-live* (reg-list %rax) live*) ,%c-call ,t0)))
+                        (%seq
+                          ; System V ABI varargs functions require count of fp regs used in %al register.
+                          ; since we don't know if the callee is a varargs function, we always set it.
+                          (set! ,%rax (immediate ,nfp))
+                          (inline ,(make-info-kill*-live* (reg-list %rax) (cons %rax live*)) ,%c-call ,t0))))
                     (nanopass-case (Ltype Type) result-type
                       [(fp-double-float)
                        (lambda (lvalue)